Insulation - 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 may help avoid water vapor condensation problems. 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.
Discussed below are various insulation options. It is important to note that insulating and finishing basement walls requires careful attention to avoid moisture problems. Moisture intrusion and condensation can cause physical damage to interior finished walls and carpeting and the growth of mold. Mold can have serious negative effects on occupant health. In new construction, proper and careful detailing of foundation and footing drainage and water proofing should avoid moisture problems. For existing basements, where moisture problems are evident, unless these problems are addressed, it may be better not to insulate and finish basement walls below grade.
Basement Walls: Installing insulation on the exterior or "dirt side" of a basement wall has the following advantages and disadvantages.
Advantages:
* Minimizes thermal bridging and reduces heat loss through the foundation
* Protects the damp proofing coating from damage during backfilling
* Serves as a capillary break to moisture intrusion
* Protects the foundation from the effects of the freeze-thaw cycle in extreme climates
* Reduces the potential for condensation on surfaces in the basement
* 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.
* Many contractors are unfamiliar with proper detailing procedures, which are critical to performance.
* 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 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:
* It's much less expensive to install than exterior insulation for existing buildings.
* There's a wider selection of 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.
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 doesn't protect the damp-proof coating like the exterior insulation.
* If the perimeter drainage is poor it 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 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 foam board 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 risk 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 and thermally 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:
* Two to 3 inches (51 to 76 millimeters [mm]) of sand
* One 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
* Four 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
* Sub flooring
* 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 (top)
* Rosin paper
* Two 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 sub flooring
* Rigid foam board insulation without the furring strips (as in the last example).
The above methods have the following advantages and disadvantages. Advantages:
* It's a relatively simple installation for retrofit work.
* It thermally isolates the floor from the earth below.
* The floor surface is approximately the ambient interior air temperature and more comfortable to stand on than concrete.
Disadvantages:
* The 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.
* 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 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 sub floor. 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. 
| Figure 85: A prototype panelized foundation being installed in a test home. |
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 headroom of the crawl space meets local code regulations if you decide to pour 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 foam board against the foundation from the sub floor 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 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 and more involved work. Adding foundation insulation during new construction is usually less expensive.
Field studies have found that foundation insulation for new houses (in the United States) have good economic outlooks, except for the warmest climates. The Builder's Foundation Handbook and Building Foundation Design Handbook are excellent references. Foundation For each foundation type-basement, slab-on-grade, and crawl space-there are several construction systems and products from which to choose:
* Cast-In-Place Concrete
* Concrete or Masonry Blocks
* Insulating Concrete Forms
* Precast Concrete
* Permanent Wood Foundation
Most of these foundation systems can be designed to meet necessary structural, thermal, radon, and moisture control requirements. Your whole-building design, including the climate, will help determine your choice. It's also important to determine how the foundation will be insulated. For more information. See Insulating Foundations section.
Cast-in-Place Concrete: Cast-in-place concrete construction involves setting up removable forms for the pouring of concrete foundation walls. It's a common method for all three types of foundations-basement, slab-on-grade, and crawl space.
Rigid foam board insulation is usually placed between the removable forms and held in place with a system of non-conductive ties. Then concrete is poured on either side of the foam. Steel rebar is generally used to add strength to the wall. Once the concrete has cured, the forms can be removed and reused as many as 3,000 times with minimum maintenance.
Concrete or Masonry Blocks: Another common foundation system is the use of concrete or masonry blocks. When using blocks for a foundation wall, the block cores should be filled with insulation. 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.
Foam inserts for the block cores are also available. 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. Even though the block cavities and special block designs can 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.
Insulating Concrete Forms: Insulating Concrete Forms (ICFs) serve as both foundation structure and insulation. They are basically forms for poured concrete walls that stay in place as a permanent part of the wall assembly. The forms, made of rigid foam insulation, are either pre-formed interlocking blocks or separate panels connected with plastic ties. The left-in-place forms not only provide continuous insulation but also a backing for drywall on the inside.
Although all ICFs are identical in principle, the various brands differ widely in the details of their shapes, cavities, and component parts. Currently, about one-third of all ICFs sold are used in residential basements. ICFs can cost up to 4% more than standard wood framing. However, houses built with ICF exterior walls require an estimated 44% less energy to heat and 32% less energy to cool than comparable frame houses.
Precast Concrete: Manufacturers construct precast concrete foundation walls or panels off-site. Most of them are preinsulated as well with rigid foam board. But additional insulation of your choice usually can be added inside the wall cavity to achieve a high R-value. The panels typically come in lengths of up to 16 feet and in standard heights of 4, 8, and 10 feet. Once constructed, they're transported to the building site. A crane is needed to lift them in to place.
Precast concrete panels not only can provide for high R-values, but also structural integrity and termite protection. They minimize air infiltration, as well.
Permanent Wood Foundation: Permanent wood foundation (PWF) construction is similar to wood-framed exterior wall construction, with some exceptions. Because PWF walls are used in below-grade applications, all lumber and plywood is pressure-preservative treated for decay and termite resistance.
Any type of insulation product can be used with a PWF. But there are some indoor air quality issues in regards to the preservatives used to treat the wood. Wood foundations also don't have the same structural integrity as concrete foundations. |
