I like the idea of external foam; 1" is good, 2" is better. But I have a question about hanging siding on the outside of it. As I see it, the foam is held in place by vertical furring strips, screwed to the studs. The furring strips in turn are used to hang the siding on, creating a drainage plane at the same time. My concern is the effect of the weight of the siding, fiber cement in particular. What keeps the weight of the siding from dragging the furring strips downward? Is the friction against the foam, coupled with the little bit of shear resistance of the fasteners enough? It would seem to me that the whole thing could sag by dragging the screws downward through the foam over time. Is there anything else used to support the wall, or has it been proven to stay put over time?

Excellent question and one I meant to ask, thanks for bring it up since I am hoping to use Hardie board over 4" of exterior foam.

In my case I am thinking of two 2" layers of foam. I would need, a 5" screw (3/8" furring strip, 4" of foam, and a reasonable penetration into the OSB or plywood sheathing).

That seems like a lot of sheer force on the screw!

Perhaps the furring strip could be glued onto the foam. If so with what?

and... if the exterior foam is supported by screws how many would be adequete? Would a 4" ledger board work to support the foam as if it was brick?

When we put the foam board around my slab, (not the same situation as it did not have to hold the weight of siding) we used tapcons - long screws with a large round washer that distributed the force of the grip. Various companies make foamboard glue that you should be able to use to glue furring strips. I think any acrylic based glue will work with foam without dissolving it. For the rest of your questions you need someone with lots more experience than I have.
-Rosalinda

Posted By kenora on 13 Feb 2011 09:28 PM
Excellent question and one I meant to ask, thanks for bring it up since I am hoping to use Hardie board over 4" of exterior foam.

In my case I am thinking of two 2" layers of foam. I would need, a 5" screw (3/8" furring strip, 4" of foam, and a reasonable penetration into the OSB or plywood sheathing).

That seems like a lot of sheer force on the screw!

Perhaps the furring strip could be glued onto the foam. If so with what?

and... if the exterior foam is supported by screws how many would be adequete? Would a 4" ledger board work to support the foam as if it was brick?

Blobs of foam-board adhesive between the first course of foam and the structural sheathing, and between successive courses of foam are usually a necessary aid to installation, but the compression of the furring against the foam is sufficient to hold the furring.  The foam isn't structural, and can't be relied upon to handle the shear load of the weight of the siding.  It's the furring & screws that hold the siding, not the foam.

The screws can handle the moment arm to support 4" of foam + 3/4" furring without any problem- it's the heft of the cement-board siding more than 4-3/4" out that becomes the issue.  With heavier cladding and thick foam bracketing or blocking to support the furring  (not the foam) may be required.  A full width ledger adds little if any strength over individual brackets, but represents a much larger thermal short bridging the foam layer, degrading thermal performance.

I understand exterior foam on concrete foundation, but not exterior foam over wood sheathing or old siding - Isn't there a significant long term likelihood of moisture-related problems inside the structure?

Posted By koberle on 07 Mar 2011 10:46 AM
I understand exterior foam on concrete foundation, but not exterior foam over wood sheathing or old siding - Isn't there a significant long term likelihood of moisture-related problems inside the structure?

Not if you do the simple math on it for your climate, and use the correct perm rating foam for the application.  The permeabilty of 2" of XPS or most closed cell spray polyurethane is still >>0.5, and 2" of unfaced EPS or 2lb Icynene is >2 perms.  The sheathing can still dry through the foam toward the exterior, but can't rapidly take up moisture from the exterior during those high vapor-drive events when the sun hits dew or rain wetted siding/masonry.  If you also allow a vented air gap between foam & siding, and use only moderate to high-perm weather resistant barriers,  the capacity of the assembly to dry toward the exterior is further enhanced as compared to siding applied tight to the housewrap or felt.

Also (and quite importantly) by insulating the sheathing at least partially from the exterior it stays warmer in winter, spending many fewer hours above the dew point of the conditioned space interior air, making it more resiliant to air leaks or vapor diffusion from the interior side.  Done right exterior foam is the SOLUTION for moisture-related problems inside the structure, not the cause.

In a cool or mixed climate, if the ratio of R value of the foam and center-cavity R value of the cavity fill is such that the mean temp at the sheathing is 37-40F or above,  the sheathing is almost completely resiliant to normal wintertime indoor humidity & temp levels.  37-40F is the dew point range of 30-35% RH, 68-70F air, so even in the event of air leakage from the interior that condenses on the sheathing, there are more drying hours than condensing hours during the winter, and the moisture content of the sheathing over a winter is well bounded. With that type of R-ratio you can skip the interior vapor retarder, which massively improves the drying capacity of the assembly toward the interior, making it far more resiliant to bulk water intrusions than an assembly that can only dry toward the exterior. 

Water can get in by any number of means, but it gets out primarily by vapor diffusion.  Exterior foam allows you to build with a higher-perm interior, increasing the drying rate.