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rigid foam sheeting Post New Topic | Post Reply
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Bryan Kwater
Posted: Mar 21, 2011 12:15 PM
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rigid foam sheeting
A few years back Michigan revised building codes to require an R-20 wall assembly in new homes. There is a portion of the code that allows 1" thick (R-5) rigid insulation on the exterior side of the wall and will allow you to use anything that is R-13 or higher in the wall cavity. This is called the R5+13 assembly. Most builders use fiberglass batts, but others use cellulose, dense packed fiberglass, etc. Rarely do these homes get spray foamed, but occasionally we get a job or two. There is almost always 7/16" OSB on the 2x4 studs which is covered by the rigid foam.This system sounds real similar to a flash and batt assembly. Have you ever seen or heard of issues with this type of wall? I understand that it works well for thermal bridging, but I would be concerned with moisture in the wall cavity. The local "energy efficient" builders are building hundreds of houses per year like this, but they care more about saving money than doing things the right way. We even have some advertising that they have the most efficient houses around with R-30 exterior walls, which is just R-5 rigid + R-25 batt in a 2x6 wall. We have noticed that a lot have now switched to 1/2" or 3/4" rigid foam to save money. |
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mason
Posted: Mar 21, 2011 01:19 PM
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When using rigid foam sheathing on the outside of the wall, the boards are to be taped and a weather, air barrier membrane system installed to the corners and perimeters. This keeps bulk water and moisture out of the sheathing and prevent it from getting in the space between the sheathing and the interior wall assembly. |
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Bryan Kwater
Posted: Mar 21, 2011 04:20 PM
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Wouldn't this essentially be the same as flash n batt? |
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Daniel X
Posted: Mar 21, 2011 11:34 PM
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No, it wouldn't. Have you compared vapor permeance of 1" of 2lb. polyurethane to 1" of rigid polystyrene? You're comparing apples to oranges. |
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Posted: Mar 22, 2011 06:34 AM
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actually the flash and batt is,,in my opinion,,probably a "safer" application for a lessor chance for the propensity for condensation in this wall cavity than is the wall system you describe,,, this wall system is,,,and will always be,,a filterglass system,,,still prone to performance loss due to convective looping,,and this wall system doen not have an effective air barrier system no matter how much tape,, caulk,,and mastic you apply to it,,,this house is still leakin,,, consider that you now have an air gap at the foam board to osb interface,,,as well as at the osb to filterglass interface and suddenly you have plenty of "next available airspace" for any moisture laden air that has met dewpoint with in the wall cavity to have a place to lay out on and condense,,, MiF,,,(you cant believe how bad i wanted to type MilF)spend some $$$,,,blower door one of their homes...blower door a similar foamed home,,the numbers will tell the story and show the value of a spray foam system... dll,,,dont you hope they are usin the rigid with the foil face,,,vb on the cold side of the wall,,brilliant,,, keep on rottin in the free world... dude |
