Replies

1. 
   Mongo | Feb 10, 2005 01:57am | #1

   Vince, the 2" foil-faced between the rafters would work nicely. That would preserve a 1-1/2" air channel for your eventual soffit-to-ridge vent, should you choose to have that installed.

   That 2" of foil-faced foam will give you an initial R-14 with polyiso...but with wood having R-1 per inch, you might have thermal telegraphing through the 2x4 rafters. Depends on your climate. I'd recommend running horizontal strapping or furring strips across the bottom face of the rafters, 16" on center. That will give you a 3/4" air space between the polyiso and the drywall so the foil will act as a raidant barrier and not just as a conducting film. It will also give you better spacing for screwing the drywall, as well as giving a slight thermal break between the interior and exterior.

   Another 1" of foam across the face of the rafters, gapped and then the gaps sealed with canned foam, and then furring, would be nice. That would give you a tremendous air seal as well as a total thermal break...but I understand the desire to preserve headroom.

   I'd also put 1" foil-faced on the gable walls.

   Do remember, though...before you seal the lid on your house, address any moisture sources lower in the thermal envelope. Cooking/bathing sources, as well as sources from the basement.

1. 
   VinceNW | Feb 11, 2005 03:24am | #2

   Thanks for the help, Mongo!I'm going to be working in parts so that I can address the ventilation issues you mentioned in the first floor as I go, but it is nice to get the plan together before I begin.I've never worked with the rigid foam before so bear with me, but does the foil side go towards the living space like normal FG? And how would you best attach the board while leaving that space there for the air channel?Since I do have a true 4 inches there I might be able to squeeze 3" board into those cavities...I see that some makers do have a 3" board foil-faced...have you ever used any that thick? Seems like it might be a real pain to cut down :)Thanks!Vince

1. 
   rez | Feb 11, 2005 04:19am | #3

   Greetings Vince,

   It might be advantageous  to familiarize yourself with the various cathedral insulation concerns.

   If you scroll down in the lower left corner of your screen there is a search function that will take you to previous threads dealing with whatever you type in the search bar.

   If you type in 'cathedral insulation' or other keywords of the subject matter you'll get a good supply of data from those old threads.

   Cheers"Live Free,      not Die"

2. 
   csnow | Feb 11, 2005 11:50pm | #4

   "I've never worked with the rigid foam before so bear with me, but does the foil side go towards the living space like normal FG? And how would you best attach the board while leaving that space there for the air channel?"

   In a heating climate, the foil should face the interior.  Not that the foil adds much.  If you want to leave a space between the roof deck and the foam, you could lay down a length of strapping next to each rafter, and attach the foam to that.  Foam up to 2 inches thick can be cut pretty well with a 'break-off-blade' utility knife, sometimes called an "Olfa" knife.  A hand saw with fine teeth works pretty well for thicker foam, though less neat.  Pros use special hot wire knives.

3. 
   Mongo | Feb 13, 2005 08:20am | #5

   Vince,

   The thickest I've used is 2" thick polyiso, and it came with foil on both sides. But as csnow wrote, if it's only on one side the foil should face inward in a heating climate.

   When running rigid foam between rafters, I cut it about 1/2" narrower than the space between the rafters. I'll run some of the strapping, but leave some off so I can slide the 8' lengths of RFBI up and between the rafters, then slide it down the rafter bay to where you want it. Once the last piece is in run the rest of the strapping.

   Center the foam laterally in the rafter bay, then use canned foam to foam the gap between the RFBI and the rafter. Air tight.

   If you don't want to strap, you can tack a few nails into the sides of the rafters to hold the rigid foam up the bay while the canned foam is setting.

   Some of the 2" polyiso I get has fibers in it, it doesn't score and snap as cleanly as the other manufacturer's board does. If I have a lot to do I'll actually rip the boards to width on a tablesaw. Fast, precise, and the dust collection keeps things clean.

   For hand cutting I use a utility knife on thinner stuff and a very sharp, thin-bladed filet knife on the thicker stuff.

2. 
   jrnbj | Feb 13, 2005 08:57am | #6

   2x4 rafters 24" O.C. Hunh?!? are you sure you're living in a house......

1. 
   VinceNW | Feb 14, 2005 12:57am | #7

   Built in 1915 by the 1st owner...they used true dimension 2x4s and they are SOLID pieces of timber...most of the rafters are 24" o/c but some are a few inches less. When I had the roof redone we put up plenty of plywood sheathing and there's no problems holding it all up...but most of the rafters are probably supported a bit by the kneewalls. I don't think that is unheard of for older houses is it?

1. 
   MisterSteve | Feb 14, 2005 03:33am | #8

   I have the same spacing on my 2x4 rafters. I am in a 19-teens kit built bungalow. Like you, I suspect that the knee walls are carring much of the load. I am going to need a new roof in a few years. What size ply did you use? I

1. 
   VinceNW | Feb 15, 2005 02:31am | #9

   Mrsteve,I'm not totally sure...but I think it was like 1/4" or 3/8" ply that would normally be used for roof sheathing. Since it was a tear off (of cedar shingles + 3 layers of composite) the old skip sheathing was left and the new ply placed over the top. I imagine that the weight of all that old roofing material was just as much as the new ply and a single layer of shingles, but either way I have't see any bowing or stability problems.I'm a tad concerned about adding drywall on the inside but honestly some of those rafters don't have kneewall support (they did every other one down one side) and I don't see any problems with those either. So hopefully it won't bring the house down :)Vince

1. 
   MisterSteve | Feb 15, 2005 02:39am | #10

    
   53927.10 in reply to 53927.9 Vince,
   Thanks for the info. I was planning on putting 3/8 over the exisiting 1x6 skip sheating (replacing any 1x6 that was rotted)
   My rafters have drywall on them (The second floor is within the roof) Actually, its 1/2 drywall over some 3/8 thick early version of drywall- multiple layers of paper, no plaster at all. Anyway, no sags. But, in the summer, it gots hot up there, esp. where the wall slants (between the knee wall and ceiling) near your head when you are standing up. Its only the thickness of that rafter between the drywall and roof.
   -Steve
   http://www.lukeworks.comMrsteve,I'm not totally sure...but I think it was like 1/4" or 3/8" ply that would normally be used for roof sheathing. Since it was a tear off (of cedar shingles + 3 layers of composite) the old skip sheathing was left and the new ply placed over the top. I imagine that the weight of all that old roofing material was just as much as the new ply and a single layer of shingles, but either way I have't see any bowing or stability problems.I'm a tad concerned about adding drywall on the inside but honestly some of those rafters don't have kneewall support (they did every other one down one side) and I don't see any problems with those either. So hopefully it won't bring the house down :)Vince

2. 
   MisterSteve | Feb 15, 2005 02:40am | #11

   sorry to all that I did not delete Vince's post at the bottom of my reply.
   -SJ