Basement sub-slab insulation
Michael Maines
| Posted in Building Code Questions on
I should know the answer to this already…. I’m in climate zone 6, using the IRC 2009 code. Table N1102.1 requires R-10 insulation below a slab. I’ve been taking this to mean under the basement floor, but in re-reading it for the 100th time today, I think I’ve been misunderstanding the requirement. I know it’s best practice to insulate under the poured concrete floor in a full basement, but is it required?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Depends on your local authority / the state's adoption of IRC 09 - see http://www.iccsafe.org/gr/documents/stateadoptions.pdf.
But keep in mind, codes generally enforce *minimum* levels of safety & performance...
Cramer, I'm in Maine, and our statewide energy code is the IECC 2009. We work in many different municipalities, and I can ask the CEO for each project, but it seems like this shouldn't be something left up to interpretation. (I'm a designer and project manager at a residential design/build firm.)
I guess the broader question is, does the meaning of "slab" include the floor of a basement? I assumed it did, until today when I noticed that conditioned crawlspaces are not required to have insulated floors. Almost all of our projects have full basements so I hadn't even read the crawlspace requirements before.
On the project I'm working on now we will keep the insulation I have already spec'd for under the basement floor, and going forward I'll keep trying to spec better than code solutions. I'd just like to know what's elective and what's required.
Michael,
Section N1102.2.7 is one of the worst written paragraphs of the 2009 IRC -- and that's saying something, since so much of the IRC is ambiguous and poorly written.
Here's the paragraph:
"N1102.2.8 Slab-on-grade floors. Slab-on-grade floors with a floor surface less than 12 inches below grade shall be insulated in accordance with Table N1102.1. 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 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 code official as having a very heavy termite infestation."
This is maddening. A key sentence reads, "Slab-on-grade floors with a floor surface less than 12 inches below grade shall be insulated in accordance with Table N1102.1."
So builders wonder, "What about slabs that are more than 12 inches below grade? How are they insulated?" On this question, the code is silent. Does silence mean that no insulation is required? Who knows?
Let's take this one step further, what if it is a heated slab? I don't know about the 2009 code, but 2012 says R-5 will be added to the slab edge. So now, I'm taking concrete slab from 72 degrees to 105+, but I only need to insulate the slab edge? We now have a 55 degree + delta T, and we're not going to insulate the heated slab except at the perimeter?
I'm in zone 4, so an uninsulated basement floor that is below grade doesn't get me too excited as we get the cooling gain in the summer. But a heated one? And the way I read it, if its all below grade there is no insulation requirement (I read it the same, that unless it is 12" or less below grade, no insulation is required.)
I just had a situation yesterday they are going to use slab heat with pex tubing at 110 degrees and had installed R-10 below the entire slab. They thought they were doing an over-kill, until I explained the delta-T. Myself, I prefer to see at least an R-20 for heated slabs.
Bryce,
The building code gives you a list of things you have to do to stay out of court (or out of jail). It doesn't tell you how to build a good house.
Of course a slab with hydonic heating tubes needs a continuous layer of sub-slab insulation -- even if that isn't required by code. It's just plain common sense.
1. They are specific about insulating crawlspace walls.
2. They are sort of specific about NOT having to insulate crawlspace floors in N1102.2.9.
3. They are specific about insulating basement walls.
4. They are specific about insulating slabs less than 12" below grade.
5. They are NOT specific about insulating slabs more than 12" below grade.
I am now fairly confident that sub-slab insulation is not required in a basement.
Of course I could have just called the CEO and asked him but this is more fun.
Mike,
I'm not an expert on this topic, but your question interested me and I looked-up the resources mentioned in this thread. I believe the answer lies in your question. Table N1102.1, which you reference, shows that in Zone 6, sub-slab insulation of R-10 is required down to 4' below grade.
My interpretation of this is that when an unheated slab is more than 4' below grade, no sub-slab insulation is required. Am I understanding this correctly? Hopefully the gurus on the forum can verify this.
Source: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002.htm
I met a carpenter named Mike Maines at BRB, 15-20 years ago. Same fellow?
Roland,
The heading for the table column you reference is "Slab R-value and depth" and the entry for zone 6 is "10, 4ft." Footnote "d" does not clarify what is meant by "depth." I believe you are right, but it's frustrating that it's not more clearly spelled out.
I'm not sure what BRB is. 20 years ago next month I graduated from high school, and 15 years ago I was working as a carpenter in Boston.
A slab on grade is less than 12" below grade at that section of the slab., If it's in a basement, the wall, rising from it, is 'above grade' Around here, it is usually a walkout section of a basement or a whole house slab. In that area of the foundation, place foam boards to the footer, and anything left over from the 4' requirement can be horizontal. Actually, it can all be horizontal, or any combination. The thermal break separating the slab from the wall is also important. The bevel cut never seems to get don correctly, see if your inspector, or your states alternative will allow an R-5 strip as a thermal break