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Old 01-01-2014, 08:04 PM   #1
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2" foamboard insulation before siding


Hi, I've been looking at doing lp smartside for my project, none of it I'm doing myself. My contractor told me it's not advisable to use 2" foamboard insulation on the exterior of the wall because it doesn't allow the siding to hang as well. He said it's better to limit it to 1".

Any siding guys here have an opinion on this? Does it matter how old the house is? I may have to inspect the sheathing for mold and rot when I get the old siding off. I'm not sure if the windows and doors were flashed at all.

Thanks for your time.

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Old 01-01-2014, 08:10 PM   #2
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2" foamboard insulation before siding


if your trying to nail thru 2'' of foam then into the house your contractor is correct imo

i don't think i would advise 2'' of exterior foam without changing all the windows and doors out

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Old 01-01-2014, 08:12 PM   #3
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2" foamboard insulation before siding


Sorry I forgot to mention that I will be changing most of the windows out along with my front door.
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Old 01-01-2014, 08:29 PM   #4
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2" foamboard insulation before siding


visit the LP website for the details you need
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Old 01-02-2014, 12:14 AM   #5
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2" foamboard insulation before siding


Do you want 2" for your location; http://energycode.pnl.gov/EnergyCode...jsp?state=Iowa

Preventing condensation; http://www.buildingscience.com/docum...r-requirements

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Old 01-02-2014, 02:30 AM   #6
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2" foamboard insulation before siding


Gary,

I have blown in cellulose in a 2x4 wall now. I'm not sure what kind of r value that is but it's not dense packed. Would 1" foamboard be better for my climate? It just seems weak. We get some very extreme temps here. I thought 2" would be better to keep the wall temp above dew point while allowing drying to occur to the inside.
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Old 01-02-2014, 08:03 AM   #7
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2" foamboard insulation before siding


According to LP, Insulation that is 1" or less you can nail the siding directly to the sheathing. 1" or greater and you need to install strapping over the insulation.
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Old 01-02-2014, 09:12 AM   #8
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2" foamboard insulation before siding


there is much more to consider than just dewpoint when you start going over 3/4'' of foam

hopefully your installer has experience with the product and flashing details
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Old 01-02-2014, 09:31 PM   #9
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2" foamboard insulation before siding


Nails get all weird when you try to drive them through 2" of xps first.

This may get a good laugh, but I would pre-drill for them first if I were to try to do it. I've never tried it that way, but it may help.
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Old 01-03-2014, 09:42 AM   #10
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2" foamboard insulation before siding


You can do 2" of foam if you want.

At that point, hang the siding from furring strips that are screwed to the structure of the home. Rainscreening the wall is a great idea.

Depending on where the home is, 1" might work. 2" is usually better and the more insulation you can put on the wall, the better the dewpoint and condensation resistance is.
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Old 01-03-2014, 12:59 PM   #11
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2" foamboard insulation before siding


Quote:
Originally Posted by strategery View Post
Gary,

I have blown in cellulose in a 2x4 wall now. I'm not sure what kind of r value that is but it's not dense packed. Would 1" foamboard be better for my climate? It just seems weak. We get some very extreme temps here. I thought 2" would be better to keep the wall temp above dew point while allowing drying to occur to the inside.
Give me the name of a city close to you and I'll figure the dew-point temp for you. Others are correct, use an installer that knows flashing, sealants, and house wraps and install (at least) to code minimums for you. Is there an interior vapor barrier/retarder (poly) next to drywall, creating a double vapor barrier with thicker foam board?

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Old 01-03-2014, 01:07 PM   #12
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2" foamboard insulation before siding


Des Moines IA

No vaper barrier I can see.
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Old 01-03-2014, 08:41 PM   #13
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2" foamboard insulation before siding


Using the 3 coldest months averages, with R-10 you would be safe to 42%RH in the room at 68*F. With R-5 and R-13 cavity again, dew-point is at 32%RH and higher. (Code minimum) Your dew-points are almost exactly as for Chicago, so read the info on pp.10-14, here: http://www.buildingscience.com/docum...nd-wall-design

Notice with R-5 unfaced; Fig. 3b.1 no vapor barrier is required because the board is not faced allowing moisture out. Same with Fig. 3d.1. The difference between fig. 3.d.1 and fig. 3.e.1 is only 1/4" thickness but because of the foil-facing, the thickness is critical to warm the cavity enough to prevent condensation hence- a lesser degree of vapor retarder is required.

If it is sheathed already (holding the cellulose in...), IMO, pull the windows/door and add sill pans (probably lacking) and sticky wrap/head flash to rough openings. Due to your moderate rainfall (36") area, a rainscreen is not absolutely needed, but... because the CB siding is a reservoir siding, at least use a "stucco wrap" or similar to help it drain--- or use foil-faced FB to eliminate the air space; http://www.buildingscience.com/docum...voir-claddings

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Old 01-03-2014, 11:58 PM   #14
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2" foamboard insulation before siding


i would be leary of using a foil-faced foam directly under any wood based product
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Old 01-04-2014, 01:29 AM   #15
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2" foamboard insulation before siding


Good catch, Tom, thanks! For some reason I was thinking cement board panels.... another forum perhaps.

The unfaced is better with wood panels or add a product like HomeSlicker for drainage: http://www.jlconline.com/coastal-con...-shingles.aspx

The foil will keep any backside moisture in the sheathing, only allowing front face drying- slowed by the paint application.

Gary

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