Any thoughts on this detail with epoxy rebar? im having a hard time posting the photo. its just a standard ICF with 2" of EPS in the center resting on the footing. http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/construction-begins-and-we-encounter-few-snafus

Pic...

[URL=http://s660.photobucket.com/user/craigtoo/media/ICF_XPS.png.html][/URL]

Posted By fallguy on 06 Nov 2014 09:09 PM
Any thoughts on this detail with epoxy rebar? im having a hard time posting the photo. its just a standard ICF with 2" of EPS in the center resting on the footing. http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/construction-begins-and-we-encounter-few-snafus

Unless you live in Arctic Zones 6 or higher, I don't see the benefit vs risk of having your concrete wall sitting on top of EPS instead of it sitting on a solid concrete footing. You are chasing absurdities in trying to reduce the energy loss in that type of design. How comfortable are you with a concrete wall crushing and causing the EPS to creep and have your wall shift and crack because you saved $5 a year on energy costs?

Your inspector will also freak out seeing foam on the footing and then having a concrete wall bearing thousands of pounds of weight on foam instead of the bare concrete footing.

Also, the steel rebar footing dowels passing through the foam on the footing would likely negate any thermal benefit provided by the foam.

Some vertical rebar won't negate much of the savings from the foam but it might support all the weight. If not, I would do the calculations for foam creep. I suspect you want pretty high density foam. As I recall, about 3x what you would calculate is needed.

Near or at grade, I'd guess the foam is important. Many feet below the surface and it's just a nice to have.

Seems like it would be easier to put the extra foam across the entire of width of the ICF vs just in the center. But maybe ultra high density foam is too expensive for that.

According to the blog, they used the incorrect foam and needed to re-do it with High Density foam.... and -for some reason- had to recalculate the re-bar schedule. DOH! Little alarm bells would be going off in my head. :/

juice is not worth the squeeze?

Insulate the slab, on top of the footing so EPS is direct contact with the ICF wall, place basement slab over this, you now have a thermally broken it all

Posted By Chris Johnson on 07 Nov 2014 09:16 PM
Insulate the slab, on top of the footing so EPS is direct contact with the ICF wall, place basement slab over this, you now have a thermally broken it all

I was thinking more about the transfer running up the wall bypassing the outside layer of foam.

The place for the foam is under the footing. This will give much less psi on the foam. After all, that's why we put in footings, to spread the weight.

In the case above, under and around the footing would be about 1/4 the PSI load. On the other hand, if it's easier/cheaper to put it under the ICF block and you can get it strong enough with the right foam or other details (like standing on vertical rebar), then why not. I would think that ICF vendors would have specific recommendations.

STOP! HALT! NONONO! do not place foam inside the wall like that picture shows.....catastrophic wall failure and worse? this is one of those ideas that at first glance seems ok but the basic laws of physics have been discarded, no type of foam in that application is going to withstand the load of the CONCRETE wall ...it will compress the foam and who knows what direction it will lean, crack or worse fall!!

so a much better method is called a Capillary Break! please google this method we use it all the time in our ICF applications, where we want to stop the wicking effect of a concrete footer sucking up the water then placing it into the ICF walls, the one guy who explained about placing foam against the ICF wall-on the slab side of the wall works, that would be under the interior house slab on top of the footer....BUT NOT INSIDE THE WALL!!!!!!!!!

Notice that the footing in the picture has a capillary break which is why it is white.

Making a foam (of some type) ~4x stronger than what is successfully being used under footings is doable and doesn't ignore any laws of physics. The only interesting questions are things like "how strong is needed", "is it readily available", "will someone approve it".

capillary breaks are usually an epdm or plastic layer, is that paint? i guess that will work? no engineer is ever going to approve that detail, there is a reason why no manufacturers have ever suggested that detail, 4,000 lbs p/cubic yard roughly speaking of course, you wouldn't build a wood wall on 2" of xps foam why do you think you can with concrete? the reason it works on slabs is the TOTAL load of the slab is spread out equivalent to the layer of foam say 20ft x 20ft horizontally....now you are talking about setting a 8-9-10ft vertical wall on an 8" wide by 2" thick piece of foam, no engineer will touch that idea, not in this world.
if you dont believe me ask your ICF block representative, those are LOGIX blocks you are using, you dont want to void the warranty.

Posted By fallguy on 06 Nov 2014 09:09 PM
Any thoughts on this detail with epoxy rebar? im having a hard time posting the photo. its just a standard ICF with 2" of EPS in the center resting on the footing. http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/construction-begins-and-we-encounter-few-snafus

You really need to rethink your reasons for doing this, IMO. How deep will the footing be, 8', or thereabouts? And where are you building, Maine also? If so the ground temp is going to stay somewhere in the 50° vicinity. In winter, if the temp at the top of the foundation wall is -10° F outside, and 65° F inside, the concrete will be something like 30° in the wall between those two temps. That's 20° LESS than the footing temp. Instead of heat going from the interior down into the ground through the wall and footing you'll have heat moving up from the earth via the footing into the wall. That is exactly what you want!

You really, really, need to study the temperature profiles of the ground throughout the year, the temps the footing is likely to experience, the temp profiles through the wall, and get a realistic assessment of which direction heat is going to move throughout the year. Without getting into a detailed study I'm going to postulate that foam at the bottom of the wall such as you are asking about may be your enemy, not friend!

And for God's sake don't do like that blogger did. Put your rebar into the footing before placing the concrete. Don't waste time and effort drilling and putting it in afterwards. And don't waste money on epoxy rebar. You only need that when corrosion is a likely problem, which won't be the case for you.

If so the ground temp is going to stay somewhere in the 50° vicinity.

Unless you extract heat from said ground as your calculation indicates. Then it drops significantly. This seems to be a common soil thermal analysis error - the ground isn't an infinite source of 50° without resistance (R value).

There is no doubt that footing thermal bridging wastes some energy. But with many feet of insulating soil and one side of an ICF between the interior and the cold ground surface, it isn't much.

dmaceld - "something like 30º"
Even in Calgary, I have never had "frost" inside my walls. Some time so I hope to set up some instrumentation to document the heat transfer through ICF but it is clear that it is not well understood.

You would need a steady state interior and exterior temperature to have an average mid point.

There is a lot more shoulder season time working on the walls then there is design temperature (either hot or cold) so most all the time a 68º basement would be giving off heat to the 50º footing.

From a practical purpose, totally encapsulating the concrete in foam will give you the best behaviour.

Posted By jonr on 10 Nov 2014 10:45 AM

If so the ground temp is going to stay somewhere in the 50° vicinity.

Unless you extract heat from said ground as your calculation indicates. Then it drops significantly. This seems to be a common soil thermal analysis error - the ground isn't an infinite source of 50° without resistance (R value).

True, probably most of the time. However, if there is a good water flow through the soil then the source would be almost unlimited. Cooling of the ground is one of the considerations in designing geo source heat pump systems, and has in fact been a problem for some. That's one of the reasons the OP's "simple" question is not so simple, although the quantity of heat moving through a footer would be miniscule compared to what is extracted with a heat pump.

Posted By FBBP on 10 Nov 2014 11:23 AM
dmaceld - "something like 30º"
Even in Calgary, I have never had "frost" inside my walls. Some time so I hope to set up some instrumentation to document the heat transfer through ICF but it is clear that it is not well understood.

You would need a steady state interior and exterior temperature to have an average mid point.

There is a lot more shoulder season time working on the walls then there is design temperature (either hot or cold) so most all the time a 68º basement would be giving off heat to the 50º footing.

From a practical purpose, totally encapsulating the concrete in foam will give you the best behaviour.

My example is on the extreme side, but some winter climates have a long enough period of low enough temps that I'm sure you have a continual, albeit fluctuating, flow of heat to the outside. During the shoulder season there probably would be very little heat moving in either direction. I am referring to an ICF foundation wall, not a bare concrete foundation wall.

All I was really trying to do is drive home the point that a close look at the reality of the issue is needed, that it is not a straightforward case of insulation is always worthwhile. The benefit and payback of insulation underneath a deep footing may be similar in benefit to 12" of exterior foam vs. 6" of exterior foam. I put 3/4" blue foam (I think it was 3/4") under the rat slab in my crawl space which is my supply plenum for heating and cooling. My best estimate was 4", or even 2", would have been a waste of money because the lower heat loss would be many, many, years in providing a payback.
 
To be clear, a footing that is only 2' below grade more likely would benefit from insulation underneath it, both in winter in a cold climate and summer in a hot climate.