Insulation and Vapor Barrier


Old 12-29-03, 12:51 PM
Scott McDonald
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Insulation and Vapor Barrier

I live in Wisconsin and I'm converting my attic into additional living space. I have 2x6 rafters and have installed faced insulation with an R-value of 19. I would like to increase the value by adding rigid foam insulation panels. Depending on the thickness I could get an increased R-value of 5 - 7.5. I'll have to furr out the rafters with 2x2s.

Is it worth it to do this, and is this a correct application of rigid panels?

My next question has to do with vapor barrier installation. I have a continuous ridge vent and I've installed rafter vents from soffit to ridge. I will have the aforementioned insulation from ridge to soffit. I will have insulated knee walls approximately 5 feet from the exterior walls. The space behind the knee wall will be unheated/uncooled, but it is insulated.

When installing the vapor barrier do I go all the way to the exterior wall, or do I just need to go to the knee wall?

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Old 12-29-03, 01:08 PM
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I'm not sure if I am reading your post right, but you will need an air space between the insulation and the underside of the roof sheathing to permit airflow between the soffit and ridge vents. Failure to have a vented roof in this manner will reduce the lifespan of the roofing, resulting in premature roof failure.

2 x 6s have a nominal height of 5.5 inches. If you use rigid foam board (extruded polystyrene) with a 1.5 inch space between the insulation and the roof sheathing, that will give you 4 inches or an R-value of 20.

What R-value does the local building inspector require for your finished space?

The vapor barrier goes between the insulation and drywall. It does not extend into the unheated portion of the building.
Old 12-29-03, 03:34 PM
Scott McDonald
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Thanks. I do have durovents providing a channel for air to pass from soffits to ridge vent. I do not know the local code on R-value. I'll look into that.

As mentioned earlier, I have 5.5" faced batts (R-19) over the durovents and would like to add 1" rigid foam panels over the batts. The manufacturer rates the panels R-5 (1.5" = R-7.5). This would give me a total of R - 24. Am I figuring it correctly?
Old 12-30-03, 06:26 AM
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Your math sounds correct, but may over estimate the insulative value of the fiberglass. If it is the least bit compressed, which it sounds it may be due to the roof venting system, its R-value will be reduced.

You should consider running your straping for the extruded polystyrene foam board layer perpendicular to the roof rafters. This helps reduce the amount of thermal short circuiting, (kind of like a bridging effect) between the roof and drywall that the wood rafters provide. Given that this is a roof, I thing I'd run 2x4s, so that the 3.5 inch nominal dimension can be completely filled with faom board, giving you and extra R-7.5 (over 2 inch foam board). In your area, R-49 is the recommended insulation value.

I would ask the local building building inspector about what you want to do. Generally, you are permitted to do anyting that he or she will approve. What construction methods did you specify in your building permit application?
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