# Slab-on-grade Insulation



## fj80 (Feb 13, 2017)

Designing a single-family house using IRC 2012/ 2012 Virginia Residential Code. Climate Zone 4 (not Marine).

I have a conditioned space with a slab-on-grade that is less than 12" below grade, so I know it needs to be insulated per N1102.2.9. But I could have sworn that in addition to providing insulation on either the inside or outside of the foundation wall, that you also had to provide under-slab insulation horizontally for a minimum of 36" from the perimeter of the slab. Is this requirement somewhere else in the code? Or is that an old requirement that has changed?


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## Francis Vineyard (Feb 13, 2017)

In accordance with Section N1102.2.8 and TABLE N1102.1.1 (R402.1.1) Column 10; SLAB _R_-VALUE & DEPTH

Insulation located below _grade _shall be extended the distance of 24 inches by any combination of vertical and horizontal insulation extending under the slab or insulation extending out from the building.


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## rogerpa (Feb 14, 2017)

N1102.2.9 (R402.2.9) Slab-on-grade floors. Slab-on-grade
floors with a floor surface less than 12 inches (305
mm) below grade shall be insulated in accordance with
Table N1102.1.1. (R-10). The insulation shall extend downward
from the top of the slab on the outside or inside of the
foundation wall. Insulation located below grade shall be
extended the distance provided in Table N1102.1.1 (2') by any
combination of vertical insulation, insulation extending
under the slab or insulation extending out from the building.
Insulation extending away from the building shall be
protected by pavement or by a minimum of 10 inches (254
mm) of soil. The top edge of the insulation installed
between the exterior wall and the edge of the interior slab
shall be permitted to be cut at a 45-degree (0.79 rad) angle
away from the exterior wall. Slab-edge insulation is not
required in jurisdictions designated by the building official
as having a very heavy termite infestation. Stated values
based on Zone 4/5. If a heated slab add R-5 2' becomes 4'
if located in Zones 6-8. See footnote d of Table N1102.1.1
for heated slabs in Zones 1-3.


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## fj80 (May 17, 2017)

Since the code says "The insulation shall extend downward from the *top of the slab*..." does this mean you have to place the insulation between the interior face of the foundation wall and the basement slab? If so, then the slab never contacts the foundation wall. Does the slab not need to be tied into the foundation wall with rebar? Or is it fine for the slab to be structurally independent from the foundation wall?


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## Francis Vineyard (May 17, 2017)

It depends how it's done. Pre-engineered wall already have the insulation. The below example apparently is no longer linked online:


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## tmurray (May 24, 2017)

fj80 said:


> Since the code says "The insulation shall extend downward from the *top of the slab*..." does this mean you have to place the insulation between the interior face of the foundation wall and the basement slab? If so, then the slab never contacts the foundation wall. Does the slab not need to be tied into the foundation wall with rebar? Or is it fine for the slab to be structurally independent from the foundation wall?



We generally see a break between the slab and the wall for insulation. These walls are generally laterally supported on both sides, so I'm not sure what rebar would do...


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## mark handler (May 24, 2017)

tmurray said:


> We generally see a break between the slab and the wall for insulation. These walls are generally laterally supported on both sides, so I'm not sure what rebar would do...


During Dynamic action, Earthquakes and tornados, the slab is used as a horizontal diaphragm, to restrain and transfer loads from the walls.


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## Mark K (May 24, 2017)

On many smaller buildings  the slab on grade is not formally considered as part of the lateral system.  This is a call that should be made by the engineer.

When the slab on grade is needed to tie the building together any break between the wall and the slab is a problem.


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## tmurray (May 24, 2017)

mark handler said:


> During Dynamic action, Earthquakes and tornados, the slab is used as a horizontal diaphragm, to restrain and transfer loads from the walls.



AHA, we don't get those here, at least not that many. Thanks!


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