Insulation For Slab-On-Grade

[TSJHD1]

Could someone point me to the code reference in the IBC that deals with how much insulation to use behind a foundation wall, when the floor is a slab?

Our plans call for 1" rigid board, from bottom of slab to footing, which is 16", then 24" laid flat.

Building inspector is questioning it b/c the former inspector says it's not enough. But he got fired.

Thanks.

Tom

[Dick Seibert]

Tom:

You better question both inspectors, Maryland is in the "Moderate to Heavy" Termite Zone", foam can't be used within 6" of the ground, see IBC map below.

[JMS BUILDERS]

True Dick,

But XPS has resistance to those "buggers" as well as the "foam" xps is on the interior of the block and under the slab. not quite the same as having it to the outside of the wall.

[ThingOfBeauty]

I don't know from the IBC, but I believe it's the same as the IRC which requires R-10 for 24" from the top edge of the slab. Since each county in Maryland adopts the code with its own revisions, you'll need to get with your AHJ, of course. In Montgomery County, they adopted the IECC 2009 for commercial which appears to require R-10 for 24" in Group R buildings. IECC 2009 table 502.2(1).
You can go straight down two feet, or down and under the slab for a total of 2'.
R-10 is 2" of XPS, or about 3" of EPS.
We (Maryland) are in the "moderate-to-heavy" termite range, which means (in the IRC anyway) we are allowed to use foam plastic below grade. Dick's comment above applies only to "very heavy" areas. R318.4
In residential, you can cut the very top of the foam board at a 45-degree angle, so the slab covers the foam except the very little point at the top. That gives you a bit more physical & termite protection. IECC 402.2.8

Good luck, I'm going to say your first inspector was right, you are supposed to have 2" of foam board under there.

[TSJHD1]

...or down and under the slab for a total of 2'.
R-10 is 2" of XPS, ...
In residential, you can cut the very top of the foam board at a 45-degree angle, so the slab covers the foam except the very little point at the top. That gives you a bit more physical & termite protection. IECC 402.2.8

Good luck, I'm going to say your first inspector was right, you are supposed to have 2" of foam board under there.

The plans show it installed vertically and horizontally (flat), each 1" thick. Together they add up to R-10, but not sure how they look at that.

Thing that gets me, the plans were approved for 1" each way. Maybe he just doesn't realize we simply have not yet installed the flat piece (the inspection was for plumbing backfill, not slab--he just jumped the gun...I had not yet called for the slab inspection...can't until more work is done.)

Thanks Doug for looking into this for me.

Tom

[eric_anderson]

Tom,
I know the budget may not allow it, but why not use 3-4" EPS or 2"xps under the whole slab? Do you know how deep the water table is? If you are in a high water table area, the heat loss through the slab can be very high, and insulation here can be cost effective and provide a warmer more comfortable slab.
Eric

[Dick Seibert]

I'll Post the code:

2603.8 Protection against termites. In areas where the probability of termite infestation is very heavy in accordance with Figure 2603.8, extruded and expanded polystyrene, polyisocyanurate and other foam plastics shall not be installed on the exterior face or under interior or exterior foundation walls or slab foundations located below grade. The clearance between foam plastics installed above grade and exposed earth shall be at least 6 inches (152 mm).

Exceptions:

1. Buildings where the structural members of walls floors, ceilings and roofs are entirely of noncombustible materials or preservative-treated wood.

2. An approved method of protecting the foam plastic and structure from subterranean termite damage is provided.

3. On the interior side of basement walls.

Note: Figure 2603.8 referenced is attached to my post above. In addition, if you have any Environmentalists in your area, they are trying to ban all foams worldwide for clogging up the oceans and killing the fish and birds, and they can get violent.

[NW Architect]

Works out just fine that the places where you can't use foam under slabs/foundations are mild climates where soil temperatures aren't going to need much insulation anyway (although the folks on sierra mountops in CA might think a little insulation would be nice).

[ThingOfBeauty]

I'll Post the code:...Note: Figure 2603.8 referenced is attached to my post above.

In figure 2603.8, there are seven states that are in the "VERY heavy" zone, where the code restrictions apply. Maryland is not among them.

Still makes me nervous to use foam near or in the ground, but it is not contrary to code in Maryland.

[Dick Seibert]

Doug:

In the early 70s we went to pier and grade beam foundations, one of the requirements was that the grade beams had to be 2" off the ground so expansive soils heaving wouldn't crack the grade beams. We started cutting 2" styrofoam into strips and laying them under the grade beams, after we stripped we were supposed to dig out the styrofoam. It was such a difficult job that I argued with inspectors and my structural engineers that I should be able to leave them in, the styrofoam would crush if the soils heaved, and termites would remove them in a couple of years anyway; no dice, they said that the styrofoam was a magnet for termites and presence of termites would allow them to build mud tunnels up the grade beams and access the structure long after they had digested the styrofoam. After that we went to cardboard boxes under the grade beams, ti was easier to dig out, but they still wouldn't let me leave it in for the termites

[TSJHD1]

OK, interesting comments.

I'll agree about the foam board attracting termites...to some degree. But it's not my call.

Expansive clay: We have lots of that on the eastern shore of MD, and in many cases it must be removed. If you dig it up for a foundation, you cannot use it for back fill.

But getting back to my issue...

Called the arch...he read the IECC, which states the R value can be achieved by a combination of vertical and horizontal placement. I didn't ask him what section he was reading, because I don't have a copy, but I should have so I could post it here. He's making a call to the BI as we speak.

Dick, do you have a reference in the IBC that speaks to the use of XPS near/at grade, re termites? (Or Doug, is it just the fact that the term used is "very", and since MD is not in the "very" range, we are excluded?) I saw it yesterday and indeed MD is not applicable to the restriction, but then I could not find it again! (I'll bookmark that stuff in the future!)

Thanks.

Tom

[ThingOfBeauty]

The termite restriction in the IRC is section 318.4. I don't know where it is in the IBC.

My local counties over here have links to free copies of the code on the internet...if your county's website doesn't you may want to click over to Montgomery County's page and find a copy.

My understanding about the "R-10 for two feet" is you start at the upper, outer edge of the slab, start drawing downward, and either go straight down, or down and then turn under the slab, for a total of 24". All that insulation should be R-10. If you have a layer only 1" thick (R-5), you'd need another layer 5" thick between the slab edge and the outside.
There's a good thread going on GBA about "foundation edge insulation" if you want to explore a lot of ideas about it:
http://www.greenbuildingadvisor.com/...ion-insulation

[Dick Seibert]

Tom:

I posted it above, it's §2603.8 Protection against termites in the IBC

[manhattan42]

Could someone point me to the code reference in the IBC that deals with how much insulation to use behind a foundation wall, when the floor is a slab?

Our plans call for 1" rigid board, from bottom of slab to footing, which is 16", then 24" laid flat.

Building inspector is questioning it b/c the former inspector says it's not enough. But he got fired.

Thanks.

Tom

Chapter 13 of the IBC says that buildings must be designed and constructed in accordance with the International Energy Conservation Code.

The IECC Table 502.2(1) requires NO INSULATION for foundation walls in Zone 4 (Maryland) for unheated slab-on-grade buildings.

Foundation insulation is required only if the slab is heated.

-----------------
Restriction on foam insulation along foundations from IBC 2603.8 does NOT APPLY in Maryland.

Restriction only applies where probability of termite infestation is VERY HEAVY in the states of CA, MS, LA, AL, GA, SC and FL and there are even exception for those states.

[Sweep8]

No insulation is required unless the building use is residential (R-2, 3 or 4 of any height) in which case the required R value is 10 for a distance of 24" as stated earlier by others. Since the inspector is requiring insulation I suspect the use is Residential but that should be confirmed.

The vertical and horizontal distances can be added to meet the 24" requirement. The insulation must run from the top of the slab downward 24" (or to the footing) OR from the top of the slab downward and then horizontally under the slab so it would be logical to assume that if it did both, the two layers of insulation would only need to be R-5. I suspect that before it splits into two layers it would need to be R-10 (at the edge of the slab). It seems it would be easier to install one layer of R-10 insulation.

Since it appears that this insulation will be installed inside the foundation wall, there should be no code requirement regarding termites even if it were in a very heavy zone.