Siding And Insulation

Jaketoo

I have a customer who wants new James Hardie Siding, new roof, and new windows. He has electric heat, and wants the walls insulated very well. I am thinking of using 2" Rigid Polyiso Foam Board (12.8 R-Value) over existing sheathing. My first thought was to pad out over each stud with a 2x rip and a strip of 1/2" plywood.
After looking at the Hardie website, I am now thinking about installing the foam in full sheets vs cutting foam to fit between nailers.
From what I understand, after looking at the Hardie website, all you need is 7/16" for nailing. I am wondering if I should install foam in full sheets, tape all the joints with some Tyvek Tape, install 1/2" CDX or OSB board over that with 16d nails, install Tyvek, and nail the Siding to the plywood.
Any suggestions?

Jaketoo

The more I think about it, I will use 4 1/2" screws to fasten sheathing.

Meetre

longer fasteners through the foam, full sheets of foam, seal the seams. If it's got sheathing, no need to add extra.

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Jaketoo

The Hardie website says you can go over up to 1" of foam.

Chris Johnson

Look at a product called 'Ceiling Technology' made by a company called 'Nudura' www.nudura.com

Forget the name 'Ceiling' and look at applying it to the wall, it comes in 2 1/2 and 3 1/2" thichness and has 1x built in, I am wondering if you could apply it and fasten to the studs through the 1x and attach the siding to the 1x once installed. I know this would work for vertical siding, for horizontal you would probably need to strap it out.

Now that I think about it, you could probably make it yourself with a bit of patience.

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maceycon

Left me speechless. 2" insulation over sheathing. Good luck with that. I tried to bite my tongue.

twilson

I have worked on several houses where we have used 1.5 " sm styrofoam on the exterior of the house. We did not tape or seal the joints because we were advised not to. This was because where we live the vapour barror is on the inside of the 2*6 frame. The sm sytrofoam acts as a vopour barrier when sealed. this would lead to moisture being trapped in the walls resulting in rotted walls. The exterior finish on the wall was stucco and vinal siding.

Jaketoo

Wow, that's a good point. I'm sure these 2x4 walls have a vapor barrier on the inside. Probably Kraft Face R-11. I was planning on using the double foil face foam because that's how you get the best R-Value. I will have to give this some more thought.

sawyerEd

Don't assume vapor barrier is installed correctly when you are adding more insulation. Check all service entrances plumbing, heating, electrical boxes and upgrade the seals. Upgrade the vapor barrier if it is at all questionable.
2 inches of styrofoam seems excessive for Massachusetts unless there is no insulation in the existing wall. You need to check this out. Do not seal joints in the foam board and trap moisture in the wall.

maceycon

Ma Building code. 5314.2.5 Siding backer board Foam Plastic board of not more than 1/2-inch maybe used as a siding backer board when separated from interior space of not more less than 2 inches of insulation or installed over existing exterior wall finish.

Jaketoo

I never thought to check the code book. It is also listed in the One And Two Family section 3603.17.2.5
I will have to check with the building inspector. I wonder if there is any chance he will say it's o.k. with the James Hardie Siding, because the manufacturer says it's o.k. ? I guess it's worth asking.
From what I understand after looking around, the foil face (Polyiso Foam Board) provides a vapor barrier, but the Extruding Polystrene (Dow blue stuff) does not.
I was going to use foil face, because the R-Value is better. But I read that the foil face does not really give a much better R-Value when used under siding, and we don't want a second vapor barrier. There is an existing foil vapor barrier on the inside of these walls. So it looks like the T&G 2x8 blue stuff (no tape on seams) may be a good way to go.

maceycon

I am sure this is in place because of fire spread or smoke ratings. The building inspector may not even know. When I read your post the first thing that stood out to me not even considering the codes. I know what a pain it can be to build the house out with 3/8 insulations. Matching up to the rakes, fascia and so on. The windows and doors are going to appear sunk in quit a bit with 2 in insulation. I think it is going to look ridiculous. I don't know how your gable ends are if there isn't an over hang. You are going to have your siding sticking way out beyond your rake return etc. Same as your corners. You are going to have to cap everything. I personally would show the home owner the building code and use 1/2. I think the finished product is going to come out a lot nicer. But what do I know.

Jaketoo

I asked the customer to call the building inspector and ask him what he recommends. I would not ask most homeowners to do that, but I know this guy will be happy to do it.
The job is only 1 small 1 story wing of the house. It's getting new windows so I would build out the framing around the window openings. It's also getting a new roof. It's only got 1 small gable (16' wide building with 5/12 pitch) so padding out the rake is no big deal.
If this was a bigger job, I would agree 2" sounds a little crazy. The homeowner said he already got an estimate for 2" insulation, and asked me to do the same. He said the electric heat in this addition to the house is very expensive. Getting the best R-Value under the siding is very important to him.