HI all. Its been a few years since I posted. Since then we have had 2 more kids (3 total). Now we have no choice, we have finish off the attic. I apologize in advance for the length of this post.
First the facts. We have a 2 story, 2050 sq.ft. colonial style home in CT built in 1922 with a walk up 1,000 sq. ft. attic. The roof has a 9.5″ pitch with full dimensional 2×6 rafters spaced @22″ O.C. with 2 small dormers in the front. The roof was stripped and redone 7 years ago and we installed full ridge vent (on the dormers too.) The roof has @ 1’+ overhang with no soffit vents, which we will install. While the house is balloon framed, there is a double 2X4 plate all around the perimeter of the attic foor. The attic floor is T&G southern yellow pine. We spaced and drilled holes in the floor and blew in celluose isulation several years ago. The plan is to install a cathedral ceiling with a 2-3′ tray/flat(?) ceiling at the ridge. We will install 4-5′ knee walls. We are also planning to install central air/heat and we would like to cool the second floor with the system as well.
Based on my searches here, the consensus seems to insulate from the ridge to floor plate so that the area behind the knee walls are within the building envelope. Obviously the problem is coming up with the right solution to achieve enough R-value (R-38 around here). Assuming we leave 1″ rafter airspace for venting (is that enough?), we could install two layers of 1 7/8″ and one layer of 1″ Dow Tuff-R polyiso (foil faced both sides) between the rafters and 1″ Tuff-R over the rafters, for a total R value of 37. Another option using the Tuff-R polyiso is to install two layers of 1 7/8″ between the rafters and a third layer of 1 7/8″ over the rafters, for a total R value of 36. The second option allows for some additional air spacing (1″ or so) between the layers of polyiso in the rafters. Is there any reason for doing this? I would like to avoid having almost 2″ of polyiso on the rafters to preserve head room. Either way we will then install 1X3” furring strips, level off the rafters, then drywall.
Are these options viable? Any issues or porblems? Assuming 1,400 sq.ft. of roof, I have estimated that either option will cost me @$3,500!!!! for materials alone, excluding the various other costs (canned foam, beer, etc.). I don’t even want to think about the time and labor it will take to friction fit either system. I have heard about the sprayed in foam they always use on This Old House. Does anyone know what the options are for that system and what it might cost to have it done? I’ve heard that the R-value is 7 per inch.
All questions/comments/opinions/reality checks are greatly appreciated.
Replies
You sure the way to go is up? I added a 2nd story. Assessor was out in a flash to jack up the taxes. I had to drill caissons about 16' to carry the extra weight. Neighbor with same style house just had addition added over a basement he had dug. Looks good from the street. This was all planned so you could mess with the house....right. Let's see. How are you going to heat that, add plumbing for the NECESSARY bath, get the required natual ventilation in the new living space and will that ceiling turned into the new floor be stout enough to hold live loads....ahhhh.....its all in the fun right? Get the $$ lined up in advance! Tyr
Have you looked into spray foam? The new version of the IRC allows for unvented roofs if you spray a closed cell foam directly on the bottom of the roof sheathing. Doing so seals out the air that would carry the moisture that would cause the condensation that would worry the inspector in the house that Jack built. The stuff's not particularly cheap, but I bet it's not a lot more than the options you're considering. BTW, gaps between sheets of foam as you described would contribute nothing to the R-value, and might in fact contribute to heat loss.
Andy
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