Replies

1. 
   bd342 | Dec 18, 2002 02:47am | #1

   option #4 , its by far the best you dont squash the isulation that way and end up with a much more efficient system.

2. 
   Mongo | Dec 18, 2002 06:31am | #2

   Pete,

   My preference, and DIY-friendly to boot:

   You want to preserve vent channles in the rafter bays? Then fill the rest of the bay with FG batts (R-21).

   On the face of the rafters, put up 2" thick polyiso foil-faced RFBI (R14.4). Simply tack the 4x8 sheets to the rafters with a couple of nails w caps. when you hang the sheets, gap each sheet form its neighbor by about 3/8ths of an inch. Use a sharpie to put tick markes on the face of the insulation that indicate where the rafter's centerlines are located. When done, used canned foam to fill the gaps.

   This will give you the tightest ceiling you can possibly get. Even though the the total R-value is about R-35 it'll outperform its R-rating.

   Furr 16" oc over the 2" RFBI, screwing the furring strips through the polyiso and into the rafters. You know where to screw from the tick marks. Drywall in standard fashion.

   Polyiso is nasty in a fire, so cover it with 5/8ths "X" drywall.

1. 
   PeteBradley | Dec 22, 2002 07:28pm | #3

   Mongo,

   Thanks.  That sounds like a great solution.  Just one question -- what's "X" drywall?

   Pete

1. 
   calvin | Dec 23, 2002 12:43am | #4

   Pete, mongo is advising you use 5/8"- Type X "Firecode" drywall.__________________________________________

   Remodeling Contractor just outside the Glass City.

   http://www.quittintime.com/

2. 
   Piffin | Dec 23, 2002 03:48am | #5

   I'm with Mongos proposal. Your #3 was the worst and most wasteful. Before the r21 in #1, I would go with R19 and baffles.

   Best would be to have ureathene foam sprayed in but that's not a DIY option.

   Another would be to nail up an inch of foam per Mongo instructions and then blow cellulose full into the space, renting the blower..

   Excellence is its own reward!

   "The first rule is to keep an untroubled spirit.

   The second is to look things in the face and know them for what they are."

   --Marcus Aurelius

3. 
   dabonds | Dec 23, 2002 06:54am | #6

   5 other. Go to this web site http://WWW.fomo.com and order their two part poly-urethane foam. Pifin is wrong. You can order their two part faom in refillable tanks and spray the bays full of foam. Install baffles first to allow for ventilation. This will cost more but be much faster and easier and for the DIYer. I have used the stuff and it works great.

1. 
   Piffin | Dec 23, 2002 07:01am | #7

   Now we're both wrong. When you spray foam in, you don't need ventilation. The urethyne foam is a vaporbar and is such good insulation that it prevents cold points for condensation to occur. The baffles only make it hard for the foam to have firm adherance and can fall away easier. Even with the kits, it isn't an easy typical DIY project tho, IMO.

   Excellence is its own reward!

   "The first rule is to keep an untroubled spirit.

   The second is to look things in the face and know them for what they are."

   --Marcus Aurelius

1. 
   dabonds | Dec 23, 2002 05:53pm | #9

   I knew that would get a response. I live in New Orleans where the occasional hurricane blows through. I still think that the baffles are desireable as all roofs eventually fail and without the baffles it will be impossible to tell when one has done so. With the baffles, it will be possible to see water marks where water has gotten through the sheathing and traveled down the baffles. I consider the baffles to be ventilation for the sheathing and not part of any condensation solution.

1. 
   Piffin | Dec 24, 2002 12:48am | #12

   and to brad

   You are right. It's been so long since I roofed in the south, I wasn't thinking about that point. Almost every roof I replaced in Florida needed some sheathing replaced, even tho it didn't always show as leaks inside. It would be hard to do with foam stuck to the backsides and shingles do have a shorter life with foam immediatley behind them.

   Sorry, and thanks for the correction..

   Excellence is its own reward!

   "The first rule is to keep an untroubled spirit.

   The second is to look things in the face and know them for what they are."

   --Marcus Aurelius

1. 
   PeteBradley | Dec 24, 2002 05:45pm | #13

   Piffin,

   Thanks.  What do you think about my offset framing idea?  Any pitfalls?

   Pete

2. 
   Piffin | Dec 25, 2002 01:07am | #14

   I've done it but it's a lot of work and cost..

   Excellence is its own reward!

   "The first rule is to keep an untroubled spirit.

   The second is to look things in the face and know them for what they are."

   --Marcus Aurelius

2. 
   PeteBradley | Dec 23, 2002 06:33pm | #10

   I thought one of the reasons for the rafter vent was to help keep the shingles from getting barbequed in the summer time.   (This house has asphalt shingle).  It does seem like houses that I've seen around here have the shingles go first in any areas where there wasn't venting.

   Couple more questions on rigid foam and fire.  In the case of foamboard, doesn't furring it out create chimneys between the rock and the foam?  I suppose you could fill the gaps with more board though.  Is the sprayed in foam less toxic/flammable?

   It also occurred to me that I could modify option 4 by offsetting the 2x4s from the rafters (just nail 'em on the other side of the kneewall studs and collar ties).  Then (I could put unfaced R21 in the rafter bays with r15 between the 2Xs.  This would increase the insulating effect, but the 2X4s would only be attached at the ends.  Over a 4' span with nothing to do but hold up drywall, I'd think this is ok though.  True?

   Pete

   Edited 12/23/2002 1:42:01 PM ET by BRADLEPC

1. 
   Mongo | Dec 26, 2002 06:51am | #15

   "Couple more questions on rigid foam and fire.  In the case of foamboard, doesn't furring it out create chimneys between the rock and the foam?  I suppose you could fill the gaps with more board though.  Is the sprayed in foam less toxic/flammable?"

   Your furring will run horizontally, so your "chimneys" will be horizontal, about 14 1/2" wide and running the length of the house. Between the horizontal furring strips, put a 14 1/2" long (or slightly shorter, the length depends on the width of your horizontal strips) by 3" wide piece of 3/4" thick furring vertically behind the breaks on the butt joints of the drywall. This will create "firestops" in the "chimneys" that you are concerned with.

   In your climate, with your desire to have air baffles, I'd stick with air baffles, then stuffing the rafter bays with FG, then RFBI, then furring, then 5/8ths "X" board.

   Solid performance and easily accomplished by the DIYer. Easy and economical. No tools to rent, no learning curve required.

   In other scenarios I'd prefer dense-pack cells over the FG batts.

   Been eating schnitzel and enjoying fine brews for the past week. Sorry for the late reply.

1. 
   PeteBradley | Dec 28, 2002 08:50pm | #16

   Got it.  Wasn't thinking too clearly there.

   Schnitzel and fine brews.  Hmmm...that sounds pretty good!  My wife just had a baby and I've got the mother in law in town, so I've had about 10 minutes (not consecutive of course!) to think about this lately.

   Thanks to all for the information.  I'm either going to go with the FG/RFBI approach (simple & effective) or the deep FG option (more work, no foam).

   Thanks again!

   Pete

4. 
   xMikeSmith | Dec 23, 2002 08:09am | #8

   if you want more than 7" .. furr the ceiling down..

    me... i'd put up 1" of EPS foam, furr the ceiling below that and blow the cavity with

   7 " of dens-pak cellulose

   Mike Smith   Rhode Island : Design / Build / Repair / Restore

5. 
   PeteBradley | Dec 23, 2002 09:43pm | #11

   Just noticed a typo -- choice #4 should have read R30c.  R38c wouldn't leave room for ventilation.

   Pete