Hi, I’m seeking a sanity check on our basement/slab insulation. We’re owner-building in south-central PA and plan to break ground in July (yikes).

Project:
For the basement, ~7’ of the 9’ poured walls will be below ground with a 4” slab, V.B. and 4” stone. Footers are form-a-drain. The garage will have footers (in a trench, not form-a-drain) and poured stem wall and slab same as basement. After reviewing forum input here and then actually getting real $ quotes on ICF, pre-cast and poured, we’re decided on poured walls with future plans to DIY upgrade interior basement wall insulation, when money allows.
We have no desire to ever finish the basement for a living area. It will be storage and possibly a project work area. There are NO plans for radiant heat in the basement slab. We will hire-out the pour, but the slab prep [insulation(?), VB, rebar] are DIY materials and labor.

Sanity Checks:
I was thinking 2” of Type II (1.5 lb density) EPS under the basement slab.

Would you cover the entire basement area (~1600 sq ft) with insulation? Or would you just do a 2’ or 4’ perimeter?

In general terms, are we talking a real energy ($) savings here (over time) or is this more of a personal comfort issue (as in basement as living space). It is interesting to note that no local builders, even of high end homes, seem to do under-slab insulation. I believe our deep ground temps are about 58 deg F.


How about the attached unconditioned garage? It will have insulated walls and attic and I was thinking of insulating inside of the stem wall down to the footer with Type I (~1lb density) EPS, and then either 2' or 4' perimeter of two inches Type II (1.5 lb density) EPS. Or should I just skip the foam in the garage altogether since it's outside the building envelop? I was just trying to avoid freezing temps in the garage. 


Buying Perform Guard - I’m trying to work with a PA distributor that gets their borate treated rigid foam from a manufacturing in Winchester VA. It’s not been that easy to get pricing and place an order. And forget about trying to actually compare prices with other sources! Apparently the PA distributor doesn’t really carry the under-slab foam, and only use a much thicker borate treated foam in their in-house SIPs products. So, it will be a special order for the distributor, and that’s never really a good sign (price wise). I’m still working through it, but wondering how others came to buy the Perform Guard they used in their projects.

Thanks for your help.

I'd use Dow's BlueGuard instead of borate treated foam. Not having the garage freeze will require air sealing and insulation. Personally, I prefer some air exchange to let things dry.

[Deleted. Double posted via editing]

For basement slab, I'd go with 2" under the whole slab. Layers, top down, would be slab, poly vapor barrier (seams taped), foam, stone layer. Besides being a large-area year-round heat loss, you'll avoid most or all of the condensation of summer humidity on the cold concrete if you insulate the slab, and things will be that much drier down there. If you were superinsulating the house, I'd say use 3-4" of foam.

Still, there will be more heat loss and condensation at the vertical foundation walls than at the slab. Even if you have to put off insulating the walls until later, at least have the outside waterproofed (not "dampproofed") before backfilling. For comfort and dryness, I would recommend getting those walls insulated (with foam) as soon as you can. It can be a DIY job, so it's only material cost. You'd have foam fired or glued on, then some sort of nailing frame, and finally sheetrock for required thermal protection of the foam. For sheetrock frame, I've got rips of 2x PT fired into the slab, studs on the flat, and top plate into the floor joists.

In the garage, with walls and ceiling insulated, think about an insulated door. You'll have to divide claimed R value by3 or 4 to get likely performance, but that's a large area. Also, the exposed foundation wall will be another substantial heat loss. Granted you're not heating it, but you can help keep the temperature inside moderated by insulating the inside of the stem walls, down to the footings as you suggest. Without a source of heat, other than the short term effect of cooling car engines, you can make a case for not insulating under the slab, so that ground heat can provide a little benefit.

If you are keeping the house, I'd do the 2" insulation under the entire slab, and at the edges (and of course, vapor barrier and gravel or stone beneath).
AND, I'd take one extra day to do a DIY radiant tubing job in the slab (for the future, just in case).

If you are not planning to keep the home and feel you have to cut upfront costs, you might get away with rips of the 2" foam to isolate the slab edges from the foundation walls. However, an uninsulated slab or basement is just a continuous heat sink to some degree or another.

Thanks for the input - Since we plan to stay in this house until they carry us away, I’ll go ahead and do the 2” under the entire basement slab and isolate the edges from the foundation. And doing it under the entire slab, vs just the perimeter, takes away a nitpicky detail of getting a 2” difference for the underslab stone which will be easier for both me and the excavator.

I agree the vertical walls of the basement need to be done - and not 10 years after construction! I hope we can do this within two yrs of moving in. As a DIY project, and with the possibility of getting polyiso from local building auctions, it won’t be a monster cost (when compared to the overall project). The only reason I put a 2 yr window on it is because I know we’ll be so busy putting out other ‘fires’ that the basement walls will take that long to rise to the top of the list!

In the garage, we’ll definitely have insulated garage doors. For the shop we put up last summer, I bought a good quality insulated door and because of the commercial size, it sure cost a pretty penny. So now that I’ve had my hair blown back once on overhead doors prices, I’ll be better prepared to hear the quotes on the garage doors for the house. But definitely insulated, I agree.

Regarding Dick’s comment: ”Also, the exposed foundation wall will be another substantial heat loss. Granted you're not heating it, but you can help keep the temperature inside moderated by insulating the inside of the stem walls, down to the footings as you suggest. Without a source of heat, other than the short term effect of cooling car engines, you can make a case for not insulating under the slab, so that ground heat can provide a little benefit.” We did that very same thing in our shop, in large part because of input I got on this board. I guess I was so focused on “house and garage” as a single item, I was thinking they would be insulated the same way - which is really silly. For the shop, although we have it closed in and the ceiling in, we haven’t yet insulated it. Therefore I’m not certain how well the shop’s winter temps will be moderated by the 6” slab (with foundation wall and perimeter insulation). I think it did help this past winter, but until we insulate the shop (we’re waiting until we do the house, economies of scale) we won’t really know. I did make a mistake on the shop though…I used XPS for the under-slab and inside foundation insulation. I don’t know if or how much this will be a future problem with termites, so I’m trying to learn from my mistakes and do a better job on the house.

Posted By jonr on 25 May 2011 06:45 PM
I'd use Dow's BlueGuard instead of borate treated foam. Not having the garage freeze will require air sealing and insulation. Personally, I prefer some air exchange to let things dry.

Hi Jonr, in doing my original research I had the same thought as you - Dow BlueGuard. When I called Dow to get info on where I could buy it I was surprised to hear that they've discontinued this product and have no replacement offering (i.e. termite treated foam). That's what put me on to looking for PerformGuard, although actually acquiring it hasn't been that easy ... still working on it. 

When I saw your post here, and since the PerformGuard is proving hard to get, I called Dow again just to double check. And sure 'nough they said it was discontinued in Oct. 2008. Bummer.

When I read articles on green building and passivhaus and super-insulated and all, and see how much rigid foam is going into some of these homes - and often in termite areas - I wonder, "Are these folks using termite treated rigid foam and how are they purchasing it, from manufacturers, distributors, other?"

Regarding your comment on possible moisture issues in a well sealed, insulated garage - maybe that is something I should take better note on. I read an article somewhere (I think on a GBA blog) about "issues" with attached garages - mostly in terms of IAQ. I believe they mentioned having a ventilation fan (I guess with a fresh air intake (?)) that vents the garage for X minutes after the doors have open/closed. Maybe that would be a help for moisture issues too (for us in winter, since in summer the outside air is very humid). So - I don't know the answer - but you raise a good point and I'll give it some thought as we move forward with our project.

Interesting point about BlueGuard. Given that there are lots of areas with termites, I wonder why getting termite resistant foam is so hard.

For the garage, mostly I just want the cars to get and stay dry. That can be hard if they come in with rain and snow and then sit in a sealed room (that will quickly go to ~100% humidity). I run a circulation fan and that helps a lot. In a sealed and heated garage I guess you need a HRV - perhaps a humidity controlled one.

Shouldn't the vapor barrier go under the insulation? So you have from bottom up - stone, vapor barrier, foam (seams taped and gaps foamed), slab.

-Rosalinda

Shouldn't the vapor barrier go under the insulation? So you have from bottom up - stone, vapor barrier, foam (seams taped and gaps foamed), slab.

That's how I'm doing it - right now, in fact.

In most comments on VB under vs over the slab foam, I see over the foam recommended. That's the way I did it. One benefit was that I had a flat foam surface under the VB, a real help when tape-sealing the VB overlap joints. I suppose one can argue that since XPS itself is a vapor retarder and isn't a sponge for liquid water then it serves the VR function by itself, although certainly the joints in it ought to be taped.

Either way, if a sheet material is used between footing and foundation wall as a capillary break, then I think it is good practice to have that sheet overlap the VB under the slab with those seams taped also.

bbuilder

we've been using  foam products  under slabs and inside and outside for about 25 years...
long enough to take some  projects apart and see the damage  that occurs  by not using  treated foam..

the only  foam we use now is Performguard
and we get it from  Branch River Foam in Smithfield, RI

under slabs we use 2"  of 2 lb/cf on top  of 12" of 3/4" crushed stone and drainage systems

RI is not that far from Pa.... get a quote from Branch River and see if it's worthwhile to pay for trucking ....or pick it up yourself....your whole basement would fit in a box truck

Kermit,
Thanks for the very useful info - I will definately call them for a quote.
I'll let you all know how it goes, dig date looming.

Hi, O.P. with a follow up Q. This is a down in the details question on the underslab insulation. Attached, if it worked, is a little drawing of what I’m proposing (we’re doing the slab prep work). Note that there are some funky details and this drawing is mine (i.e. not a designer or contractor’s). I would appreciate any advice from construction knowledgeable folks here. If the drawing looks OK, I’ll make a better one to scale and show it to the excavation contractor so he can see what/how deep to dig.

- The footer is 8” high, but the FAD is only 6”. This is what the concrete contractor wants to use. Said he’s does it this way all the time with on problems.

- Designer's Drwg calls for 4” stone, VB, 4” slab, AND I’ve got some very tight head-room clearance issues on an egress stairs out of the basement … so I can not be making the slab any thicker/higher (if that makes sense).

- If I use 2” foam underslab, I’m not sure how to handle the area directly over the footer. So my drwg shows only 1” foam there and 3” concrete in order to NOT lose any headroom.

Does this look OK? Or am I asking for trouble with a 3” slab over the footer than thickening to the 4” slab off the footer?

Re: previous discussions above, still working on getting the foam and nothing final to report yet.

Thanks!


OOPS - that foam is supposed to say EPS, not XPS.

The 3" of concrete is a code violation. Redesign or excavate to accommodate required depth.

Posted By acwizard on 17 Jun 2011 04:12 PM
The 3" of concrete is a code violation. Redesign or excavate to accommodate required depth.

Poster states"with a 3” slab over the footer than thickening to the 4” slab off the footer"  this is not a violation as long as it is in direct contact with the footer. A 4" slab is considered a "nominal 4" , typically 3-1/2" and not unusual to vary from 3" - 5" due to imperfect grading

I would pour the footer and the slab as one piece - and put insulation outside of both. Ie, a thickened edge monolithic slab. Consider moving insulation to the basement ceiling and having an unheated basement.

Garage - frost protected monolithic slab. I probably wouldn't seal it up tight because of moisture issues. Warmth plus 100% humidity and maybe a little road salt and you have great conditions for rust.

Uniform Building Code section 1904.4 states minimum must be 3.5". There are no exceptions to the rule

Posted By acwizard on 18 Jun 2011 10:33 AM
Uniform Building Code section 1904.4 states minimum must be 3.5". There are no exceptions to the rule

I agree and a 3" slab poured directly on an 8" footing is an 11" slab with a cold joint. 

1. From a practical standpoint, there are no guarantees that a 3-1/2" formed slab is a consistent thickness thruout. What I can guarantee is it will be less than 3-1/2" in spots

2. It is perfectly acceptable to top off an existing slab with a 2" overpour cold joint , the slab in essence is still  over 4" in thickness (with a cold joint)

I’m glad it was pointed out that I’m either in violation of the code, or dancing around the hairy edge it. I don’t want to do that, or to accidentally mess with anything structural.

Re: the insulation, I’ve had a 2nd go at the sketch, and took out the 1” EPS foam over the footer. There is still 2” under the rest of the slab. The slab is isolated from the footer by a 15 mil VB and I am still thinking of isolating the slab edge from the poured wall, as shown.

This new drawing keeps the slab a full 4” everywhere, so that’s good. But there is no insulation under the slab for about 6” where it is over the footer. When we upgrade the basement (not finish it) to have interior wall insulation, that would come in off of the poured wall about 4” - more if we have to add studs and drywall for code, depending on what type of wall insulation I can get.

Seem OK to you?