Hardie over foamboard with rain screen

Old 08-23-08, 05:52 PM
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Hardie over foamboard with rain screen

Hello. I've been pouring over these forums for a while now and have been very impressed by the knowledge and with the level of expertise that is so generously offered here.

I have a few questions regarding the installation of 7-1/4" HardiePlank over foamboard and the incorporation of a rain screen (properly, a drainage plane).

My intent is to remove the existing siding from my house, cover the exterior walls with 1" of polyisocyanurate foamboard, followed by a housewrap.

I'd also like to incorporate a rain screen/drainage plane into the installation and it looks like there are a couple of ways to accomplish this, namely by either using a manufactured system such as Obdyke's Home Slicker, or installing vertical furring strips to create a vented airspace behind the siding.

I'm leaning toward the second option. I've spoken to a local contractor who has an excellent reputation, and this is the method that he uses when installing siding over foamboard. He believes that the furring strips provide a better base for the siding than the foamboard and helps to eliminate the waviness that can occur when installing siding over foamboard. The resulting rain screen/drainage plane that's created is, in his opinion, is a fortunate bonus.

Sounds good so far...until you actually read the Hardie installation instructions. They allow for installation of Hardie products over foamboard up to 1'' in thickness, as long as you use a nail of appropriate length to achieve the required 1" of stud penetration.

But what about installing Hardie over 1" of foamboard and a 1/2" thick furring strip? The contractor that I spoke with is pretty firm in his belief that by installing a well nailed furring strip, then using a 3-1/2" siding nail to attach the siding (through the furring, the foam, exterior sheathing and 1"+ into the studs), you are actually getting a stronger (and better) job than simply nailing the siding directly over the applied foamboard.

I've already asked Hardie for a ruling. After a bunch of unanswered emails and unreturned phone calls, I've decided to broaden my search for opinions. (Not that I'll stop trying Hardie. If anything, I'm persistent.)

Really, any input would be greatly appreciated, and thank you for taking the time to read this.

Old 08-25-08, 05:42 AM
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As far as Hardie goes, whatever warranty there is may be null and void if not installed according to their specs.
Your contractor with the excellent reputation, ask for references on jobs he has completed using this method. Talk to the people who's homes he has used this method. Go look at them. See for yourself. Then, if your happy with all this, by all means..... go ahead with your project.
I'm of the opinion that the people who develop these products know exactly that. The people who install them, through trial and error, end up finding the best methods for their applications. Good luck!
Old 09-28-08, 07:26 PM
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Siding over Foam

I’ve also been reading up on siding over foam insulation. And I saw where a LEED built home used 4” of foam insulation over the exterior walls. They used plywood strips lag bolted to the studs. Then they sided to the plywood strip they lag bolted to the studs. This made Rain Screen, which allowed them to use wood over the foam insulation.

The thing is now weight of your siding on the foam. It’s going to give some over time if it’s too heavy. So as I see it we’re only talking vinyl or cedar siding at max.

I don’t like vinyl all that much. And with wood I worry about the stud layout. (If I go with wood I like a see threw finish.)

As for Hardie they don’t really want to way in on any How To’s over foam. The foam companies don’t want to talk about siding over the thick stuff unless it vinyl. I know your talking only 1” of foam.

Hey the HardiePlanks you’re looking at maybe much weight for 1” thick foam. Trust me I gave it some thought myself.

I like to go out 2” for the insulation factor and to hide my foundation that 2” out. My father built the house. And to make the house look shorter he played with a foundation cap. Now all the exterior walls are 2” inside of the edge of the foundation. Bug me from birth.

I’m talking 1” to 1.5” of ridge foam panels, lag bolted 1” plywood strip (1" to sink the bolt heads in), and then siding. Oh yea, tarpaper at lease over the plywood and before the siding. And a vent screen top and bottom.

The lag bolt idea has some merit. The bolts would care the weight factor better then nails. And I don’t like the idea of 100s of nail holes in my insulation. Especially when I’m counting on the foam as a water barrier.

Feed back please, I’m not proud. I’d like this one shot deal at this.
Old 09-30-08, 06:24 AM
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tyvek over foam insulation?
1)usually you put tyvek over the sheathing then the insulation over the tyvek.
2) usually when using hardi plank you only use tyvek,no insulation boardyou DEFINITELY need the surface to be flat but it can be shimmed if needed. i am confused about this guys methods.
Old 10-01-08, 09:15 PM
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Foam Panel Insulation


For those of you can see the need for the insulation on the out side of the building frame. Its like this, my last oil fill up was $475 for one of my 4 up this year. Now add the 5 tons of pellets that Iíll burn this winter. I think the 2x4 wall with the 1956 Kimsol insulation need some help. Yet Iím going to replace the oil coated paper towel back with tin-foil that they called Kimsol Insulation in my walls soon with better.

Even with new windows the walls need an improvement. Maybe if I wanted to go around and up grade to 2x6 studs in all my out sidewalls. I could get more fiberglass insulation to rises the R-value. But wood doesnít do well in the R-value. So by covering the house frame with solid foam insulation help keep the framing a more even temp. This reduces the hot cold cycles on the framing members.

The rain screen help hold the paint on the non-vinyl siding some of us love. The wood siding donít like foam panel insulation or Iíve even seen a problem with normal tyvek cover house. The tyvek cover house was repaint over and over like every 2 years. An engineer was call to find that capillary action pull water in and wash the oil off the paper you call tyvek. Capillary action is like what pulls blood into blood tester machine. The lap on the siding works much the same way. Siding got wet and didnít dry out. Plywood rotted and the engineer call for rain screen to fix it.

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