I want to cover a CMU building with EPS foam, then attach Cedar Shakes to the foam. I'd like some input, to be sure I am covering my bases. So far all I can think of is to attach wood 1x2 furring strips to the foam and then staple cedar shakes to the furring strips.

I have looked at a couple of retrofit foam systems made by ICF companies, and at just buying sheets of EPS. At this point, in my education, I am leaning toward the sheets - not the retrofit.

I'd like feedback from anybody familiar with attaching sheets of foam to CMU and then attaching shingles to the foam.

Here's what I've come up with so far:
1. Glue & screw a metal drip edge to bottom of the CMU wall to rest the foam panels on. This might also help as a barrier against carpenter ants.
2. Glue the foam sheets to the CMU with either Green Glue (noise countering and expensive), Construction Adhesive (way cheaper than Green Glue), or Polyurethane Glue (way cheaper than Green Glue). I don't know if Green Glue is really worth the cost.
3. Tape all seams with Dow foam board tape, or an equivalent tape.
4. Glue the furring strips to the foam and Tapcon screw the furring strips through the foam and into the CMU.
5. Staple the cedar shingles to the 1x2 furring strips.

Now the pro's & con's:
a. I don't like the Tapcon screws going into the CMU as they will carry heat/cold/sound. Is there a better way to attach the furring strips? I would love to just glue the furring strips to the foam, but I have doubts about the strength of the foam holding the overall weight of the rows of furring strips & shingles.
b. What about carpenter ants? Keeping the ants out of the foam, on both the ICF portion of the house (my new addition) and on the retrofitted portion (the existing CMU building) seems like a future problem waiting to happen. Any suggestions about "easy" ant control? I read that a copper drip edge (that would hold the foam panels at the bottom) is a deterrent against ants, but have not verified that.

Thanks for any & all comments.

carpenter ants only eat wet wood. keep it dry and no ants

To get value out of Green Glue, I would use it only exactly as they specify. For example, between two sheets of drywall (there may be others ways too, but don't guess when it comes to sound transmission).

A few screws won't have a big effect on heat loss although you could use stainless steel to reduce it by almost 2/3.

3" wide strips will increase the weight the foam can support by 50%. What is the worst case weight per square foot of the attached shingles?

Posted By smartwall on 30 Jul 2013 10:45 AM
carpenter ants only eat wet wood. keep it dry and no ants

Smartwall,
I meant the problem with carpenter ants eating away at the foam.

Posted By jonr on 30 Jul 2013 11:37 AM

3" wide strips will increase the weight the foam can support by 50%. What is the worst case weight per square foot of the attached shingles?

Jon,
Average dry weight per sq foot: 1.5 - 2.0 lbs. Average wet weight per sq foot: 3.0 - 4.0 lbs.

Say you use a very conservative 5 psi shear strength to account for long term creep. A 1x12 strip of wood fully glued to foam should hold up 60 pounds. So check with a mechanical engineer, but it looks to me like it would be more than 10x stronger than needed to support the shingles. More if you use 3" plywood strips and a few screws.

Posted By jonr on 30 Jul 2013 02:41 PM
Say you use a very conservative 5 psi shear strength to account for long term creep. A 1x12 strip of wood fully glued to foam should hold up 60 pounds. So check with a mechanical engineer, but it looks to me like it would be more than 10x stronger than needed to support the shingles. More if you use 3" plywood strips and a few screws.

Are you saying that you think the furring strips could be glued to the foam only - with no screws to the wall, and then shingles stapled to the furring strips?

You'll definitely be needing to TapCon the furring to the CMU- glue alone won't cut it.

See my comments on your other thread about weather resistant barriers & window flashing issues when retrofitting foam to CMU. The flashing needs to be continuous with the drain-plane, and in a retrofit that will often be the exterior surface of the CMU, in which case you'd need at least a micro-gap between the foam & CMU for drainage.

Are you saying that you think the furring strips could be glued to the foam only - with no screws to the wall, and then shingles stapled to the furring strips?

Let's say that some quick figures indicate that it's worth investigating. But I'd use some screws too - if nothing else, you need to hold the foam and strip tightly in place while the glue dries.

I suggest doing a small test (let us know how it goes).

Posted By Roger R on 29 Jul 2013 10:37 PM

b. What about carpenter ants? Keeping the ants out of the foam, on both the ICF portion of the house (my new addition) and on the retrofitted portion (the existing CMU building) seems like a future problem waiting to happen. Any suggestions about "easy" ant control? I read that a copper drip edge (that would hold the foam panels at the bottom) is a deterrent against ants, but have not verified that.

Thanks for any & all comments.

Posted By smartwall on 30 Jul 2013 10:45 AM
carpenter ants only eat wet wood. keep it dry and no ants

Carpenter ants do not eat wood.  They dig out galleries in wood.  Carpenter ants are also very happy to dig out galleries in sound wood.  (Dampwood termites will only attack wood.)


Carpenter ants and velvety tree ants both like to dig out galleries in foam insulation.  In the California Sierras velvety tree ants are very commonly found digging out galleries in foam insulation.

Carpenter ants and velvety tree ant control is a bit different than many/most urban structural ants.   Baits are one of the best long term products for carpenter ants, though for the long term - an ongoing service contract with a good local pest management company is probably a good bet.