Using Housewrap for Insulation Bafflescomments (5) October 25th, 2012
A bargain-hunting builder searches for an alternative to commercially available insulation baffles for use in a cathedral ceiling
Building on a tight budget? That's a situation many of us have faced, and bwkwood is one of them. He's in the process of putting up an addition with as little cash outlay as possible. He's rounded up some bargain insulation and is now looking for a cheaper alternative to insulation baffles for a cathedral ceiling.
"The R19 kraft faced fiberglass for the ceilings and walls (1000 sf) cost me only $200," bwkwood writes in a post at the Breaktime Construction Techniques forum. "It is going to drive me crazy to spend another $200 for the plastic insulation baffles at Home Depot."
He's come up with an alternative. For only $30, he can buy a 9-ft. by 100-ft. roll of housewrap. His plan is to cut the housewrap into strips 16 in. to 18 in. wide and fit them into the rafter bays in place of commercially available baffles.
"This would certainly fill the role as an air barrier in my rafter bays," bwkwood says. "Does anyone out there have a nifty idea on how I could maintain a 1 1/2-in. vent chute between sheathing and house wrap? It needs to be simple, time effective and cost effective."
Bwkwood's quest for economy and performance is the subject of this month's Breaktime Spotlight.
Is housewrap really an air barrier?
The gap immediately below the sheathing would allow air to travel from soffit vents to a ridge vent, carrying away any moisture that worked its way up through the insulation. He hopes the housewrap works as an air barrier to reduce air infiltration (and unwanted moisture) into roof cavities while maintaining the performance of the insulation.
But would housewrap accomplish what bwkwood hopes? Opinions are divided.
"Housewrap is not an air barrier material and will give you virtually zero benefit if installed as you describe it," davidmeiland tells bwkwood.
"Housewrap is most definitely an air barrier," DanH replies.
"Air barrier is generally a rigid material...drywall, plywood, rigid foam, etc." says davidmeiland. "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."
In fact, he adds, builders who are aiming for tight houses are giving up on housewrap as an air barrier, even when all seams are taped. "Air moves through it easily," he says.
Wrap is possible, but foam is better
PaulCP replies that while housewrap is cheap (and assuming it works as an air barrier), there's still a lot of labor involved in bwkwood's plan. If he's determined to use it, PaulCP suggests starting with bundles of lath 3/4 in. thick, 1 1/2 in. wide and 8 ft. long.
"Tape [the lath] down to a clean concrete floor every 16 in. or 24 in.," he says. "Each center will have a lath border on both sides. Unroll the paper over it and staple it down to each of the lath pieces every 6 in. 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."
PaulCP adds that the assembly will leak "unless you tape the paper well at the seams, and caulk or foam where the lath and rafters meet."
An alternative, PaulCP says, would be to use sheets of rigid foam insulation, cut scored on a tablesaw so it could be bent to form a chute.
"You use a tablesaw to rip a sheet in 16-in. or 24-in. wide strips (depending on your rafter spacing)," he writes. "Then you set your blade height at 7/8 in. if you are using 1-in. board, and run the fence at 3/4 in. from the blade.
"Now you will score both long sides on the same face. Here comes the trick: Fold them over. Now you have a wide flat chute that is spaced exactly 1 in. 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."
Two other possibilities
Two other posts suggest other cheap alternatives to foam. Junkhound faced the same dilemma and also was unwilling to spend the money for foam baffles. His solution was to scavenge 1/4-in. plywood from old crates and attach it to lengths of 2x4 and 2x2 scrap.
DanH would wait a couple of weeks and take advantage of the political season.
"Wait until the second week in November, then collect all the corrugated plastic political signs you can lay your hands on," he says. "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."
Not only will the signs be free, he adds, they also will be more durable than the foam.
Do small gaps really make a difference?
The discussion leaves bwkwood confident that housewrap could be used as an air barrier, but it raises another concern if he goes ahead with his plan to staple the housewrap into the rafter bays:
"Would the fact that there would not be a perfect seal (without caulking or foaming) be that big of an issue?" he asks. "We are told that insulation like fiberglass is susceptible 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?"
In a word, replies PaulCP, yes.
"To act as a proper insulator, air needs to be still," he writes. "Otherwise, it keeps bumping into solid objects and transferring 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 its effectiveness. It will also allow new moisture into the insulation space and provide new opportunities for condensation, rot, and mold."
He adds that vent chutes are typically only used in area immediately over the exterior wall. They're not designed for installing in the full length of rafter bays in a cathedral ceiling.
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