Vinyl siding over styrofoam - OK?


Old 02-12-01, 09:10 AM
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I'm having an addition put on my house and need to decide how to have the exterior walls built. There are considerable differences between contractors: one is suggesting styrofoam over plywood, one is suggesting plywood at the corners for bracing with styrofoam in between.

One contractor is suggesting plywood over the whole structure with no foam. He says that vinyl manufacturers do not recommend putting vinyl over styrofoam since it can cause the siding to warp.

Has anyone heard of this? My preference would be for both plywood and vinyl on the entire structure for both strength and insulation.

When I resided the existing house, I put Tyvek and 1" Dowguard under vinyl siding and have had no problems.

Can anyone confirm if it is bad to put vinyl over styrofoam?

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Old 02-12-01, 09:45 PM
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If it were my house, I would put OSB over the studs, then wrap it with Tyvek. I would then use vinyl siding with Thermo-Wall insulation behind it. This is EPS that is cut to match the contour of the siding, and can also be installed in the corner posts to make them "solid".

'Styrofoam can cause the siding to warp'?? Never heard that one!
Old 02-13-01, 03:41 AM
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Lefty has told you correct. Listen to him. We will not steer you wrong.
Old 02-13-01, 07:30 AM
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I have seen vinyl siding where each piece has EPS already attached to it, matching the contour of the siding. Is this what you are talking about, or does Thermo-Wall come in large sheets with the exterior side molded to match a particular siding profile, e.g., 4-inch Boardlap?

I checked Certainteed's web site and the siding that I used on the rest of the house is not available with EPS inside it. Naturally, I want to match that siding so choosing siding with EPS is not an option.

So, if Thermo-Wall is something that comes pre-attached to the siding I will need another option. Any thoughts?

Old 02-13-01, 10:59 AM
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Which Certainteed siding do you have? Mainstreet? Monogram? Is it a 5" dutchlap, or one of the other shapes they offer? The ThermoWall insulation is seperate from the siding. We installed a different brand of siding once that has the foam glued directly to the siding. Went back six months later and took it off. The foam, being glued to the siding, didn't allow the siding to expand and contract properly, and the homeowner wasn't happy.

E-mail me directly (just click on the 'mail' icon in the bottom of this post) and I will get you some info on the ThermoWall, and where you can get it. Pretty sure if your siding installer (or source) can get Certainteed, they can get the ThermoWall for you.
Old 02-14-01, 11:41 AM
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I contacted CertainTeed about installing over Styrofoam and warping and this is what a Sales Support Rep emailed me:

"We do not recommend against using Styrofoam sheathing underneath our vinyl siding. We do not specify what type of sheathing to use under the siding, as long as it provides a smooth rigid surface, and is no more than 1" thick. Products often used as sheathing include plywood, OSB board, rigid foam, and fiber sheathing. We have no information about Styrofoam specifically causing our siding to warp.
One thing we do specify against using are contoured foam drop in backers. These products can restrict the expansion and contraction of the siding and cause the siding to warp. These products should not be confused with foam sheathing."

When the contractor told me about warping, I assume this is what he was talking about.

When I told him that this forum has recommended ThermoWall, he responded:

"Yes, Thermo-wall is a drop in backer. We do not recommend these products being used with our siding.
Thermo-wall however is an exeption. It has been tested with our siding and has been approved for use with our products. In fact, it is listed in our installation manual as the only drop in backer that can be used. The product is manufactured by Progressive Foam Products. They have several sizes that are contoured to fit various siding profiles."

So, there you have it. Vinyl over drop in backers can cause the warping that the contractor was referring to, and ThermoWall is good stuff, like lefty said.
And, like Jack said, you didn't steer me wrong.
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