Real estate

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Tips for a finished basement that’s mold-free

Q. I want to get your input on the correct way to finish, in this case refinish, a basement. I believe the previous owners missed one or two critical steps, causing mold to grow on the backside of the drywall.

The house is about 60 years old with a poured-concrete basement foundation whose walls are in good shape. The concrete walls were not painted/waterproofed on the inside. There are no signs of water leaks or holes in the walls.

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The previous finishing was done by putting in untreated 2-by-3s as studs ¼ of an inch from the concrete walls, stapling R13-value pink insulation to the studs, then putting up regular drywall on top of that. Twenty-five years or so later, mold appeared on the bottom portion of the drywall (on the back ), and some of the untreated-wood sill rotted. These items are now gone. There is an appropriate-sized dehumidifier in the basement.

To insulate, seal, and finish the basement properly, what are the best steps for the money that meet the necessary building code standards? It seems there are a number of ways to go about this.

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For instance, should I paint the concrete walls on the inside with Drylok Extreme, follow that with R10-value XPS tongue-and-groove rigid insulation, apply strapping that’s pressure treated at the bottom, and then add mold-resistant green board or DensArmor Plus? It sounds like XPS has a vapor-quality to it, so I’m not sure if the above would be overkill.

Not sure whether it is best, and to code, if instead I paint the concrete walls with Drylok Extreme, hang studs (using pressure-treated wood for the bottom sill), staple R13 pink insulation (or should I use mineral-wool insulation instead?), and then cover that with mold-resistant green board that has been painted with Caliwel Industrial Antimicrobial Coating for Behind Walls and Basements on the back and Zinsser Perma-White?

Ultimately, as before, only about half the basement will be finished. I am presuming that Drylok Extreme would need to go on all of the inside foundation, regardless of whether that area is going to be finished or not.

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A. I’m a big fan of humidifiers, but not of applying a coating to concrete. As long as you don’t have water infiltration here’s what I’d do:

Insulating a basement is far more complicated than insulating regular wood-framed walls. In fact, improperly insulated basement walls are one of the biggest reasons for mold in homes. I prefer framing basement walls rather than installing furring strips because the walls can be plumbed for a better look, allow for the installation of electrical outlets, and are easy to insulate.

Walls can be framed using steel or wood studs. Wood studs should be framed with a pressure-treated bottom plate, allowing 1 to 2 inches of airspace between the concrete and stud walls if you are not insulating. But if you are insulating:

Insulation

The best basement insulation is spray foam or rigid polystyrene glued directly to the foundation wall. Insulation joints should be sealed with special tape so the warm, moist basement air will not come into contact with the cool basement wall, which could lead to mold. DOW offers Weathermate construction tape, and Owens Corning has JointSealR foam joint tape. These products are more expensive, but they are specifically designed for this application and very important to the long-term performance of the insulation barrier.

Whether you spray it in place or install sheets, closed-cell foam is the answer for basement insulation. When properly installed, closed-cell foam (spray or board) provides two benefits:

First, it’s a great insulator with much better R values per inch than Fiberglas.

Second, when you use a closed-cell foamboard that is 2 inches thick, it actually acts as a vapor barrier — you are essentially getting a vapor barrier and insulation all wrapped into one easy-to-use product.

Once all the insulation is in place, frame the walls using 2-by-4s. Place the walls directly in front of the foam insulation, and then insulate the wall cavities with Fiberglas.

Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to homerepair@globe.com or tweet them to @robertrobillard.
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