Published on August 14th, 2017 | by Henri


Keeping a finished basement dry

Q.  My wife and I are in the planning stages of finishing off half of our basement into a couple of bedrooms, and a small living/TV room. I am concerned about water and water vapor creating that mildewy basement smell once all of the construction is complete. To that end before we start I am addressing any water entry into the basement, which I believe I have been successful. So that leaves water vapor. Our foundation is poured concrete with footing drains, and a coating of emulsifier on the outside. Currently I run a dehumidifier 24-7 in the basement during the summer months to control the humidity. We will be insulating the exterior walls of the basement on the inside. What are our options for doing this (including any necessary wall coatings) that will give us a vapor barrier ranked best to worst in your opinion.

For extra credit, I’ve seen on HDTV a certain program about siblings remodeling a house, and one of them, let’s call him “the builder,” insulates interior basement walls with straight unbacked fiberglass insulation. Is this a west-coast thing, and even allowed in Vermont? Thanks.

A.  I assume that you have corrected any grade issues that would encourage basement leakage. Don’t count on the emulsifier coating applied on the exterior of the foundation walls to prevent leakage; it’s not foolproof and loses any effectiveness over time.

Once you are positive that potential basement leakage has been taken care of after being subjected to several severe rains and one significant snowmelt, the only concern is a high relative humidity (RH) that would cause the mildewy smell you are right to be concerned about. High RH is common in basement rooms. Your running a dehumidifier in the summer is the best strategy, and you might need to continue doing so after the construction is finished.

Be aware that building codes require means of egress from below grade bedrooms. This could be a problem when you sell the house; a qualified inspector would have to flag it.

If you are positive that the footing drains are properly functioning and you know that the foundation backfill is made of well-draining, coarse material topped by good soil gently sloping away from the foundation and planted with grass (no shrubs or flowerbeds within four feet), you can safely insulate the foundation’s walls from top to bottom. But if you are not sure, only insulate from the top of the basement down to three feet below grade in order to let some heat convect into the soil outside to keep frost that could damage the walls from penetrating too deeply.

There is no need to coat the inside of the concrete walls if you have not noticed any dampness on them following persistent or heavy rains and snowmelt.

One good insulation option, if you are planning to do the job yourselves, is to apply 2-inch thick XPS (extruded polystyrene – Styrofoam and FoamulaR are the two major brands) to clean concrete walls with dabs of Styrobond or polyurethane caulking. Make sure that the walls are thoroughly vacuumed to remove any dust, apply the adhesive every two feet and firmly press the insulation into the adhesive. Then build a 2-inch by 4-inch partition, set on a pressure-treated plate, tight to the foam. This will allow running electrical wiring as required by code and any hydronic piping for baseboard heat, if that is your choice. A licensed electrician and a master HVAC contractor are needed for these phases.

Once the wiring and heat piping are in, you can add R-11, R-13 or R-15 fiberglass, rockwool or cotton insulation. Fiberglass (batts or blanket) is the most common and economical; it comes in several forms: unfaced, foil-faced; Kraft-paper-faced. Choose the type that itches the least. Use gloves, skin and eye protection and an effective mask; wash your clothing and shower afterward.

Roxul is made from rock and is the best for soundproofing and fire resistance; it is now widely carried in big box stores and some building supply stores. Cotton insulation is made from recycled jeans and is more expensive, ecologically better, but harder to find. Be sure that you do not compress the insulation behind or in front of pipes and wires. Split the insulation and work half of it behind these pipes and wires and place the other half in front. Then finish with drywall.

Even if you have central air-conditioning, you may still need to use your dehumidifier or provide modicum heat even in the summer.

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