Basements can account for a significant portion of a home’s energy loss (up to 1/3 of the heating cost of an average home). To be called “energy efficient” and meet Energy Star criteria, homes that have basements must have basement insulation. And in some locations, basement insulation is a building code requirement. Secondarily, the majority of homeowners with basements eventually, if not immediately finish their basements to provide additional living and recreational space. As a result and to warm cool basement walls, most of them insulate the perimeter of their basement prior to finishing them according to their taste.
With regard to energy savings as well as comfort, insulating basement walls is logical and desireable... as long as the walls remain free of moisture problems.
Basement walls can be externally insulated, internally insulated or insulated in the middle. Most often, basement walls are insulated internally almost entirely as a result of perceived lower cost and a high perceived value to the homeowner/buyer. However, the cost of properly insulating a basement while controlling moisture should rightfully be compared with the cost of constructing additional quality living space above grade.
The most common approaches to internal below grade insulation used in the homebuilding industry are: interior 2” x 4” or 2” x 6” stud framing with fiberglass batt insulation placed in the cavities and then either not covered or covered with a polyethylene vapor barrier or fiberglass blanket insulation attached to 2” x 2” nailers with an impermeable vinyl or aluminum foil interior facing.
The experience with these approaches has not necessarily been good and it has been concluded that all are unsuitable for use due to the serious problems they introduce with mold, decay and odors. Why?
The problem is the accumulation of moisture within the framed wall fastened to the interior basement foundation wall. Moisture can enter the insulated framed wall assemblies as a result of the following wetting mechanisms inherent in a concrete or masonry foundation:
- Moisture of construction – because thousands of pounds of water are contained in poured concrete walls, drying in uninsulated exposed walls takes months and even longer in walls with impermeable insulation systems.
- Air leakage and moisture diffusion – air leaks into the insulated wall assembly and contacts the cold concrete of the foundation. Condensation occurs when warm moist air encounters a cold enough surface to condense. When it does it creates a wetting of the wall assembly.
- Capillary rise – moisture will rise through the foundation’s footings when the interior insulation system has an impermeable layer.
- Groundwater leakage – Normal amounts of groundwater leakage require builders to be perfect in controlling groundwater, which is an impossible requirement. Because typical interior insulating systems are made of moisture sensitive materials (lumber, nails and insulation), even minor groundwater leakage can lead to mold, decay, rust and odor.
Builders typically attempt to deal with these issues by using vapor barriers or vapor retarders. Unfortunately, both approaches are incapable of allowing foundation walls and insulated wall assemblies to dry to the interior which is the most effective way to allow wetting of the foundation and wall assembly to evaporate. Indeed, they hold the moisture in, exacerbating the opportunity for mold to grow and wall assembly materials to decay. And simply leaving them off allows vapor diffusion to wet the wall assembly. Air leakage and moisture diffusion are additional problems that cannot be solved with vapor barriers and retarders as these wall assemblies simply cannot be constructed in an airtight manner using typical building materials and trades.
From both moisture and thermal perspectives, basement walls with only rigid insulation on the exterior perform better than basements with other types of insulation only on the interior. This is because walls with insulation on the exterior are “warm” and can dry to the interior. “Warm” walls also pose little risk of condensation of interior moisture.
An effective interior basement wall insulating system must have all three of the following properties:
- It must be able to dry to the interior because should it become wet, the below grade portion of the wall will simply not be able to dry to the exterior at any time of the year. To allow it to dry to the interior, it cannot have an impermeable vapor barrier or wall finish as both will prevent evaporation of wetness that is within or on the wall.
- It must prevent any significant volume of interior air from reaching the cool interior of the foundation wall and condensing, such as rigid foam insulation installed directly on the interior concrete surface. An effective air barrier is required on interior basement walls while vapor barriers typically are not because they inhibit drying of the wall more than they prevent wetting of the wall.
- The materials placed in contact with the foundation wall, either externally or internally must be moisture tolerant; i.e. they cannot support mold growth or deteriorate if they become wet. Some materials that are moisture tolerant will still allow the passage of liquid through them, so if they do not present a capillary break, one must be placed between them and the moisture sensitive materials.
ICF lower level construction simplifies construction, reduces material and construction cost and protects the lower level living space from air leakage, moisture, rot, mildew, mold and pests.
With ICF construction, the blocks are assembled, the rebar is placed and the concrete is poured. An exterior waterproofing is sprayed on the insulation below grade and a protective barrier is applied to the above grade section of the insulation to prevent damage that can be caused during backfill around the foundation. There is no need for studs and no need for additional insulation. The interior gypsum board is attached directly to the embedded plastic webs within the insulation as a finish and a fire barrier.