Guest User
10/24/2004
I have been recommended iso board because it is "closed cell". I will be using either three quarter or inch and a half twelve inches on center over 3 dimentional shingles. 1) is a closed cell preferable 2) should I use three quarter in the form of one by four or inch and a half in form of two by four. Im concerned about the one by four following any dips in the roof as it is only half inch plywood. I dont see but a few dips and I could string those areas as i meet them.
Allan Reid
Dura-Loc Roofing Systems, Inc.
10/24/2004
Unfortunately you have provided limited information, but here are some points to consider. If it is a vented attic construction then the added insulation is of little value. You should only be looking at an insulation upgrade in a vaulted roof assembly. The building codes call for a minimum 1" vented air space in this assembly over the insulation. As well they call for an air barrier to the interior side and a moisture barrier under the roof covering (Minimum 30 lb felt). You need to watch that you do not already have a vented airspace as it will negate some of the value in the additional insulation. Also the existing shingles may have a moisture barrier under them and could trap moisture now in the centre of the new assembly which depending on your climate can freeze, reduce the R value and make exixting insulation wet. In this case you must make sure you are adding enough insulation so that the existing roof layer is at 40% of the R value from the exterior of the whole assembly. Last if the roof assembly does not have any insulation, then it probably does not have any air barrire. I would consider removing the old roof, install a 10 mil plastic with sealed joints, then the insulation and the moisture barrier. As the the closed cell vs the open, the closed cell takes slightly higher heat and is more stable and takes weight better which is why it is used in flat roofing but does cost more. Follow the assembly instructions above and open cell is just fine.
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