installing fiber cement siding over rigid foam sheathing

[albert14]

I am planning to install fiber cement siding over rigid foam sheathing. This is the plan: 2x4 wall, 1/2 OSB sheathing, 2" XPS sheathing, housewrap, and fiber cement siding. The question that I have and am trying to research an answer is, Is it necessary to use vertical furring over the 2" XPS before installing fiber cement siding or can the siding be installed directly over the XPS? The siding will be primed and factory finished on all sides. Thanks.

[Mark G]

Re: installing fiber cement siding over rigid foam sheathing

On my own home I have shot the hardi plank right through 1 1/2" foam into the framing w/o any furring and using 16d hot dipped galvy's. I was careful to mark stud location and nail into them so the nails are solid. I used Iso. foam and taped the seams in my warm dry climate.

This has been in place a couple of years now and is holding up fine.

[cdservices]

Re: installing fiber cement siding over rigid foam sheathing

Think you'd definitely want to furr it out before the siding. Much easier to nail siding into furring than going several inches back to sheathing. Plus, it's a better detail and you're supported directly by studs.

[Dancing Dan]

Re: installing fiber cement siding over rigid foam sheathing

And you get a rainscreen.

[Darrel Tenter]

Re: installing fiber cement siding over rigid foam sheathing

The air gap would also provide a slight R-value increase.

[Bob Kovacs]

Re: installing fiber cement siding over rigid foam sheathing

I'm wondering why you want housewrap over the foam?

[albert14]

Re: installing fiber cement siding over rigid foam sheathing

I want to tie the windows to the housewrap and I want the windows to be located on the exterior of the foam. Some say that XPS with sealed seams can act as a drainage plain where others say that a WRB is still needed if XPS is used. My alternative I guess is to use polyiso with sealed joints.

[ecoman713]

Re: installing fiber cement siding over rigid foam sheathing

I have completed several projects that use foil-faced polyiso as the exterior rigid insulation board. I install 1x furring strips vertically over the foam to support fiber cement lap siding. The furring strips are attached to the studs with 4" screws at 6" to 8" o.c. I do not use the foam as the drainage plane however. The foam is installed over a continuous rubber membrane which is installed over a 1/2" CDX plywood sheathing.

As mentioned the air space increases the r-value slightly, the foil facing gives me a radiant barrier (a benefit here in Austin) and a ventilated siding will maintain it's finish over time better than an unventilated siding.

Good luck with the project.

[BuildinginB'ham]

Re: installing fiber cement siding over rigid foam sheathing

I have completed several projects that use foil-faced polyiso as the exterior rigid insulation board. I install 1x furring strips vertically over the foam to support fiber cement lap siding. The furring strips are attached to the studs with 4" screws at 6" to 8" o.c. I do not use the foam as the drainage plane however. The foam is installed over a continuous rubber membrane which is installed over a 1/2" CDX plywood sheathing.

As mentioned the air space increases the r-value slightly, the foil facing gives me a radiant barrier (a benefit here in Austin) and a ventilated siding will maintain it's finish over time better than an unventilated siding.

Good luck with the project.

Is the rubber membrane a vapor barrier? I would assume that it is. Are you insulating the stud bays with fiberglass? I suppose in your climate it is better to have the vapor barrier on the outside because of the summer cooling loads.

[Wallner-Builders]

Re: installing fiber cement siding over rigid foam sheathing

EcoMan:
How thick is the foam board you've installed the siding over? 1" 2" ?

Based on others' research, I believe you are doing the right thing with your membrane. In the north, we're actually supposed to do the same -- assuming no wall insulation (we're remodelers -- deal with that all the time.) It gets dicier when there is wall insulation -- dew point calculations are critical.

[NW Architect]

Re: installing fiber cement siding over rigid foam sheathing

You can direct install over 2" foam but you run into nail gun issues. You either have to use a framing nailer of hand nail the siding. If you have the budget for the 1x furring, that is the better way to go

[clancrawford55]

Re: installing fiber cement siding over rigid foam sheathing

If you would like to skip the furring why not use a product like HomeSlicker to give yourself an air space?

[sailboatescape]

Re: installing fiber cement siding over rigid foam sheathing

Hardi install instructions will tell you 1" maximum thickness of foam that you can install over. Think of your nails as cantilevers since the foam does not support the side of a nail as the weight of the siding pulls down on it and imagine how the load increases as the cantilever gets longer. Inspector can turn you down when you don't follow the manufactures instructions.

House wrap over foam is considered by some because foam has been found to shrink over time and the joints open up.

[dgbldr]

Re: installing fiber cement siding over rigid foam sheathing

On my own home I have shot the hardi plank right through 1 1/2" foam into the framing w/o any furring and using 16d hot dipped galvy's. .

Those nails seem a bit heavy for Hardie and might cause some damage to the boards as you shoot them with a framer :)

But Mark is correct in that you need at least a 3-1/2" nail, and a fairly thick one at that, if you're hanging heavy boards cantilevered 2.25" away from the framing.

[Mark G]

Re: installing fiber cement siding over rigid foam sheathing

It's been 3 years now and everything looks and feels tight. Don't know if I would sell this to a customer but I like it. Your results may vary.
I used Trex for corner and window trim. Very little wood on the outside to rot.
Some day I'll show you my steel framed deck.