new roof on an existing home

charles samul
4/8/2013
earlier message disappeared during typing so here goes again i have a log cabin in westport ny built circa 1980. the current roof is cedar (siding) shingles and is is need of replacement. our winters are cold. our summers getting warmer. the roof is built on 2"x6" joists. the ceiling appear to be t&g i believe it is nailed to the underside of the joists. my addition and out building have metal roof and we are happy with the performance. i think its a wheeling product. the main cabin seems to stay warm and i suspect that the current roof system may not have inadequate, or non-existent insulation. when i am there in the winter with the heat on we get plenty of icicles. i am also concerned about thermal bridging. if necessary i am willing to remove the sheeting and insulate between the joists. but that wont solve thermal bridging. the house is otherwise very tight for a log home. i have heard different points of view about installing rigid foam in a roof system and am somewhat confused. some say sheeting, foam, then lath then metal roof. to make matters somewhat more complicated, i have 5 skylights in the existing roof.
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7/3/2013
The project may have completed at this point but if not, I hope this helps. You are very solidly on the right track when it comes to your thinking. Give the climate and the leaky nature (air and moisture) of the tongue and groove, I prefer to remove the sheathing and insulate the space from the top down. There will certainly be an added expense associated with the re-decking, insulation removal, new insulation, etc, but it is the best way to go. I would remove the decking and insulation. The easiest thing to do at that point would be to spray foam from the top down (preferably closed cell for the vapor barrier implications but open cell SPF will stop the air movement and therefore most of the moisture). After you re-deck the roof, I would sheet the roof with at least 3" of rigid foam, install purlins, and then create an overdeck vented roof.

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