Insulating Foundations

An uninsulated foundation can result in a large heat loss from an otherwise tightly sealed, well-insulated house. It can also make below grade rooms uncomfortable. Foundation insulation can result in lower heating requirements, and also helps in avoiding water vapor condensation problems, which is often caused by the difference in temperature between the basement interior and the earth around the foundation. However, a poorly designed foundation insulation system can cause many problems such as radon infiltration, moisture problems, and insect infestation.

The economic benefits of insulating a foundation and doing the job correctly depend on factors such as: Is it new construction or retrofit? Does the house have a full basement, crawl space or slab-on-grade foundation? Are you planning to heat the lower level? Will you have radiant floor heating? What heating climate is the house in? Does the soil drain well? Each factor narrows the choice of insulation type and installation method.

Basement Walls

Installing insulation on the exterior or "dirt side" of a basement wall has the following advantages and disadvantages:

Advantages:
* it minimizes thermal bridging and reduces heat loss through the foundation,
* it protects the damp proofing coating from damage during back-filling,
* it can serve as a capillary break to moisture intrusion,
* it protects the foundation from the effects of the freeze-thaw cycle in extreme climates,
* it reduces the potential for condensation on surfaces in the basement, and
* it reduces the loss interior space when foundation insulation is desired, and
* it conserves room area, relative to installing insulation on the interior.

Disadvantages:
* installation is expensive for an existing building unless a perimeter drainage system is also being installed,
* many exterior insulation materials are susceptible to insect infestation, and
* many contractors are unfamiliar with proper detailing procedures.

Building scientists feel that the best way to have a dry basement is to insulate the outside of exterior walls with a rigid fiberglass type "mat." Under the mat is a damp-proofing coating over the entire foundation, from footing to just below the finished grade. A carefully designed perimeter drainage system consisting of washed gravel, perforated plastic pipe, and filter fabric is also strongly recommended for locations with poor soil drainage. A waterproof paint over the room side of the foundation wall is also often recommended.

Some foam insulations are impregnated with boric acid to discourage termite infestation. However, the borate chemical often slowly leaches out of most materials when exposed to ground water.

Adding insulation to the interior of the foundation is often more cost effective for an existing building. This has the following advantages and disadvantages:

Advantages:
* much less expensive to install than exterior insulation for existing buildings,
* there is a wider choice in materials since you can use almost any insulation type,
* the threat of insect infestation is eliminated, and
* the space is isolated from the colder earth more effectively than when using exterior methods.

Disadvantages:
* many insulation types require a fire-rated covering since they release toxic gases when ignited,
* it reduces usable interior space by a few inches,
* it does not protect the damp proofing coating like the exterior insulation,
* if the perimeter drainage is poor it may become saturated by moisture weeping through the foundation walls, and
* superior air sealing details and vapor diffusion retarders are important for adequate performance.

New Methods in Foundation Systems

Some new construction systems lend themselves to both structure and foundation insulation at the same time. For example, an Insulating Concrete Form (ICF) system uses a rigid foamboard applied in the middle of a cast-in-place concrete wall, or, serves as both inner and outer concrete forms in place of steel or plywood forms.

When using masonry blocks for a foundation wall, filling the block cores with high-pressure foam works better than most other block filling methods, such as poured-in insulations like polystyrene beads and vermiculite.

There are also foam inserts for the block cores. These are installed as the blocks are mortared into place. Some concrete block manufacturers attempt to increase the thermal resistance of their product by adding materials such as polystyrene or wood chips to the concrete mix. However, even though filling the block cavities and special block designs improve a block wall's thermal characteristics, it doesn't reduce heat movement very much when compared to insulation installed over the surface of the blocks either on the exterior or interior of the foundation walls. Field studies and computer simulations have shown that core-filling of any type offers little fuel savings since the majority of heat is conducted through the solid parts of the walls such as block webs and mortar joints.

Insulating Slab-On-Grade Foundations

Slab-on-grade foundations are often insulated in one of the following ways: Over the exterior of the footing/ slab edge, or, between the interior of the footing and slab. Often the bottom of the slab is insulated from the earth to some extent as well. Each approach has its advantages and disadvantages.

On the exterior of the footing/slab edge it reduces heat loss from both the foundation and the slab. Sometimes a foamboard insulation is extended outwards beyond the foundation for several feet. This offers more protection to the footing from freezing. It also allows the builder to dig a more shallow footing without the risking of damage due to frost-heaving. All exposed parts of the insulation must be covered with metal, cement, or other type of membrane to protect it from damage.

When installing insulation on the interior of the footing/slab, it must be vertical between the footing and slab. This protects the insulation from insects and damage better than an exterior application while it isolates the slab from the colder footing.

Insulating under an existing slab is usually impractical. However, insulating under a slab in new construction consists of the following cross-section (from top to bottom):

* floor slab,
* 2 to 3 inches (51 to 76 millimeters [mm]) of sand,
* 1 to 2 inches (25-51 mm) thick rigid insulation,
* a layer of 6 mil [0.006 inch (0.15 mm)] polyethylene plastic as a moisture retarder,
* 4 inches (102 mm) of washed gravel and under-slab drainage and plumbing pipes.

Insulation may be applied on top of an existing slab in this way (from top to bottom):

* finish flooring,
* rosin paper,
* subflooring,
* rigid foam insulation laid between moisture resistant furring strips that are attached to the concrete,
* a layer of 6 mil [0.006 inch (0.15 mm) ] polyethylene plastic as a moisture retarder.

An alternative is to make a "floating floor." This consists of (from top to bottom):

* finish wooden flooring,
* rosin paper,
* 2 layers of half inch OSB or plywood screwed together, overlap all seams by several feet, hold the edge of the wood back from the walls by half an inch to be the subflooring,
* rigid foam board insulation without the furring strips (as in the last example).

The above methods have the following advantages and disadvantages:

Advantages:
* a relatively simple installation for retrofit work,
* thermally isolates the floor from the earth below, and
* floor surface is approximately the ambient interior air temperature and more comfortable to stand on than concrete.

Disadvantages:
* foamboard requires a fire rated covering,
* it may increase frost depth around the slab edge in extreme climates,
* in the summer, it separates the space above from the cooler earth, and
* there is a loss of about 2 inches (51 mm) of head room.

Crawl Spaces

How to insulate a crawl space depends on whether you vent it. Traditionally, crawl spaces have been vented to prevent problems with moisture. However, this often did not always work well. Today, building researchers are moving towards treating the crawl space the same as any other basement. This section will address both options.

If the crawl space is to be vented, carefully seal any and all holes in the floor above ("ceiling" of the crawl space) to prevent air from blowing up into the house. Insulate between the floor joists with rolled fiberglass. Install it tight against the subfloor. Cover the insulation with a house-wrap or face the fiberglass vapor barrier down. Seal all of the seams carefully to keep wind from blowing into the insulation. Also, adequately support the insulation with mechanical fasteners so that it will not fall out of the joist spaces in the years to come. DO NOT just rely on the friction between the fiberglass and wood joists to secure it in place.

Install a polyethylene vapor retarder, or equivalent material, over the dirt floor. Tape and seal all seams carefully. You may also cover the polyethylene with a thin layer of sand or a thin slab of concrete to protect it from damage. Do not cover the plastic with anything that could make holes in it, such as crushed gravel. Be sure the head-room of the crawl space meets local code regulations if you are considering pouring a concrete slab.

If the crawl space is to be unvented, seal all holes in the foundation where outside air can enter. Install the plastic ground cover as described above for an unvented crawl space. Run the plastic up the walls and attach it to the first piece of wood (the mudsill). Install rigid insulation foamboard against the foundation from the subfloor to the plastic (or concrete slab) on the floor of the crawl space. Do this all the way around the perimeter of the foundation. An alternative to foamboard is to drape fiberglass roll insulation down the foundation walls with the edges butted tightly together. This is an acceptable alternative to foamboard insulation as long as the crawl space stays dry.

Installation Cost and Performance

Although you can achieve considerable savings in space conditioning costs by insulating the foundation, the installation costs can become relatively high, especially for retrofit projects. The type of materials used, the application method, and the extent of work all affect the overall cost. Simple payback is typically in the range of 6 months for a simple do-it-yourself installation to 20 years for "professionally" installed and more involved work. Adding foundation insulation during new construction is usually less expensive.