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DIY Insulation Baffles

bwkwood's picture

      I am currently building a cathedral ceiling room addition on a very limited budget. I have been fortunate to obtain many building materials at low cost. The R19 kraft faced fiberglass for the ceilings and walls (1000 sf) cost me only $200. It is going to drive me crazy to spend another $200. for the plastic insulation baffles at Home Depot. I can purchase a roll of housewrap ( 9' x 100') for $30. This would certainly fill the role as an air barrier in my rafter bays. Does anyone out there have a nifty idea on how I could maintain a 1 1/2" vent chute between sheathing and house wrap? It needs to be simple, time effective and cost effective. Other particulars :  2 x 8 rafters, 16" oc, 5/8" OSB sheathing; plan to cut housewrap 16"-18" x 140" to fit each rafter bay.

Wait until the second week in (post #206905, reply #1 of 14)

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.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Are you sure about that price (post #206905, reply #2 of 14)

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?

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insulation baffles (post #206905, reply #3 of 14)

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. 

Using wrap is cheap, but (post #206905, reply #4 of 14)

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

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Like I said, you can do the (post #206905, reply #5 of 14)

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


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Same problem, but for 24" (post #206905, reply #6 of 14)

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.

Seems to me (post #206905, reply #7 of 14)

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.

Housewrap is most definitely (post #206905, reply #8 of 14)

Housewrap is most definitely an air barrier.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Don't agree (post #206905, reply #9 of 14)

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.

While the paper IS an air (post #206905, reply #10 of 14)

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

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Builders that are trying to build tight (post #206905, reply #11 of 14)

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

I don't know what you mean by (post #206905, reply #12 of 14)

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).


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

   It has been great getting (post #206905, reply #13 of 14)

   It has been great getting all the feedback from my original post. Several comments: first, I agree that housewrap is definitely an air barrier but there seems to be concern that  I would have sealing issues if stapled to the rafter bays. Would the fact that  there would not be a perfect seal (without caulking or foaming) be that big of an issue? We are told that insulation like fiberglass is suceptible to R value degradation if cold air flows through it, but the very small cracks that would result from stapling the housewrap, are they enough to cause any significant loss of R value? I understand that with vapor barriers a perfect seal is necessary, but for this barrier application I am not sure it is critical. The commercial products out there are 2,3 & 4 ft long so you are going to have many seams to deal with in these products. DanH's idea about the foam political signs got me thinking, so I started canvassing the area for how many are available, how many I would need and whether I could wait another 3 weeks before I could gather them. In my rural area I would guess at least 4-5 hrs and 25 miles to get enough, but I thought it was a GREAT idea; perfect material, perfect price.

      

Would the fact that  there (post #206905, reply #14 of 14)

Would the fact that  there would not be a perfect seal (without caulking or foaming) be that big of an issue? We are told that insulation like fiberglass is suceptible to R value degradation if cold air flows through it, but the very small cracks that would result from stapling the housewrap, are they enough to cause any significant loss of R value?

Yes.

To act as a proper insulator, air needs to be still.  Otherwise, it keeps bumping into solid objects and transffering heat energy which is what you DON'T want.  Unconditioned air from outside that is washing the underside of your roof deck will pull and push air in and out of your insulated space, lessening it's effectiveness.  It will also allow new moisture into the insulation space and provide new opportunities for condensation, rot, and mold.

Those comercial vent chutes are usually only used for the 3'-4' area from the soffit over the exterior wall... it's not designed for a catherdral ceiling application, it's designed to keep your insulation from blocking the air channel in an otherwise open attic.

Air sealing is an important part of insulating.

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