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billybarty 04-15-2008 12:29 PM

Sheathing with foamboard under siding
I have a place that is 4" tongue and groove cedar siding and is just 2x4 outside wall construction. I want to add foamboard insulation on the outside for added insulation as well as a more uniform surface for the siding. Would I be better off putting OSB over the existing siding to get a good surface and then add the foamboard? And if so can I use adhesive to attach the foam to the OSB? No matter what surface I add over the siding there will still be a little bit of a void between the old siding and OSB/foam, does this matter? And I use felt over the foam under the siding, correct, not house wrap?

Termite 04-15-2008 03:47 PM

What type of siding is on there now? If you're going over the top of it, good luck making it look good or help you home's value.

The OSB or plywood (often in the form of siding) on your house is often needed to provide wind load and rack resistance to the walls. It works by attaching to each stud, as well as the top and bottom plates. I always frown on people that don't take that into consideration. The amount of load a stiff breeze puts against the side wall of a home is staggering, and even more impressive if you have a two story. There are ways to rack brace the walls with X-bracing, but nothing beats OSB sheathing.

The foamboard should be applied with cap nails, not adhesive. Cap nails have 1" (more or less) caps made of plastic, and are typically ring shank nails.

You're not going to gain much R value at all by adding that foam. It won't make much of a difference and isn't worth what you're going to spend.

Ideally, you'll fasten your siding to the OSB directly. I'd suggest either felt paper or a membrane such as Tyvek direcly under the siding.

billybarty 04-16-2008 08:49 AM

It's 4" tongue and groove cedar siding and I don't plan on removing the old siding just adding OSB sheeting and insulation or just the insulation so I don't see how I would be decreasing the rigidity of the place, I would think I would be adding to the strength of the structure. And would I not be adding R7 per inch of styrofoam that I put on?

Maintenance 6 04-16-2008 03:54 PM

Assuming your cedar siding is a lap style, you'll need to add something to flatten the surface. Foam board alone will do that just fine. The house is standing OK without the OSB added. Why add the extra thickness, cost and weight? When you add the foam board, you'll need to build out your window and door casings as well. Remember to use nails long enough to reach through everything. You probably want to find the studs and chalk lines vertically so you nail into them. I say that because Murphy's law says that your siding nails will fall where the wood siding is thinnest and they won't hold. Don't nail the siding tight. You'll crush the foam board into the spaces under it. Siding just needs to "hang" on the nails anyway. Good Luck!!!

billybarty 04-16-2008 06:00 PM

Thanks for the advice

billybarty 04-18-2008 10:02 AM

So now I am wondering how it will work if I put in new windows when I do the insulation. Can I install the new windows with the new foam insulation underneath it and nothing else? What I am wondering about is the length and type of nail/screw I would use. I can use a long nail to go through the insulation to secure it but can I also use a long nail to go through the window frame, foam and into the window framing. I need new windows and I thought it would be easier to frame out the added thickness inside the house instead of outside

Maintenance 6 04-18-2008 12:46 PM

I agree. You can extend the jambs on the inside, if you can't buy the windows for your wall thickness. It's common to lay the nailing flanges of the window over top of the foam board and nail through to the framing. Lay a bead of silicone sealant between the nailing flange and the foam board and make sure your windows are plumb before you nail them. It's best to dry fit the window and tack any shims in place before you permanently install the window, unless you've got a helper who can hold it in place while you shim from the inside.

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