Replies

1. 
   DanH | Oct 09, 2012 08:12pm | #1

   Wait until the second week in November, then collect all the corrugated plastic political signs you can lay your hands on.  They can be stapled to 1x2s and screwed in place or just slit halfway through and folded along the edges to allow them to be stapled in place.

2. 
   xxPaulCPxx | Oct 10, 2012 03:52pm | #2

   Are you sure about that price for paper?  Sounds like a 4'x100' roll.

   The only way to easily and effectively air gap with paper is from the top down, not stapling from the bottom.  You will live to regret that.

   Another way is to rip foam board sheets to use in your bays.  a 4x8 sheet costs $25 - is that too much?

1. 
   bwkwood | Oct 11, 2012 09:12am | #3

   insulation baffles

   Thanks for the reply. The price for the 9x100 housewrap is correct. Your idea for foam blocking is the best I have considered. If I can staple the wrap tight enough along the rafter bay sides I might be able to use 1 1/2" foam ripped as narrow as possible and cut into small pcs that could be spot glued in a grid pattern for the 14 1/2" rafter bay. My gut feeling is that even if the wrap deflects between the foam blocks as long as I have at least 1" depth of vent chute I should have adequate venting. 

1. 
   xxPaulCPxx | Oct 12, 2012 02:52pm | #4

   Using wrap is cheap, but seems like a huge labor sink.  But here goes for ideas:

   In my attached diagram, you can see how tp make a foam rafter chute.  You use a table saw to rip a sheet in 16" or 24" wide strips (depending on your rafter spacing).  Then you set you blade height at 7/8" if you are using 1" board, and run the fence at 3/4" from the blade.  Now you will score both long sides oin the same face.  Here comes the trick:  Fold them over.  Now you have a wide flat chute that is spaced EXACTLY 1" from whatever you press it against.  You would use foam to seal the edges and the seams.  THIS IS THE BEST SOLUTION as it is high R value AND waterproof if sealed correctly.

   I also illustrate how you can do similar with the wrap.  Go buy precut 3/4" x 1.5" x 8' lath, they sell it in bundles of 12.  Tape it down to a clean concrete floor every 16" or 24".  Each center will have a lath boarder on bother sides.  Unroll the paper over it and staple it down to each of the lath pieces every 6".  Now, cut the centers between the lath pieces that are side by side.  You will be left with a piece of paper with two lath strips that you can attach to the sides of the rafters.  Snap a chalk line 2.5" from the underside of the deck and line up the bottom of the lath on that.  This will leak air unless you tape the paper well at the seams, and caulk or foam where the lath and rafters meet

   
1. 
   DanH | Oct 12, 2012 06:36pm | #5

   Like I said, you can do the same thing with old lawn signs, and they're free.  And more durable than the foam.

3. 
   junkhound | Oct 13, 2012 07:51am | #6

   Same problem, but for 24"

   Same problem, but for 24" bays.  $2 for a piece of egg carton material is a ripoff.

   I simply used some old 1/4" ply that were free from old crates, stacked them on the edge of the flatbed, ripped 20 in wide pieces 36 to 48 " long (across the sheet)

   Take a few old 2x4 or 2x2 scraps, tak nail to one end, that sits on the top plate.  Other end held up between rafters and with nail gun shoot  single 16d thru ply and into rafter, all done

   Like dan said, yard sings would work fine also for insulation layer thin enough that the sign width give you enough baffle length.  I needed at least 36 inches of baffle  for blown in R-49 on 5/12 slope.

4. 
   davidmeiland | Oct 13, 2012 11:13pm | #7

   Seems to me

   that most people would not bother putting in insulation baffles when the insulation is 5-1/2" thick and the rafter bay is 7-1/4" deep. Are you not going to place the insulation so it is flush to the bottom of the rafters? Only possible issue I can see is at the eave.

   Now, a couple of issues. First of all, R19 sucks for ceiling insulation unless you are in a mild climate. And, fiberglass in a vented cathedral is going to be subject to a lot of R-value degradation due to wind washing, unless you baffle it with an air barrier material.

   The suggestion above for rigid foam installed as baffles is excellent, assuming you fit it tightly and completely seal the fiberglass from the vent space. You could install 1" rigid with a 1" air space above it and compress the 5-1/2" fiberglass into the remaining 5-1/4" space.

   Housewrap is not an air barrier material and will give you virtually zero benefit if installed as you describe it.

1. 
   DanH | Oct 14, 2012 08:10am | #8

   Housewrap is most definitely an air barrier.

1. 
   davidmeiland | Oct 14, 2012 10:32am | #9

   Don't agree

   Air barrier is generally a rigid material... drywall, plywood, rigid foam, etc. There are rolled and liquid applied materials that would also serve, but they're not under discussion here.

   If he staples tyvek into his rafter bays, he'll be the first guy I heard of who ever did it. Total waste of time, IMO.

2. 
   xxPaulCPxx | Oct 14, 2012 11:00am | #10

   While the paper IS an air barrier, the seams are not... and his idea involves nothing but seams!

1. 
   davidmeiland | Oct 14, 2012 12:50pm | #11

   Builders that are trying to build tight

   are steering away quickly from relying on housewrap as their air barrier, even with all the seams taped. Air moves through it easily. 

1. 
   DanH | Oct 14, 2012 02:42pm | #12

   I don't know what you mean by "easily".  Using housewrap is impractical not because air moves through the stuff (it doesn't if it's Tyvek, though it does, slightly, for some other types that are micro-perfed to let moisture through), but rather it's so difficult to fasten the stuff and obtain a tight seal (unless you just go straight across the face of the rafters with the full roll).

5. 
   art_yatsko_realtor | Nov 04, 2023 04:58am | #13

   Your project is probably done by now but in the interest of helping others here are a few suggestions-
   I once needed to fill the void to get the required R value for an existing roof. Laid tyvek, 1" sleepers, new decking, vented drip edge, and a ridge vent. This allowed me to fill the existing attic space with blown in.
   You could buy fanfold insulation, slice and refold it in your siding brake. Then staple in place.
   They sell corrugated cardboard baffles. Do you have a source of free boxes? Wonder if liquor store boxes( beer case?) Would be consistent enough in size that you could use them without creating a time suck.
   If you want to pursue the house wrap (or perforated plastic sheets) it could be installed from the top before/as the roof deck goes down. Laid loose if you just want to prevent wind washing or held tight to each side of the joist/truss with board ripped to size and nailed.
   What did you end up doing?

6. 
   Habilis64 | Dec 06, 2023 02:41am | #14

   Spray foam directly to the underside of the roof deck. Higher R-value, no air gap needed, no vapor barrier needed.

   Expensive? Yes. But done in a few hours. And you'll save money on heating & cooling in the long run.

7. 
   KaamDeir | Dec 06, 2023 04:23am | #15

   Consider using thin, rigid foam strips as spacers. Cut them to the desired width (1 1/2") and insert them between the sheathing and housewrap in each rafter bay. This method provides ventilation while being cost-effective and relatively simple.