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Paper or .... not Post New Topic | Post Reply
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Nolan Worley
Posted: Nov 06, 2008 04:10 PM
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Paper or .... not
I'm building a new home and we've just gotten into a discussion regarding exterior paper around the wood windows (required for warranty purposes) and this lead to a discussion on using building paper under the siding. My General contractor who sold me on using the spray foam to meet our codes, tough when using wood windows. Now he's telling me he doesn't use paper under the siding or side wall shingles due to the Corebond being a more effective vapor barrier and a layer of paper would trap moisture between the two barriers. I think if the moisture can't get there from the inside and cant get there from the outside, why not ?Whats the accpeted rule here |
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mason
Posted: Nov 08, 2008 11:30 AM
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Do you mean house wrap or the old style felt paper? Is it installed to the outside of the sheathing or the inside? If to the inside, it would reduce the effectiveness of the foam as an air barrier and to stop water intrusion. If the "paper" is installed to the exterior sheathing, it may be to act as an air barrier or a rain screen (if water gets past the flashing from the roof or windows). If the "paper" is being used as a air barrier, the foam takes care of that. If it is used as a rain screen over plywood or OSB, sprayfoam (closed cell) would stop the water intrusion into a stud wall cavity. Even the "flow through" type house wraps can create a condensing point within the building envelope when using a highly water vapor permeable insulation. Sprayfoam slows water vapor flow so that the condensation should not occur. The foam allows a slow controlled flow of moisture in both directions. Ask your supplier to do a WUFI (or other hygrothermal modeling) on the suggested design. It should let you know if the window manufacture design would get you in trouble. ( I doubt that it would unless you have a significantly high interior humidity) |
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Nolan Worley
Posted: Nov 08, 2008 12:12 PM
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The building paper would be applied on top of the OSB and underneath the sidewall shingles and siding. This is a fairly arid region, 8 months of the year. However it's located on a 55 mile long fiord like lake, and we get very high winds in the bad months with wind driven rain. The concern is wind driving water past the siding and staying between the siding and shingles and the OSB for a couple of months at a time. His position is, don't worry about it, OSB doesn't absorb much water. My position is, if it doesn't hurt, why not? And I can garantee that wind will drive water past the siding and shingles. We do see 80mph winds and rain four or five times each season. However I don't want condensation between the foam layer and the paper. We've stayed away from the Tyvek type wraps, as were hearing about lots of problems in western washington with this product, but it may or may not be the better choice. |
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mason
Posted: Nov 10, 2008 09:02 AM
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If the paper is being used as a rain screen, it should not hurt the sprayfoam application. The sprayfoam will keep and wind driven rain that gets past the paper from getting in the house. The paper can protect the OSB. Any water vapor that gets past the sheathing will be the same temperature as the outside air, The foam will prevent condensation on the inside. |
