Hi
Im putting in a new ceiling in my attic to be able to insulate and use the space for living. The old rafters are 5×5 spaced at four feet and tied together with various beams. I plan to add a sloped wall of 2x6s and fill the space with cellulose and cover with t&g. I can basically add any thickness insulation so would I be any better off with over 12 inches of cell or is it just overkill? By the way, all air infiltrations will be dealt with and I live in the cold windy Saint Lawrence River valley of Quebec. I have to DIY and have no access to a cellulose pressure blower, only the loose blower kind loaned by the home stores, so can I dense pack physically with a ram as I bring up the T&G? Also from what Ive read here over the Years, a VB is unecessary with cells… is that still ther conventional wisdom?
Basement has been dealt with: drained, polyed and slabbed. I have only minor condens. problems in the 2nd floor due to chilly 50 deg. temps.
(Attic is above 2nd floor) Thanks for answers and friendly or non-friendly advice.
Fat is good. Duck fat is better!!!
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Dear Moulard,
Always a pleasure to correspond with someone who puts moisture control first. Hand packing is worthwhile is some applications. I think you can do better.
If you sheath the underside of the rafters wih drywall or wood panels, you can load the roof system from a slot left open in the peak. Even with a whimpy blower, you can get enough density to control settling. The dw air barrier eliminates the need for higher densities we like for better air resistance. Your poly is already in the right place.
This method involves a bit more expense for the panels, but will save you a ton of labor. You'll get most reliable insulation job too.
Having enjoyed the occasional duck egg, Fred
[email protected]
Hello,
Thanks for the reply. I'm afraid Im still wondering about thickness. My wife wants me to go overkill; like 18 inches of the stuff. Sounds okay but is the return long run worth the extra expense and labor?
Also I've read one of your posts where you say you're always more careful about moisture in Vermont when you insulate. Are there ever any cases where an air gap between insulation and roofdeck is necessary? my roof is steel with an elastomere underneath, so it is 100% watertight.
Try some foiegras, its better than crack!!!
PS Did you get any ice fishing in this winter? Here the river froze barely a week. ? perch, 3 walleyes 12 pike and a sturgeon (27#)
I put it back.
Oh yes the blower... I guess there are models that will dp? are there some that just wont work no matter what?
Edited 3/24/2002 8:11:35 AM ET by MOULARD
If you can incorporate this into your design, I like adding a layer of rigid foam board insulation to the bottom face of the attic rafters. I like foil-faced polyisocyanate insulation.
In a typical attic, I'll use 2" polyiso on the ceiling and 1" polyiso on the gable-end walls.
I gap the sheets by about 3/8ths of an inch when installing them, then come back later and fill the gaps with foam. Since this is a one-time deal for you, you can buy cans of foam to accomplish this instead of a commercial foaming gun. Just be ready...once you start foaming, don't stop...the cans are single-use (ove the course of a half-hour or so) shots.
I then screw furring strips (16" oc) through the foam and into the rafters, and hang the drywall from the furring strips. I also add pieces of 3/4" ply wherever I plan on hanging anything from the attic ceiling...a light, a smoke alarm, backing blocks for faux beams, etc. I don't want ANYTHING to penetrate the polyiso other than the screws that secure the furring strips to the rafters. You want to keep that foil-faced plane intact and unpenetrated. No can lights, for instance.
Polyiso is R14.4 for the 2" foam...but with the air-sealing capability it's much more effective than 14.4 R's of anything else, especially FG. My opinion.
You can pack cells into the rafter bays behind the polyiso for added R's. Definitely furr the polyiso before blowing the cells, or the blown in cells will blow the polyiso right off the rafters.
A tight, well-insulated attic that has heated livng space below will require very little supplemental heat to keep the attic temp comfy. I'm in a warmer clime than you and Fred (Connecticut), but typically once a few incandescent lights are turned on, and once a few warm bodies go into the attic, that's all the heat that's needed.
As Fred mentioned, excellent job addressing moisture concerns below. That's a rarity these days to see someone proactive instead of reactive.
Regards, Mongo
Thanks Mongo,
I suppose the rafters in your plkan are 16oc too?
My bays at best can be 24 inches but Id rather stay with
32 inches just because its a little savings on framing.
As far as moisture goes, ive got 22 batten windows none of them thermal,and formerly an earth floor basement, so condensation was a problem from day one. The second floor never got above 55 degrees no matter how hot the 1st floor got. Windows are to be changed this summer after the attic work is done. But I can only hope that I get around to doing it all. The roofing season will be starting in april and Ill be putting in my regular 12 a day...
Moulard,
Yes, my roofs are framed 16" oc.
If you went 24" oc with your framing, you could still furr 16" oc with no problems that I can see. If you went 32" oc and you're going to dense-pak, for a little extra blowout protection you may want to use larger furring strips (1x3, or I've even seen some people use a 2x4!). The larger furring strip will give more resistance to flwex as well as a larger target for screwing the drywall. You could also close up the spacing on the furring strips to 12" oc...though that may not be neccessary.
Smaller furring strips, closer spacing. Larger furring strips, wider spacing. How dense you pack the cells is the deciding factor.
A couple of tidbits:
I usually use 5/8th inch thick drywall (fire board) on ceilings.
When I can't get furring strips the size I need, I simply rip them to the required width out of 3/4" sheets of CDX ply.
When (if?) you hang 4'x8' sheets of polyiso insulation on your rafters, the ends of the sheets do not have to break over the rafters...they can break in the middle of a rafter bay. That makes it easy, since gapping them is going to throw the 8' sheets off of conventional framing after a few panels.
Ok im going for the poli-iso.
I can get it at discount from the company I work at, so Im into the air barrier thing big time.
2 questions though:
1. Since it is basically a moisture Barrier due to the foil, should I leave an air space btween roofdeck and insulation? I did mention that the roof has continuous welded membrane under the tin, so its not going to breathe that way.
2. If I do leave an air space do I have to open the space through soffit to ridge? I know Fred thinks its unwise in the northn but the home energy site recommends it. What to do, what to do?