Sealing a concrete slab to block humidity from entering basement?

Tomorrow is moving day, saying goodbye to Cambridge and hello to Lincoln, Massachusetts.  Our new house requires a little history lesson.  In the bad old days people dug down into the earth to make a basement foundation for a house.  For hundreds if not thousands of years these primitive people stored roots, boxes, and other junk down in this basement while they lived in various floors built well above the ground level.  Above the top floor they built an attic in which to store more junk.  This attic became very handy in the 1970s when people looked for a place to put insulation, a whole-house fan, and other energy-saving devices.

Fortunately for mankind, in the 1960s a group of MIT and Harvard architecture geniuses came along and designed the Deck House kits.  Why have a contractor assemble building materials and the local drunkards on-site to frame a house when the thing could be precision-cut in a factory?  Storing carrots and boxes in your basement?  Why not live in the basement instead?  The typical Deck House, including the one we are moving into, is cut into a hillside so that part of the basement is walk-out level.  Half of the living space is in this “first floor”, directly on top of a concrete slab.

How about an attic?  Wasteful!  Just make the living room ceiling the underside of the roof.  The roof in this house is about 5 inches thick from the bottom of the living room ceiling to the top of the asphalt shingles.  I’m hoping that there is some insulation in there, but am not confident.  In any case, there is no way to add insulation without redesigning the roof to be about twice as thick.  There is also no obvious place for a whole-house fan.  Storage?  Dump it all in the garage.

What happens when you live in your basement?  It tends to be kind of damp.  How about cranking up the central A/C to dehumidify it?  The geniuses behind the original Deck House didn’t see the point in running ductwork.  As there are no conventional floors or walls, it is not obvious how to run ducts after the fact.

I am now on a fanatical quest to dehumidify the basement, partly because I want to store some slides and negatives down there.

My first idea is to minimize the amount of moisture coming into the basement.  A landscaping company is coming over to dig away a small portion of earth against the foundation and see if it is moist enough to warrant a “French drain” (pipe with holes in it).

I know that concrete breathes and am wondering if humidity is coming up through the slab, which was poured in 1968 as part of an outrageously expensive construction project ($38,000 to build 4000 square feet; thank God that the government assures us inflation is minimal or I would be upset by the insurance company’s estimate of $800,000 to rebuild the place today).  I have instrumented the house with a Honeywell weather station from Costco (am now a true suburbanite).  The Honeywell is saying that the humidity is about the same throughout the house, at least this evening.  Without the heat being on, it is a dank 57 percent relative humidity upstairs and down.  So we have some evidence that water vapor is NOT coming up through the slab.  A civil engineer friend says that basements are damped simply because they are cooler than the rest of the house and the humidity mostly comes from the outside air.  Nonetheless, I feel compelled to attempt to seal the slab.

The slab is currently covered by some glue, a porous carpet pad, and a medium-thick carpet.  She who has the last word on home decor says “carpet is disgusting; I would pay as much as $10,000 to have the carpet replaced by hardwood.”  I replied “So I guess we won’t do it, then, because the quote for engineered hardwood was $15,000.”  It seems that we are doing it…

The standard way that flooring companies put down hardwood over a basement slab is… by not using hardwood.  A solid chunk of wood tends to warp.  The solution for potentially humid conditions is particle board or plywood topped with a thin wood veneer.  It looks like regular hardwood, but is resistant to warping.  The installation starts by taping down a closed-cell foam underlayment, which is more or less impermeable to water vapor, and then floating the floor on top of the foam.  An engineer friend recommends 6 mil polyethylene sheeting underneath the foam, overlapped by at least one foot at the seams.

My question to the suburban heroes reading this is… Why not seal the slab with a chemical before putting down the sheeting and foam?  If we don’t do it now we will not have another chance for 20 years.

Then the question becomes how to seal concrete.  Drylok is the standard product, but I’m not sure what value it would be on a 40-year-old slab.  I talked to a company down in Texas, where they should know something about humidity, called Enduroseal.  They recommended

  1. cleaning to remove the adhesive with a product from
  2. treating with their own Hydraloc product
  3. waiting 10 days
  4. treating with their NCS-20 product (not sure why they recommended this on the phone; the current slab does not have any flowing water on it and actually feels dry to the touch)

Anyone out there competent to evaluate this company’s claims or have personal experience with this stuff?  Is what they are selling better than Drylok from Home Depot?

Stepping back the larger question is whether it makes sense to seal the concrete slab at all.  It is dry to the touch and the carpet feels mostly cool rather than damp.  There is a slight musty smell in the basement and slight mold growth on a couple of beams.

[This is outside the scope of the current blog posting, but the next planned step after the sealing and hardwood is to install some sort of central plumbed-in dehumidifier.  There is a small utility room at the center of the basement.  One idea is to put a standard HVAC air handler in this room.  Run ducts up to the top floor and discharge air up there.  Have the intakes in a couple of the rooms on the ground floor, thus forcing the air to circulate from top to bottom of the house.  Honeywell and Lennox both make whole-house dehumidifiers that plug into air handlers.  Run A/C in the summer and the dehumidifier in fall and spring.  While we’re at it, add an air filter and a humidifier for December through March.]

33 thoughts on “Sealing a concrete slab to block humidity from entering basement?

  1. I have a finished basement with the slab covered by thick plastic sheets, plywood and hardwood strips anchored to the plywood and it is completely dry and warm. The work was done about 20yrs ago by the previous owner and having seen a portion of the slab itself doubt it received any treatment. The fact that the basement’s walls are covered by sheet rock may also contribute significantly to the feeling of dryness.

    My understanding of french drains is that they provide a path of least resistance to the moisture in the surrounding soil so that it doesn’t want to go in places like foundation cracks. I don’t think they will prevent soil from being damp; they just redirect any excess water flow.

  2. You don’t want to solve moisture problems by sealing concrete as they all will fail, according to many guys I had out to look at my basement. We had actual water coming up in several places and moisture for three seasons. I put in a perimeter drain and a sump pump which has solved the water seepage. I am overlaying the floor with dry-core which is a product with a dimpled plastic laminated to 3/4 OSB. This creates an air gap where water vapor can escape into the living space. I still need to run a dehumidifier. The reason for this over the poly directly on the floor is to prevent mold and trapped water if it does come in for some reason. The part of the basement where I have used the dry-core is noticeably warmer in the winter than the office area with only carpet over the concrete. I will be putting dry-core down in the office next.

  3. You’ll need to keep air moving continuously throug hthe basementto keep it dry. one of the problems is warm air and exhalating + cooler concrete = condensation. Keeping your film archive in the basement is not a good idea. Unfortunately I happen to know this from experience.

    Have fun moving! Sorry you are having to do this.

  4. The other condo in our row-house is “garden level” (aka half-under basement) and first floor. When the old house was renovated into condos, the renovator (a sometimes architecture professor) put in a wood laminate floor in the basement. By the time we looked at the condos, the floor was hopelessly warped, and she had to take that one temporarily off the market to refloor it. The new flooring warped too. They have carpet now.

  5. You may also want to look into Dricore (or a competing product). It’s a waffle of hard plastic on the bottom and OSB or plywood that serves as a subfloor. The waffle of plastic is to allow airflow under the wood to permit the dehumidification to “reach” the concrete directly, rather than only after it’s come through your $15K wood floot.

  6. Considering all the trouble you’re already going through, I’d consider an underlayment with dimpled plastic that creates an airspace immediately next to the floor (for insulation and air circulation or at least isolation), then put your foam underlayment over that.

  7. Moving water away from the house is critical. In most cases the water in your basement comes from your own roof. Keep the gutters clean so they don’t overflow onto the foundation. Route the downspout extensions far, far away from the foundation using long extension tubes. You can always run them underground if your wife doesn’t want to look at them. Ideally the ends of the downspout extensions will be a foot or more below the foundation so you have positive flow.

    You want soil up against the foundation, sloping away from the house.

    When you install water heaters, washers, driers, etc. on a concrete slab, be sure to put plastic pans under them. Most people think the appliance pans are to catch water. They’re actually to keep moisture from the concrete from rusting the equipment.

    You got good advice about not using hardwood on a slab. Hardwood on concrete is a recipe for heartache, particularly in a basement.

    The usual alternative is Pergo, which is an engineered wood with a plastic top from Sweden. It’s installed on top of plastic sheeting and thin foam insulation, pretty much as you described. The stuff in my house is 10 years old and has no problems. (For all other applications, however, I think hardwood is preferable. Pergo eventually gets scuffed up and unlike wood Pergo can’t be refinished.) Another option is ceramic tile.

  8. also be careful what kinds of adhesive they use to put down your engineered wood product…some of them release some serious VOC’s which are not at all good for you!

  9. Phillip,
    I think you are generally on the right track.

    You need the Building Science Corp book:
    Builder’s Guide to Cold Climates
    That will help a lot. or call their consulting branch:

    For a house without ductwork, consider “mini-split” air-source heat pumps from Daikin or Fujitsu or Mitsubishi (these are the best brands). AC in summer, 3x efficient heat in the winter (with green electricity from your electricity co)

    The typical inexpensive approach for basement dehumidifying is a standalone dehumidifier connected to a condensate pump with pumps the water up and outside thru a tiny clear flexible pvc tubing. but it’s loud if you want to use the space for more than storage.

  10. I think Les nailed it. We had real hardwood floors in our basement in Brookline. One flood and the floor warped and buckled, requiring a complete, messy resanding. In our current house, we installed Pergo. However our installer was not that experienced and the floor buckles a little in the hot, humid summer months, even with the upstairs A/C left on. These floating floors do have limits on the distances and weight of furniture since they really do move slightly with the changing atmospherics.

    Les is further correct that the source of moisture is usually your roof drainage. It could be the local topology but that’s more rare. Fix the gutters and downspouts before you do significant work on the basement.

    We have a portable dehumidifier for the winter months that can be installed permanently but we don’t like it. It makes the basement uncomfortably hot, as it is effectively a reverse A/C or heat pump. It’s also noisy. I suggest you try a portable (or borrow mine) and see the results before you hardwire one.

    The humidity is quite tolerable for most of the year. It’s the hottest days in the cool basement that cause the most problem. Having an extra a/c for just that would probably suffice.

    If you are most concerned about the longevity of your negatives, why not store them off-site in a reputable, professional temperature controlled environment? It won’t be that expensive and it will be much easier to maintain a stable environment than any reasonable priced home solution.

  11. The first, cheap method to determine if water is coming through the slab, is to take a piece of plastic with no holes in it, and lay it down over the concrete, sealing the ends with something heavy like 2x4s, concrete blocks, etc. After a day or two, lift up the plastic and see if it is wet or not.

    Concerning french drain construction, they are simple to build, and I believe there is special equipment (similar to the kind used for laying fiber-optic cable) that can “shoot” the pipe into the ground without the laborious digging , i.e. boring a hole instead of digging / laying the pipe / covering.

    Definitely checking all drains around the house is an inexpensive thing to do and possibly cleaning them out will fix the issue.

  12. We renovated our basement earlier this year, we looked at many different options but here are a few suggestions that worked in our case. This will only work if you have enough head room in your basement.
    1. Make sure you don’t have a leaky foundation. If there is a drainage problem, fix it from the outside, not the inside.
    2. Lay down 1.5″ or 2.5″ of blue XPS board, seal all joints with tuck tape or equivalent. This will create a vapour barrier.
    3. Lay down a radiant heating system, electric or hotwater depending on your preference. Since our house has a hotwater radiant system we used hotwater radiant heat.
    4. Pour another 1.5″ to 2″ of colored concrete.
    5. Polish concrete.
    6. Add a fan going to the outside controlled by a humidistat.

    Result is a dry, warm floor that looks like one big piece of marble flooring. The radiant heat if properly sized to the space should minimize any humidity problems.

  13. [I hit the submit button too soon, please remove my post above]


    After dealing with a partial basement addition put in during the late 1970s, a period of ‘anything goes’ construction here on the CA coast, all I can offer you are my warm wishes.

    Seriously, though, concrete does allow moisture to pass through. In our case, we were literally inches above ground water during the winter rains, and therefore any ‘cut’ of natural grade is an invitation for a seasonal spring to develop, such as in the basement. The floor, even without the springs providing water from above, shows signs of water transmission via effervescence on the concrete surface, slight buckling/ridging (its no longer flat and level), etc. In this condition, about the only remedy is to remove it all, excavate, and build anew.

    Ours is an extreme case, but I would suggest extreme caution, and study through the various seasons of the year to see what you’re working with before committing to laying down expensive flooring, which could easily be ruined with excess moisture. (is there a decidedly more wet season near Boston? Perhaps not a consideration).

    If its in fundamentally good shape, I’m wondering if some sort of epoxy like coating that you’d see on a hangar floor would be appropriate?

  14. You have yourself a fine project. If your contractor is doing extensive excavation around the below grade exterior walls to install the french drain, you might consider applying plenty of styrofoam or other type sheet insulation and a new tar based seal to the exterior of the wall. This insulation is typical construction for below the slab here in the South, and should help you retain heat if applied to your vertical walls.

    Previous poster is spot on right about the importance of directing rain water away from the house. What is the configuration/condition of the seal where the floor slab meets the wall slab? Possible moisture ingress point here.

    I see you want to use a “drylok” type seal on the inside of the concrete, and I think you should, for peace of mind if nothing else.

    The carpet will, in my view, make you sick sooner or later due to mold, which is sometimes invisible, but a real hazard.

    I am not fond of pergo. We have had two houses with a gypsum concrete floor over plywood subfloor, and used hardwood flooring – type where an oak (or whatever) veneer is applied to tounge and groove plywood. The sections (8″ x 4′ typical) are edge glued together and the whole floor floats on the slab or subfloor. Provide plastic sheeting and a high quality thin foam pad (specific to this application) as previously suggested. The veneer over T&G plywood product has a factory applied aluminum oxide finish that is extremely scratch resistant, and there is no smell from site applied finishes. the plywood (about 5/8″ also insulates better than the pergo substrate. Properly installed, space is left around the perimeter, and moisture will not likely make any sections bow upward due to expansion.

    If you want to install central HVAC (a good decision), you can, ceiling height permitting, install a grid type drop ceiling with insulation above – architectural versions of this product are attractive and stylish. The drop ceiling will provide space for the ductwork – perhaps the existing ceiling can be demolished to provide room for ducts or additional insulartion between the roof joists. Messy.

    I have taken a lot of information from your wonderful web site over the past few years, hope I have given a bit back. Hope this info is some use. Many thanks.

  15. As background, my advice on building matters comes as a designer / generalist who teams up with true experts — and I have experienced several whole-house renovations-with-problems that yielded cost-effective results, great satisfaction, no regrets, a supportive home base for creative living, and even some publication requests.

    As Christopher Alexander has written, the most critical part of any creative project is identifying the optimum sequence of events. So, first things first.

    1. Health. Radon exists at a need-to-know level in 30% of houses in Middlesex County, your new County. You can easily test for radon yourself with one $14.95 test kit from 1-800-AIR-CHEK. Their general information website is Test kit instructions are simple and clear. The results will be as accurate as the state-of-the-art equipment used by my radon mentor, who was the senior health physicist at EG&G.

    Given his experience with the cumulative health effects of radon, Dr. Dowell Martz set a low threshold for radon tolerance. I have met workshop-trained, certified inspectors who argue confidently for a higher threshold. Radon is the heaviest gas and, absent convection currents, will concentrate near the floor, where dogs and children breathe. So, do test as soon as you have finished with all the airflow of moving in, and let us know the results. Please know that if there is radon present, effective remedies are simple but very specific. With your flooring plans upcoming anyway, you are at the perfect stage to easily employ best practices. More if you find radon …

    2. Maximize health, comfort & air quality with one solution?! You are also at the perfect stage to consider retrofitting radiant heat over your first floor slab, using a specific small-caliper tubing product. I never like to use the concept “expensive” to disqualify a solution before I consider the actual cost-benefit analysis over the life of the installation.

    Why radiant heat?

    – You have a choice location and you have projected a 20 year occupancy.
    – You have 1968 construction, oil heat — whether by air or by radiators — and some sense of dankness downstairs.
    – Your stage-of-development prior to new flooring is opportune.
    – You want to achieve a photography-friendly humidity level.
    – BTW, what is your outdoor humidity level when indoors is 57%?
    – You will want to eliminate any allergens from mustiness and mold.
    – You want to be truly comfortable at home in the New England climate, even as you enjoy indoor creative interests that are physically sedentary.

    – Radiant heat warms your core temperature so that air temperatures need
    not be so warm as to be doze-inducing. An inspiring little book: Thermal Delight in Architecture, by Lisa Hershong, is worth knowing.
    – I have lived with radiant heat in zone 4 for decades and find that it actually makes your body less sensitive to cold overall — much like the effect you get from raising your core temperature in a hot springs or sauna in winter — but the effect becomes systemic & lasting.
    – A radiant floor creates air convection currents that contribute to better air quality and support optimum humidity levels.
    – We retrofitted radiant heating over existing floors in an historic house — without having to reset door frames — by using a small-caliper tubing product. Tube caliper is ~ ¼-3/8,” and the array is configured as two tubes linked every foot or so, one for outflow and one for return, both connected to a manifold. My expert on this, David Reel of By Design, has left the planet, but if you like I can research a product link from his colleagues or clients around Port Townsend, on the Olympic Peninsula. Upon query, Boston contractors who have seen & used only the single, hose-sized tubing may imply that you must be nuts, a response I met with at first in Newton Highlands when seeking products found in other parts of the country. Here in Sun Valley, Idaho, my regular plumber took me aside and said, “I hate to tell you this, but this is never going to work! That tubing is too small to pump out the heat!” When asked, David responded with a smile, “The smaller the caliper, the slower the flow, the greater the heat transfer.” Done. The system is still working beautifully after 25 years!
    – The cost in the mid 80s was $10,000 and that included the finished floor of 1.5” poured concrete in a 4’ tile grid, sans rebar, and professionally wet-ground off [don’t even think about dry-grinding!] to show the terrazzo-like grain — and topped with an impervious, waxed-looking finish that I found when we used it on hardwood floors in Massachusetts.
    – The math is excellent on cost-effectiveness and cost-benefits.
    – I have clear photographs of this installation process if you are interested. It was initiated as part of a whole-house radon remediation — about which I have never written the book until now …
    – Apropos of your potential retrofit considerations, a friend who is a retired
    bridge builder twice built his favorite 2-storey house plan with radiant heated floors and he used only an ordinary water heater as a heat source. That was in northern coastal California where winters are cold and damp.

    3. Ground your electronics systems against nearby lightening strikes. According to David Reel, whole house, computerized systems should be grounded more extensively than the ready-made lightning-rod or surge-protector products that were available mid 80s. He recommended using heavier copper and a much wider outreach than they anticipated. We never did that and twice I lost the systems computer or pumps when the transformer on a utility pole in the alley was struck by lightening. That feels expensive!

    4. Do consider finding a local expert who knows who does great work. Ethan’s suggestion sounds good. And there is Frank Olney, of Johnson Olney Associates Inc, in Boston: (617) 482-2806. He had a lot of pilot friends back when he was the intelligence officer on a carrier, and he has probably never seen some of his Boston area projects from a helicopter.

    5. Air quality, dehumidification & cost-effective heat enhancement can be addressed all together with an air-to-air heat exchanger. You need an expert for current recommendations. David Reel’s choice for all of the above in the mid 80s was a “Star Heat Exchanger” and I do not see a relevant link to that product today. Ours was instrumented to kick on silently with high humidity and/or from radon. The dehumidification was dramatic and we got rid of the ice dams at the south window wall in the garden hallway, where we had radiant heat under the bricks and the growing greens zones, but used only solar gain as it turned out – a working example of Thermal Delight in zone 4. It was mid-winter when David first turned on the heat-exchanger and the incoming fresh air was arctic-bracing. I asked, “Can we temper that cold air?” and after a beat David dashed out, got an old car radiator, put it in an insulated box in the attic – could be outside – piped hot water through the radiator and directed the incoming air through the radiator. An hour later we had incoming tropical fresh air and the system is still working perfectly. What a great soul!

    6. Health again: please acquaint yourself with the bau-biology movement.
    Foremost, you want to critically assess the off-gassing potential of any glue
    laminated wood product, especially one as big as a new floor, and especially going into winter when the house is closed to ample natural ventilation. You have to dig for the details. New carpeting also needs to be assessed. My case in point there hurts me: at age 63 my father was the foremost track & field athlete for his age group in the world. And ten years later, after he moved into a new home in paradise – a new home laden with pressed wood construction & products – he developed chronic fatigue syndrome. I have seen similar effects in younger people who started with vibrant health and have read sad stories about pets who became the canary in the coal mine.

    And second, in any wired house, it is good to be aware of EMF patterns from wiring & appliances and to know how to ensure an EMF-free zone for a healthful sleeping area.

    Congratulations on your new home! I checked Deck Houses on Google images and the look is promising. 4000 sq ft makes a wonderful indoor world in the wintertime.

  16. I would give up the idea of sealing your concrete. As you mentioned, the slab has glue on it already which makes application of a sealer difficult at best. Scraping the current felt pad/glue off will have the hardwood installers teaching you all types of new vocabulary as they labor to pull tarry old glue off an old slab. At best, a smooth, tarry surface will be left and trying to have a perfect moisture barrier painted on over it will be nearly impossible. The plastic sheeting idea suggested by your CE pal is much better and easier to do. The extra plastic sheet (6-8 mil) under your foamy layer may help you sleep better at night. I would pick a corner basement room which is already carpeted and may feel damp to you. Yank a good size piece up (hey you are replacing it anyways so who cares?) and throw a mouse pad or tape plastic on the bare concrete. Come back 24 hours later and see if it is moist underneath to get a feel how much moisture is from beneath before doing too much outside digging as I have heard of a house foundation caving in during excavation for a similar reason.
    French Drain is old and very effective. Actually consists of a deep trench filled with gravel and dirt placed over the top to grow your grass/roses on. Modern tech is a 6 inch corrugated, perforated plastic tube performing a similar duty but with a slightly lower efficacy. If you go that route make sure gravel surrounds the tube to reduce chance of clogging. Lichen and ferns will tell you the areas outside which are damp year round.

    Storing film in a relatively anhydrous basement environ is possible. Really just an inverse of an above ground wine cellar which uses vapor barriers and a humidifier. A dedicated room to film preservation may only need a small room dehumidifier draining outside through a small hole.

    I am currently pondering your attic fan desire as well..

    Welcome to your new home!!!
    Steve & M

  17. David: Why not store the negs and slides in a warehouse? In theory I need access to them to get them scanned, have them printed, etc. In practice I seldom do open those file drawers, so you’re probably right. Maybe I can find a friend with a big office building who will let me park my file cabinets there. 🙂

    Seriously, though, I don’t think that in the long run I want to live in a house that is too humid for negs and slides. Half of the living space of this house is in the basement and it doesn’t make sense to accept mold and a musty smell when we can simply declare war on the environment and run a monster HVAC system 24/7. I won’t be satisfied until I am as sensitive an environmentalist as Al Gore (i.e., I need to spend at least $30,000 per year on household electricity and ride around in a Gulfstream).

  18. Also, check out the excellent article:
    The Stay-Dry, No-Mold Finished Basement
    Fine Homebuilding, Feb/Mar 2005

  19. Phil,

    The plastic sheet sealing approach is the better. Getting the carpet glue off would be a nightmare. Also, for a wood floor consider a bamboo floor (engineered). This is quite durable and attractive. It’s also a renewable resource and can be installed as a “floating” floor which you will have to do plastic or no. Don’t forget to run the plastic up the walls to outside ground level. Walls will sometimes leak more than slabs will.

    Absolutely do the french drain. And, if you don’t have one yet, install a sump pump and check for radon as well. (Radon’s been a problem in the NH, MA, VT, and ME areas due to the prevalence of granite.) That slab may be dry to the touch now, but it won’t always be. This can be true especially in the early spring when the snow is melting and the ground is still frozen. Well, still frozen except right against the house’s basement where your central heat warms it.

    Welcome to suburbia. It sounds as if you – as Al Gore does – will be running an office in your house so that it’s a business place and a home both.

  20. Getting the carpet glue off is relatively easy. It can either be removed with an acid wash, or power sanded off. Sanding is more effective, but dusty. If you can find someone who will include clean up after the sanding you’ll be set. Just be sure to seal the rest of the house off if you go this route.

  21. I don’t know much about basements. My parents had basements in Kansas and Nebraska, and they always had sump pumps. If you don’t have a sump pump, perhaps the drainage really is that good. I can tell you one thing: if there’s water coming from somewhere besides the slab, and you seal that slab, you’ve got no where else for the water to go. Then you get moldy hardwood.

    Porous materials are generally better for coping with humidity. Really good tool boxes and tool cabinets are made of wood and not sealed, precisely because the machinist with his 1/10,000″ micrometers want’s *balanced* humidty, so no water condenses out. Because water doesn’t ruin anything. Condensation will ruin just about everything.

    So before you go sealing that basement, you may want to see how it behaves for a year, or at least half a year. I find it very productive to ask neighbors these sorts of things, since odds are at least one of them already entered the experimental arm of the study several years before you.

  22. BTW, “open basement windows and use a fan” on dry New England days is good advice too…

    Article 390748 of misc.consumers.frugal-living:
    From: (Nick Pine)
    Subject: Re: Dehumidify Basement?
    Date: 14 May 2002 12:08:01 -0400
    Organization: Villanova University

    max wrote:

    >An electric dehumidifier will yank between 30 and 65(!) pints/day from
    >the air. Power consumption isn’t as high as you _might_ think, but it’s
    >definitely up there.

    I measured a new efficient dehumidifier as a heat pump. It produced
    1.6 kWh of heat for every kWh consumed, ie 1 kWh from the motor plus
    0.6 kWh to condense about 2 pints of water, ie it consumes 0.5 kWh per
    pound of water, eg 15 kWh for 30 pints/day, $1.50/day at 10 cents/kWh.

    >The second best technique would be to open basement windows and run a
    >fan or set up a fan+duct arrangement to exchange drier air from out of
    >doors. This will definitely work — “almost” as well as a dehumidifier,
    >on nice dry days…

    A 70 F basement with 60% RH and humidity ratio w = 0.00947 pounds of water
    per pound of dry air. Air with a 50 F dewpoint has w = 0.00787, so a 36 W
    560 cfm fan could remove 24x60x560x0.075(0.00947-0.00787) = 97 pints in 24
    hours, at a cost of $0.1x24x36/1000 = 8.6 cents, ie 0.089 cents/pint, 1/56th
    the cost of the dehumidifier. It would be nice to have an automatic way
    to control the fan.


  23. If you choose to air condition [say with the mini split system suggested by Erik] undersize the units so they run longer. This will lower the humidity more reliably. If an installer oversizes the unit you get instant gratification – immediate cooling, but less moisture removed. Better to run the units longer [forget about putting the ac on a timer set for an hour before you return home] and get more moisture out.

  24. The French drain was a huge help for us. If you have drainage issues, it is necessary and not too labor intensive. It took about two days to remove the dirt, install the drainpipe, hook it up to the gutters and lead the water out to the front of the house where the pipes end in a pseudo-dry well (gravel under the landscape fabric, topsoil and mulch in the front yard).

  25. Les’ answer was correct, except IMO your downspouts should drain far from the house, to where it can’t run back towards the house, not just a few feet.

    As others stated, you must do the french drain. The essentials are: 1) perforated pipe must dip below foundation at all points 2) plastic sheeting all along foundation, wrapping under pipe in a J shape 3) do not trust the laborers. Examine the pipe before it’s backfilled. Put a level on a long 2×4 and make sure you have adequate fall, I think it’s 1/4″ per foot.

    I had a fourplex once which had mold in one basement unit. I did a french drain on that side, and moldy unit dried up. However the water then washed towards the neighboring unit and it got moldy. Do the whole house.

    If you get a dehumidifier, plumb in a drain. The tanks are a PITA.

    I have a beautiful hardwood floor in my home. I loathe it. Every time I drop something on it I curse because it has a new dent. Every time water falls onto the floor I curse and fret that I might get more swelling. Refinishing a hardwood floor essentially requires you to move out of your home for a week while troglodytes stomp about and pee in your sinks. Hardwood owns you. My wife once went on a vicious multi-day rampage concerning the need to refinish our floor, but now denies that this happened. Mazel tov.

    Pull up a carpet and pad sometime. Make sure you wear a mask though, it is nauseating. Carpeting is honestly barbaric. The beater bar on a vacuum cleaner consistently drives some the dust into the pad. Someday people will marvel at how unsanitary our lives were with carpeting.

    For my money pergo and throw rugs are unbeatable. Someone mentioned that it scuffs. Yes, but slower than hardwood. You can literally hammer it. I buy it for next to nothing from Costco, install it in a day, and throw it out when I’m tired of it. All you have to do is pull up the baseboards then snap the pieces together. Don’t bother with the glue. Admittedly I’m not familiar with the tongue and groove stuff the other fellow mentioned. Since the dawn of the internet you can buy extraordinary Persian rugs for a few hundred dollars. Some ought to be on the walls they are so charming. You won’t even notice the Pergo.

    I have a friend who had perfect results in his basement by putting down a thin layer of self-leveling cement plus what he described as a “rubberized paint”. This might be preferable to scraping up the glue that’s on your cement.

    Hydronic heating is righteous, if nothing else because it’s so efficient. If you have the ceiling height you could pour maybe an inch and a half of self-leveling concrete over the existing basement floor, embedding pex tubing for hydronic heat. A warm floor is unbeatable for comfort. Heating the slab will dry it out better than anything else. You won’ t need ugly registers on your floor in every room. Hydronic heat ties in with your existing hot water system. It’s amenable to all sorts of coolness such as co-generation (burning natural gas for electricity while using the engine’s waste heat for personal warmth), thermal flywheels (huge water reservoir for storing heat), heat exchangers for reclaiming heat from your waste water line, heat pump to cool the floor for AC, etc. The whole house fan is nice, but I don’t use mine at all except to blow heat around.

  26. First of all, outside measures are the first step, though it sounds to me like you don’t have particularly bad issues.

    Whether the slab feels wet or not is not a valid test of whether there is water coming in. The standard test is to tape down a 2×2′ piece of 6-mil plastic and let it sit for a few days. If it’s damp underneath, you are getting water transport through the slab. If not, you probably aren’t getting much.

    There are various kinds of coatings that can reduce water vapor transport (they’ll likely not solve the problem of water flowing in). Simple paint-on ones that are DIY up to more expensive professional ones that are thicker and have some give to them. The second kind is more effective, especially if you have cracks in the slab (and very few slabs that size don’t have cracks).

    I love hardwood, but have laminate (alloc domestic) in my basement. I think it makes a lot more sense over concrete. Since it’s floating, you can put it down over 6-mil poly (seams overlapped and taped), and you should have no moisture problems coming through the slab.

    A few more thoughts:

    1) you don’t mention what kind of walls you have. If they are concrete block, make sure the tops of the blocks are sealed – this can be a huge contributor to moisture issues. If it’s poured concrete, you will also want to get some sort of moisture seal there.

    2) You will want to insulate the concrete walls. The best way to do it is with either spray or sheet foam glued to the walls and then normal walls on the inside.

    For reference, take a trip to You have to search a little (try “basement” in the search box, but you will find some good analysis of good ways to finish under-ground space.

  27. We live in Japan and had a moldy bedroom in the last apartment we lived in. In Japan they sell floor standing dehumidifiers. You set the relative humidity you want, and turn it on. Every day or so you empty out the bucket of water. If you’re away for a week or so it will stop working, because the bucket will fill.

    We bought the biggest, most powerful one around and put it in the room and turned it on. Problem solved. We just left it on for a couple of years. Our electric bill was higher, but the mold problem was solved.

  28. Heyo,

    One thing that jumped out at me regarding your concerns vis-a-vis the roof and insulation, is sprayed polyurethane foam (or non-ceramic fiber blankets, if you’re rich) under a standing seam metal roof. The asphalt shingles seem to be nothing more than a cheap sealant, but in our little bungalow, we found that a mere 4 inches of SPUF radically altered the amount of cooling and heating required to make the house comfortable. If we’d realized how light a standing seam roof is (compared to the alternatives), I think we’d have ripped off the asphalt and gravel, and gone for the whole 9 yards like our neighbor. 3 out of the 4 houses on our little pipestem now have foam insulation on the roofs. Our air conditioners are silent while the neighbors’ run. The fourth house on the pipestem has a ginormous A/C unit on the roof. D’oh.

    Something worth considering, since standing seam roofs are basically bombproof (there are some in Australia that are over a century old; ask any roofer) and their light weight is conducive to non-structural foam insulation (very high R-value) underneath. Not quite as nice as an isolated airspace (attic) and a fan, but close. And it’s a lot more expensive to retro-fit an attic onto a prefab house than a metal roof.

    I felt morally obligated to mention this when your gentle readers’ comments about Pergo vs. hardwood gave my spirits a boost. We installed hardwood in our DC townhouse and it looked great, but became a little un-level after a few years. The buyer did not care, but that was back in 2004. Out here, we have Pergo over a cement slab and… an Indian throw rug which my folks brought back from Rajasthan. It seems like my wife and I tend to bumble our way into solutions that are actually pretty close to optimal, through no fault of our own… maybe they’ll be useful to you too.

  29. You don’t want to solve moisture problems by sealing concrete as they all will fail, according to many guys I had out to look at my basement. We had actual water coming up in several places and moisture for three seasons. I put in a perimeter drain and a sump pump which has solved the water seepage. I am overlaying the floor with dry-core which is a product with a dimpled plastic laminated to 3/4 OSB. This creates an air gap where water vapor can escape into the living space. I still need to run a dehumidifier. The reason for this over the poly directly on the floor is to prevent mold and trapped water if it does come in for some reason. The part of the basement where I have used the dry-core is noticeably warmer in the winter than the office area with only carpet over the concrete. I will be putting dry-core down in the office next.

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