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Where is mold-resistant drywall necessary in a daylight basement?

I am confused about where it is necessary to use mold-resistant drywall. Half of my basement is underground and is poured concrete, the other half is regular walls with 2X6 framed walls, blown cellulose insulation, OSB sheathing, Tyvek, and Hardie panel siding. Is mold resistant drywall needed on all walls in the basement, or just on the walls that are below grade? The below grade walls are insulated with 3/4 inch XPS and strapped with vertical treated 1X4s.

My reason for trying to get away from the mold-resistant board is that the walls are 9 feet and the green stuff comes in only 4 foot widths in my area. Using 54 inch drywall will make things go much faster.

Asked by Robert Sanders Jr
Posted Thu, 01/31/2013 - 20:14
Edited Fri, 02/01/2013 - 09:08

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2 Answers

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Robert,
Ideally, you have designed your basement to be perfectly dry. If you have succeeded in that goal, then ordinary drywall will work. If you haven't succeeded in that goal, and your basement is damp, then mold-resistant drywall (or fiberglass-faced drywall, or even cement backerboard covered with a skim coat of plaster or drywall compound) is a good idea.

Designing a dry basement requires attention to details and experience. (Experience helps you judge the site's riskiness.) At a minimum, you should have footing drains leading to daylight, to a distant drywall, or to the town sewer system or storm-water system. (Daylight is best.)

Your walls should be protected on the exterior with dampproofing, and your backfill should consist of coarse granular material capped with good topsoil or clay.

Better yet would be a full waterproofing system that includes a dimple mat.

Under your basement slab, you should have installed a layer of crushed stone that is at least 4 inches deep. On top of the crushed stone and directly under the concrete slab, you should have installed a layer of 6-mil poly (or perhaps something even thicker than 6-mil). Even better would be a continuous layer of rigid foam insulation above the crushed stone and 6-mil poly between the rigid foam and the concrete slab.

The grade should slope away from the foundation in all directions, and the roof water should be collected by gutters and directed to conductor pipes (separate from the footing drains) that conduct the roof water to a location far from the foundation.

Right. So you did all that. You're in good shape.

You have chosen to insulate your foundation walls with R-3.75 insulation. That's not much. It won't meet minimum code requirements anywhere north of New Orleans; Tucson, Arizona; or Jacksonville, Florida. If you live in Jackson, Mississippi, or Montgomery, Alabama, or anywhere north of there, your insulation is insufficient and probably illegal.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/01/2013 - 09:05
Edited Fri, 02/01/2013 - 09:10.

2.
Helpful? 0

Thanks! actually, I have installed another layer of 3/4 inch XPS between the 1X4s, so that gives me 1.5 inches for most of the concrete wall area. The basement is, indeed, pretty dry so I will go with regular drywall for the framed walls and mold resistant for the walls that are poured concrete. I have a heat pump water heater in the basement which helps keep things dry.

Answered by Robert Sanders Jr
Posted Mon, 02/04/2013 - 17:49

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