Healthy Living Spaces


Building a Mold-Free Home

Want to build a home that doesn’t grow mold? Build one that does not allow for water to get in and construct the walls so that if water does get in, it can dry out.

This includes avoiding vapor barriers or using them correctly. It is becoming common practice to cover the exterior of houses with “house wrap”. This is done to save money on heating and cooling bills by making the building air-tight. This should not be confused with a vapor barrier. House wraps are made of a Gore-Tex™-like material called Tyvek™. House wrap is air-tight but allows moisture to get out. Plastic vapor barriers do not breathe and do not let moisture escape.

Plastic is typically used in installing a vapor barrier. Vapor barriers should only be used in either hot and humid or very cold climates. They are used in the south to prevent humid air from reaching the interior, cool, side of air-conditioned spaces where moisture condensates inside the walls causing mold to grow. In very cold climates like Alaska, the reverse is true. Vapor barriers are placed on the inside of the walls to prevent humid air indoors from reaching cold surfaces inside exterior walls and causing mold growth.

So what do you do if you live in a climate that has both humid summers and cold winters? Unfortunately, the building codes in some regions mandate the installation of a vapor barrier and specify which side of the wall it should go on. This may prevent mold half of the year and promote it during the other.

Nowadays it seems everywhere you look contractors is wrapping houses to save energy by building an air-tight building. The potential for mold and moisture problems may not be worth the money saved in utility bills. The danger here is that unless the house wrap is installed properly, moisture may get in and be trapped. Many of the common materials used have perforations from the staples by which they are attached that can cause leakage under sustained wetting⁸. Stucco tends to stick to house warps. Stucco cracks. When stucco is applied over building paper, the traditional construction material, there is a space between the paper and the stucco by which water can drain. If house wrap is used with stucco it may be a good idea to put a layer of light building paper over the house wrap. This will allow for water to drain and for the house to be air-tight.

Building materials are not sterile. Mold spores are found everywhere. A mold spore can only grow and multiply if it gets wet. If you want to prevent mold, prevent moisture problems. Keep things dry during construction. Design and build with the assumption that no matter how hard you try, things may get wet. Design and build to allow things to out when they get wet.

Tips for Building a Mold-free Home

Choose a site location with effective site drainage away from the house. Grade the lot to drain away from house.
Under no circumstances should drywall be installed on the exterior walls of a building. Use plywood or OSB. Don’t try and save money by using drywall. It is highly prone to mold growth.
Ensure the foundation is above grade.
Install vapor barriers under concrete slabs and over soil in crawlspaces.
Consider finishing crawlspaces with concrete slabs and treating them as mini-basements.
Water proof foundations and install French drains along perimeter walls.
Don’t embed duct work in concrete slabs.
Use double paned, insulated windows to avoid condensation.
When installing drywall, leave a ¼” gap above the floor so that if there is a flood water can not wick up the wall.
Avoid carpeting, especially in high traffic areas and bathrooms.
Inspect materials as they arrive for mold and moisture content. Use a moisture meter. Wood should read less than 18%.
Allow framing to dry before applying sheet rock.

Mold loves paper. One of the favorite foods the black mold, Stachybotrys prefers is the paper backing of drywall. A new drywall product is on the market that uses fiberglass instead of paper: Paperless DensArmor® Interior Wallboard. It cost more than standard drywall. Priorities include exterior walls, behind sinks, bathrooms, dishwashers, and anywhere there is a chance for a moisture problem.

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Revised: July 05, 2017.
Information in this document is subject to change without notice.
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