Department of Energy Advice on Insulating Foundations
An uninsulated foundation can result in large heat loss from an otherwise tightly sealed and well-insulated house. It can also make below-grade rooms uncomfortable. Foundation insulation lowers heating requirements and avoids moisture condensation, which is often caused by the difference in temperature between the basement interior and the dirt around the foundation. 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 correctly depend on factors such as whether the construction is new retrofitted, and whether the house has a full basement,crawl space, or slab-on-grade foundation.
Basement Walls - Exterior
Installing insulation on the exterior basement wall presents you with some choices. Exterior insulation has the following advantages and disadvantages:
- It minimizes thermal bridging and reduces heat loss through the foundation
- It protects the damp-proof coating from damage incurred while 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 condensation
- It reduces the lost interior space
- 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
- 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 mat. A damp-proof coating is applied over the entire foundation from the footing to just below the finished grade. A carefully designed perimeter drainage system consisting of washed gravel, perforated plastic pipe, and filter fabric. It is strongly recommended for locations with poor soil drainage.
Some foam insulation is manufactured with boric acid to discourage termite infestation; however, the borate chemical often slowly leaches from most materials when it is exposed to ground water.
Basement Walls - Interior
Adding insulation to the interior of the foundation is often a better method; it is also much less expensive for an existing building. It has the following advantages and disadvantages:
- It is 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
- The space is isolated from the colder earth more effectively than when using exterior methods
- Many insulation requires a fire-rated covering it they release toxic gases when ignited
- It reduces usable interior space by several inches
- It does not protect the damp-proofing coating like the exterior insulation
- If the perimeter drainage is poor, the insulation may become saturated by moisture weeping through the foundation walls
- 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 foam board which is applied in the middle of a cast-in-place concrete wall. It serves as both the inner and outer concrete forms in place of steel or plywood forms.
Completely filling the hollow foundation block cores with high-pressure foam works better than most block filling methods. Older methods, such as poured-in insulations like polystyrene beads and vermiculite are also sometimes used. Foamed-in insulation often out-performs the latter in most climates.
There are also foam inserts for the block cores. They are installed as the blocks are mortared into place. Some concrete block manufacturers increase the thermal resistance of their product by adding materials such as polystyrene or wood chips to the concrete mix.
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 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 because 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 either over the exterior of the footing/slab edge, or between the interior of the footing and slab. The bottom of the slab is often insulated from the earth as well. Each approach has its advantages and disadvantages.
On the exterior of the footing or slab edge, it reduces heat loss from both the foundation and the slab. Sometimes a foam board insulation is extended outwards beyond the foundation for several feet. This method 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 or slab, it must be placed vertically between the footing and slab. Doing so 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-mm) of sand
- 1 to 2-inches (25 to 51-mm) thick rigid insulation
- A layer of 6-ml polyethylene plastic as a moisture retarder,
- 4-inches (102-mm) of washed gravel and under-slab drainage pipes
Insulation may be applied on top of an existing slab in this way, from top to bottom:
- Finish flooring
- Rosin paper
- Rigid foam insulation laid between moisture resistant furring strips that are attached to the concrete,
- A layer of 6-ml polyethylene plastic as a moisture retarder.
An alternative is to make a floating floor, which consists of:
- Finish wooden flooring
- Rosin paper
- 2 layers of .5-inch OSB or plywood screwed together, overlapping all seams by several feet to hold the edge of the wood from the walls by .5-inches
- Rigid foam board insulation without the furring strips, as in the previous example
The above methods have the following advantages and disadvantages:
- A relatively simple installation for retrofit work
- Thermally isolates the floor from the earth below
- Floor surface is approximately the ambient interior air temperature and more comfortable than concrete
- Foam board 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
Insulating a crawlspace depends on whether you vent it. Traditionally, crawlspaces have been vented to prevent problems with moisture; however, this method does not always work well. Today, building researchers are moving toward treating the crawlspace 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 to prevent air from blowing into the house. Insulate between the floor joists with rolled fiberglass. Install it tightly 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. Adequately support the insulation with mechanical fasteners so that it will not fall from the joist spaces in the future. Do not just rely on the friction between the fiberglass and wood joists to secure it.
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 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 crawlspace meets local code regulations if you are considering pouring a concrete slab.
If the crawlspace 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 crawlspace. Run the plastic up the walls and attach it to the first piece of wood, which is called the mudsill.
Install rigid insulation foam board against the foundation from the subfloor to the plastic or concrete slab on the floor of the crawlspace. Do so all the way around the perimeter of the foundation.
An alternative to foam board is to drape fiberglass roll insulation down the foundation walls with the edges butted tightly together. This is an acceptable alternative to foam board 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 insulation. Adding foundation insulation during new construction is usually less expensive.