Heat loss through the basement foundation accounts for 15% to 30% of the annual heat load in a two-story house. If you want to improve the energy performance of your home then you should start by air sealing and insulating. The attic can account for ~25% of a home’s total heat loss and the basement ~20%. A considerable energy saver is the insulation of the perimeter of the slab and can save 10% to 20% on heating bills.
In climates with mild winters, this saves $50 to $60/month for an 1800 sq–ft house using R–10 insulation. The cost for installation is $300 to $600 which gives a payback period of 5 to 10 years. It is assumed that basement walls that are below-grade have no infiltration and thus heat losses. However, for above-grade walls in the winter the sensible heat losses due to infiltration are ~½ to 3 times the conduction heat losses. For cooling, in the summer, these infiltration losses are halved.
Types of Insulation
There are many types of insulation based on application but the two main types are continuous and cavity. Continuous insulation runs continuously alongside the structural members, so unlike cavity insulation, it’s unaffected by thermal bridging. It is generally foam board type (i.e. rigid foam or insulating boards) made of either polystyrene (extruded or expanded), polyisocyanurate, or polyurethane material.
Cavity insulation is fitted between wood or metal structural members such as studs, joists (including rim joists), and beams. It’s a blanket type that comes in either batts or rolls that are attached to the inside surface of walls. Rigid fibreboard (i.e. rigid fiber or fibrous board) consists of flexible fibers, most commonly fiberglass. Mineral wool (i.e. ROCKWOOL®), however, has a similar R-value and beats fiberglass as a fire retarder, it is moisture resistant, and it’s more environmentally friendly.
There are three sections of the basement walls that need to be accounted for: the above-grade section; the below-grade and above the frost line; and the below-grade and below the frost line. As opposed to an outside wall that is exposed to a uniform outside air temperature, a below-grade slab is exposed to the soil which varies in temperature depending on the depth from the surface. The heat loss calculation is more complicated due to the soil temperature varying depending on the time of year, the climate, the make-up of soil, whether snow is on the ground, the amount of moisture in the soil, and the depth of the soil from the ground level.
Construction techniques such as cores in concrete blocks promote vertical convection of heat. This is beneficial but an 8–inch concrete wall without insulation has a low thermal resistance (R–1.11 & R–1.49 with air films). This warrants adding insulation.
For a 20′ x 30′ basement covered in a 2–inch beadboard (R–8), there is a 93% decrease in heat loss. Using 3.5–inch fiberglass batting (R–11) decreases the heat loss further to 95%. This type of insulation is used in warmer climates, but if the climate is colder, 4″ to 6″ (R–30) insulation is used. The addition of a reflective barrier to the wall adds 16% to the R-value and, if the air space is reflective, it adds 50% to the R-value. Insulation placed on the interior of the exterior wall will give only slightly different R-values.
For slab insulation at a 2–foot depth, an R-value that is more than R–20 is not justified because heat will go around the insulation. Also, insulating an 8–foot wall halfway down with R–10 is practically the same as insulating the 8–foot wall with R–5. With heating and cooling loads competing, this will provide earth coupling – less digging and insulation are needed.
Where To Put The Insulation
As for deciding where to put the insulation, if attached to the exterior this will be less expensive if below grade because it can be left uncovered; as such, no fire retarder such as 5/8–inch drywall is required. Secondly, the major air leak at the sill wall is prevented with a good installation. Thirdly, the thermal mass remains available to the house for heating or cooling. Lastly, the foundation is protected from stresses caused by insulating.
Reasons to go with interior insulation are that frost heaves will develop in the soil because the heat from the house is not there to warm up the soil. This can be particularly bad for block and stone foundations that cannot resist lateral forces. Foam insulation and backfilling with clean granular fill can prevent cracking because of freeze/thawing. Spalling can also occur.
The possibility of an unheated basement falling below the freezing point may be important regarding pipes freezing. Heat is added to the basement by furnaces and boilers which are in the basement. Water heaters and laundry equipment, a source of heat, can be in the basement as well. As a rule of thumb, the basement temperature should be halfway between the outside and inside temperatures. Moisture control is important, so mold or bacteria do not grow. In all but very arid climates, a vapor barrier is installed on the warm side of the wall. The insulating material should not degrade structurally or thermally after long term exposure to the elements. The barrier is usually a thin sheet of PET plastic. The barrier prevents moisture from inside migrating into the wall where it condenses somewhere in the wall and greatly diminishes the R-values of the insulation and reflective barriers.
How To Calculate Heat Gain/Loss
When using a program (i.e. REScheck) to calculate the gains/losses through a basement wall at least 50% of the wall must be below grade. Basement walls that enclose heated spaces are part of the building envelope. The area of the wall should exclude windows and doors. For increased accuracy, the basement floor area should have the exterior wall thickness subtracted. An approximate method for calculating heat loss through a basement is given by the equation: qem = EF x U x A, where EF is a correction factor for a below-grade wall and A is the area of the basement walls. Heat gain/loss coefficients for above-grade basement walls are given as U–factors and below-grade as F–factors [Btu / (hr–ft–°F)]. For the foundation, the heat loss is conducted away from the center out to the perimeter of the slab:
Qsc = F x P
- Qsc – slab edge transmission heat loss
- P – the perimeter of the slab edge in linear feet
- F – transmission heat loss per linear foot of slab edge
- Norton, Paul. “Types of Insulation”. U.S. Department Of Energy. https://www.energy.gov/energysaver/weatherize/insulation/types-insulation.
- Berner, Mike, “Common Types of Insulation, and When to Use Them”. https://www.familyhandyman.com/heating-cooling/common-types-of-insulation-and-when-to-use-them.
- “Basement Heat Loss Guide”. https://inspectapedia.com/Energy/Basement_Heat_Loss.php.
- Bobenhausen, William. “Simplified Design of HVAC Systems”. Scholarly & Professional, Wiley, 1994.