Article #1457 - REScheck Webcast Questions - Basement
This article is based on questions asked during both the May 6, 2005 REScheck Training Webcast
and the July 28, 2005 REScheck Training Rebroadcast as well as questions received through email following these events. The original language of the questions and answers has been modified to make them as generic as possible while retaining the applicable specific information.
When do you ignore the edge of a basement slab with a footing 12 inches or more below grade?
For slabs with a top edge above finished grade or 12 inches or less below finished grade, the IRC requires the insulation R-value to be applied to the outside of the foundation or the inside of the foundation wall (the required R-value of insulation is dependent upon location of the project based on heating degree days). The insulation shall extend downward from the top of the slab, or downward to the bottom of the slab and then horizontally in either direction, up to a minimum distance of 4 feet (total depth of insulation is dependent upon location).
Therefore, a slab that is greater than 12" below grade would not need to be entered in REScheck and would not be required to meet slab insulation requirements.
The following links offer additional information on this subject:
- Why is Perimeter Slab Insulation Important?
- Where and How Should Perimeter Slab Insulation be Applied?
What do you recommend for basement slab calculations with or without heated floor?
To calculate foundation heat losses, heat loss values for slabs were taken from Huang et al. (1988). In this methodology, the heat loss unit for below-grade foundations is in terms of linear feet of perimeter (F-factor) instead of square feet of surface area (Uo-factor). A Uo-factor is multiplied by a surface area and degree-days to obtain the total heat loss. An F-factor is multiplied by a perimeter length and degree-days to obtain the total heat loss. These F-factors are shown in Table A.34. The F-factors are given in the referenced paper for insulation both on the exterior and interior of the foundation wall. The F factors vary only slightly by insulation placement, so the average of the exterior and interior insulation placement was used. The same F-factors were used for heated and unheated slabs. Huang et al. (1988) did not present F-factors for insulation levels above R-10 for slab insulation 2-ft deep; therefore, F-factors were considered to be constant for insulation levels above R-10 for this configuration. Additionally, F factors were considered to be constant for all insulation levels above R-20, regardless of insulation depth. This assumption was deemed reasonable because little is gained by the additional insulation (above R-20, most of the heat loss occurs under and around the insulation).
In the REScheck software, slab perimeters can be insulated to any depth up to 4 ft. To calculate heat loss for any combination of insulation depth and R-value, quadratic curves were fit through the data in Table A.34. The resulting quadratic Equation (A.24) gives the F-factor as a function of insulation depth. The applicable coefficients for Equation (A.24) are given in Table A.35 and are determined by the insulation R-value. R-values range from R-0 to R-20.
Table A.34:
Slab-On-Grade Floor F-Factors
| Insulation R-Value | 2-ft Insulation Depth | 4-ft Insulation Depth |
|---|---|---|
| R-0 | 1.043 | 1.041 |
| R-5 | 0.804 | 0.744 |
| R-10 | 0.767 | 0.684 |
| R-15 | 0.767 | 0.654 |
| R-20 and Above | 0.767 | 0.636 |
Slab F-Factor Equation (A.24):
F-factor = intercept + coef 1 x depth + coef 2 x depth2
where depth = the distance the insulation extends downward (or downward and outward) in feet.
Table A.35:
Coefficients for Slab F-Factor Equation
| R-Value | intercept | coef 1 | coef 2 |
|---|---|---|---|
| R-0 | 1.042 | 0.0013 | -0.0004 |
| R-1 | 1.042 | -0.0967 | 0.0144 |
| R-2 | 1.042 | -0.1293 | 0.0188 |
| R-3 | 1.042 | -0.1459 | 0.0207 |
| R-4 | 1.042 | -0.1562 | 0.0217 |
| R-5 | 1.042 | -0.1635 | 0.0223 |
| R-6 | 1.042 | -0.1692 | 0.0227 |
| R-7 | 1.042 | -0.1739 | 0.0230 |
| R-8 | 1.042 | -0.1781 | 0.0233 |
| R-9 | 1.042 | -0.1819 | 0.0236 |
| R-10 | 1.042 | -0.1855 | 0.0240 |
| R-11 | 1.042 | -0.1836 | 0.0231 |
| R-12 | 1.042 | -0.1819 | 0.0222 |
| R-13 | 1.042 | -0.1805 | 0.0215 |
| R-14 | 1.042 | -0.1792 | 0.0208 |
| R-15 | 1.042 | -0.1780 | 0.0203 |
| R-16 | 1.042 | -0.1770 | 0.0197 |
| R-17 | 1.042 | -0.1760 | 0.0193 |
| R-18 | 1.042 | -0.1751 | 0.0188 |
| R-19 | 1.042 | -0.1743 | 0.0184 |
| R-20 | 1.042 | -0.1735 | 0.0180 |
How should I enter basement walls that have rigid exterior insulation that is from grade down, not from top of wall down?
Separate the wall into two different assemblies, the wall with no insulation and the part of the wall that has insulation. If the top part of the wall that has no insulation is >50% above grade, it would be considered an above grade wall and should be entered as an above grade wall. The insulated portion of the wall, if >50% below grade, would be entered as a basement wall.
The following link offers additional information on this subject:
How do I Enter Insulated Basement Walls in REScheck?
How would you enter a basement wall with outside rigid insulation that stops at grade?
When entering the depth of insulation for the basement wall, enter the depth in feet from the top plate to the grade level. Enter the insulation R-value in the Continuous Insulation column.
If I use continuous rigid insulation on the inside of the basement, do I enter this as cavity or continuous?
You would enter your insulation R-value as continuous.
Cavity insulation is insulation installed in the cavities between structural members, such as wood studs, metal framing, and Z-clips. Continuous insulation runs continuously over structural members and is typically rigid foam board. Cavity insulation is affected by thermal bridging, while continuous insulation reduces thermal bridges.
Continuous Insulation (CI)
Insulation that runs continuously over structural members and is free of significant thermal bridging; such as rigid foam insulation above the ceiling deck. It is installed on the interior, exterior, or is integral to any opaque surface of the building envelope.
The following link offers additional information on this subject:
How Do I Enter Cavity and Continuous Insulation for Basement Walls in REScheck?
Is rigid insulation on an interior basement wall (cmu) between furring considered continuous?
No, if the basement wall is being furred out, insulation installed between the furring would be considered cavity insulation.
Cavity insulation is insulation installed in the cavities between structural members, such as wood studs, metal framing, and Z-clips. Continuous insulation runs continuously over structural members and is typically rigid foam board. Cavity insulation is affected by thermal bridging, while continuous insulation reduces thermal bridges.
Insulation to be installed between furring or studs is entered in the Cavity R-value column. If the installed wall insulation will be continuous (such as with exterior rigid insulation or interior "draped" insulation), the insulation R-value is entered in the Continuous R-value column. A combination of insulation systems may be used. For example, installing R-13 batt insulation between the studs and R-5 rigid insulation on the exterior of the stud wall.
Is it possible for a contractor to show compliance without final grade shown on drawings?
No, one good example is a walk out basement. The code defines below grade walls as walls that are more than 50% below grade. If you don't know the final grade, then the best approach would be to assume the entire project is above grade or at grade level.
The following link offers additional information on this subject:
How Do I Enter Non-Uniformly Insulated Basement Walls in REScheck?
How do I calculate the depth below grade for a wall with difficult topography and before the lot is graded?
The code defines below grade walls as walls that are more than 50% below grade. If you don't know the final grade, then the best approach would be to assume the entire project is above grade or at grade level.
Can below grade insulation depth be used as a tradeoff?
In certain situations, because REScheck is heavily based on U factor x Area (UA, the heat loss/gain rate) calculations for each building assembly to determine the whole-building UA for the building design. The whole-building UA from a building conforming to the code requirements (the code building) is compared against the UA from the user's building design (the proposed building). If the total heat loss (represented as a UA) through the envelope of the user's building design does not exceed the total heat loss from the building conforming to the code, then the user's design passes.
Why do you get penalized when the depth of wall below grade is deeper into the ground? Isn't that opposite than it should be?
The REScheck UA calculations for basement walls take into account the effect of the soil surrounding the below grade walls. The larger the depth (feet) below grade the overall UA calculation is less than a below grade wall with a lesser depth (feet) below grade. The calculation takes into account the heat loss and the R-value of the soil according to the depth below grade.
How do you treat interior blanket insulation applied directly to foundation walls without studding?
The insulation would be considered continuous insulation with no thermal breaks, enter in the Continuous Insulation column.
Do you list a floor on every project? I am not sure what should be input for a basement floor.
If your basement is a walkout, you would enter the floor, at the walk out area, as slab on grade and your calculation would be the exposed portion of the slab in linear feet. The rest of the basement is entered as basement wall and not floor.
Floor components that are required to meet code are floors over unconditioned spaces and slabs on grade where the perimeter edge is exposed.
Is blanket insulation allowed for foundation walls?
Yes, blanket insulation is allowed for foundation walls, but not for use as slab insulation. You may want to consider moisture when you plan for basement insulation.
Is fire safe batt insulation that is on the entire interior face of the basement wall considered continuous insulation?
If the interior side of the basement wall is not furred out and you are draping the insulation from the top of the stem wall downward, the insulation would be considered continuous insulation. Keep in mind, the rim joist will still need to be insulated and the draped insulation is typically attached to the mud sill.