Article #1459 - REScheck Webcast Questions - Slabs
This article is based on questions asked during both the May 6, 2005
REScheck Training Webcast
and the July 28, 2005
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.
Is a heated slab "heated" or is it a slab in a conditioned space?
The definition of a heated slab is a slab that is directly heated by using hydronic coils or other means within the slab itself.
Is slab edge insulation only for a walk-out situation?
No, slab on grade floors, whether a walk-out or the entire house must meet insulation requirements dependent upon location of the project.
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).
In Texas I am not positive that it is necessary to insulate slab on grade. However if not necessary and need to get proper energy compliance can this be performed or does this need to be consulted with building inspectors?
Most areas of Texas do not have slab on grade insulation requirements. However, even if the slab is not required to be insulated and the slab is part of the building envelope, it must be shown on the compliance report.
The following link offers additional information on this subject:
Why is Perimeter Slab Insulation Important?
For slabs with exterior rigid insulation with a kicked out wing; there is a joint that is sometimes not supported correctly; which often breaks when back filled thus breeching the continuous insulation.
This is an application problem. The code wants to see the full R-value at the slab edge. Following this approach leaves you without a nailer for carpet tack strip. There is language in the code when slab insulation is installed between the exterior wall and the edge of the interior slab, the top edge of the insulation shall be permitted to be cut at a 45 degree (0.79 rad) angle away from the exterior wall. Insulation extending horizontally away from the building shall be protected by pavement or by a minimum of 10 inches (254 mm) of soil However, this does tend to be a weak edge and often times can break.
Regarding slab on grade: how do you do the turned-up insulation if the slab actually sits on the foundation wall?
You need to either provide a thermal break or insulate the outside edge of the slab or trade it off with another building envelope component.