I'm an amateur builder working on my dream home, an off-the-grid medieval castle. Yes, strange but true. You can see pics of the project here: http://www.keenecastle.com Been working on this for 20 years now and I'll be doing the first-floor ICF walls this summer.



My question is about thermal bridging. I'm doing the entire castle in ICFs for energy efficiency. But I'm building on top of solid granite. I fear that the heat is going to be sucked out of the walls into the granite in the winter. Surely where must be a way to slow down thermal bridging.

I'm considering using a 2 part 8 LB Density Polyurethane Expanding Liquid Foam that will create a 1/4" thermal break between the foundation and the first floor walls. When this stuff sets up it is rock hard but full of tiny bubbles and works well as an insulator. (See: https://www.amazon.com/gp/product/B007V23Y94/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1)

Is there any structural issue of not having the concrete of the first floor touch the concrete of the foundation? The rebar is pretty beefy per the structural engineer, #8 rebar, 9 inch on center. The rebar pegs go 16" into the granite. So I can't see this shifting around under any circumstances. And we are not prone to earthquakes in NH.

This one is out of my league, I don't build in those conditions. But good luck sounds like a fun project.

I don't think the wall is going to be transferring much heat down through the foundation wall to the granite. What's the situation for the basement? It looks like there are some crawlspace floors that are bare granite. Will the floor between the crawlspace and first floor be insulated?

In one place, the first-floor wall is only 8" off the granite. We will definitely see some heat loss in that corner. The basement crawl space is all exposed granite. The first floor will consist of 3/4" sub floor, Nudura HYDROFOAM (4" of EPS) the 3" of concrete for radiant heating.

I would ask your structural engineer about placing anything between the foundation and the ICF walls.

Given the rebar dowel size/schedule, I would expect significant heat transfer into the granite in any event. You might want to determine the lowest temp of the granite so as to better quantify this heat loss to see if it is a real problem. If it does create a problem, increasing the wall interior side R-value with more insulation would likely be best solution.

We love the Nudura 4” R16 Hydrofoam and we even used it in our home when the 2” would have been more than adequate (photos on the About Us tab of our website). If it were me, I would eliminate the crawl space, place level self-compacting pea gravel on top of granite, then vapor barrier, then Hydrofoam, and then reinforced concrete floor pad.

We also built an ICF single tower castle home for a client several years too, but it was not built on solid granite... We had to build a jig to cut and glue the block back then so as to create the radius. Our own home has a 24" thick interior castle wall (finished in stone on one side and brick on the other) and a masonry heater (finished in stone) that provides our passive cooling negating any need for AC. There's nothing better than a castle feel to a home...

Good stuff and good luck!

Yes. The granite's going to act as a big heat sink and incorporate your walls into it (possibly causing condensation problems) if you have direct contact).
If you're going all concrete, you may still stay below 20 PSI for your walls. EPS can be engineered with a compressive strength of 10-60 PSI, making it more stable than some compacted fill types.
With XPS, you can find it in ratings up to 100 PSI. Talk with an architect/structural engineer to get their input. If you need extra strength 2-3 sheets of Foamular 400/600 will run you about $100-150.

Going this way would, unfortunately, require some digging/chipping/etc to flatten the top of the granite interface.

I wouldn't know about spray foam. You'd have to talk with the company doing the work or the company selling the service to get compressive strength ratings out of them. 2 pound may or may not be strong enough to handle the downward pressure of the wall and you might need to go to a heavier product.

If I'm reading this right , why not increase the insulation on the inside walls ?

Posted By smartwall on 07 Apr 2020 01:05 PM
If I'm reading this right , why not increase the insulation on the inside walls ?

Because the concrete wall core is in direct contact with the granite outcropping.
You could pack on 3 feet of R5/inch , and you'd STILL have a problem requiring a thermal break and still have problems with condensation on the interior of the wall.

No you would not. As you add insulation you move the dew point in. Ever heard of a vapor barrier? Tends to keep moisture out of a wall assembly.

I agree with Smartwall...and I suggested adding additional interior wall insulation as needed previously. I would think that just placing vapor barrier behind the interior wall drywall and designing the assembly to dry to the exterior side should address this issue. However, I would first want to know the granite temp profile and perform the Glasser dew point analysis to determine where in the assembly and how much moisture will be generated.

https://www.borstengineeringconstruction.com/Building_Assembly_Moisture_Analysis_Calculator.html

I feel good . I have an engineer on my side. Thank You. Actually his own statement makes him out to be a fool. The only way to get to an R-5 per inch would be to use to use extruded, which has a really low perm rating. I've done some work in NH and by the title, Keene is in the south of the state. I don't know how far along he is since he is talking about isolating the foundation from the first floor. The no.8 rebar with the 9" schedule will still be a problem. Would not use poly between foundation and 1st floor. The stuff soaks up water like a sponge, which would cause deterioration over time. I wonder if he has access to 4" icf. less concrete and if you add 2" of extruded foam which would add an R-10 to the wall, without increasing the depth of the wall. Even switching to a different system would be smart, if possible. I use an old clunky system that right now I'm putting together wall sections in my warehouse with 2- 3.25" thick panels of reclaimed Iso board.

Well Smartwall, you have provided me and others with so much great ICF advice over the years, it’s the least I can do... Yeah, if it were me, I think I would have built an engineered pad over all that irregular granite to minimize the granite heat transfer into the foundation/walls and create a nice level surface of consistent loading on which to build. As we all know, it is NOT settling that causes foundations to fail, it is uneven settling... I also think a good structural engineer would have fully considered and easily resolved all this...20 years ago.