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Best insulation technique for 2x6 cathedral ceiling


chimpywrench's Avatar
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06-13-16, 10:20 AM   #1  
Best insulation technique for 2x6 cathedral ceiling

I've just finished tearing out a very poorly done finished attic. It's a roughly 30x24 space with a 45deg roof built of ~16ft 2x6 spaced 16" oc. They had it insulated with R-19 but didn't put in the necessary venting. At first I thought I'd do spray foam but given the cost, and that I can reused all the fiberglass I took down, I thought I might do fiberglass, then 2" xps, then drywall. I'm wondering if this is a good way to go, assuming I vent the spaces.

As for venting, I've used the egg crate foam stuff for other houses with 24" oc rafters with unfinished attics. The vents pretty much just act as tubes to let air in from the soffits and keep the cellulose from clogging things up. In this case, the space will be finished and I feel like I can do something better. The foam hardware store vents don't span the full bay. In fact for 16" on center you're supposed to break the 24" ones in half and you end up with a 9-10" vent within a 14.5" space. I feel like a full width vent will perform much better and seal the fiberglass within it's own space so it performs better.

I'm trying to figure out what material to use, and the best thing that comes to mind is corrugated plastic (coroplast) which could be cut, edges bent and stapled, but I can't find it locally and it might be too expensive. I think I'd need about 30 4x8 sheets worth. Next I though of 1/8" hardboard ($6-7/sheet), but I'd really prefer a more moisture sound material up there. Maybe 1/2" foamboard?

Will the coroplast or foamboard cause me to have a double vapor barrier or is it permeable enough considering that moisture can still travel through the wood rafters?

I guess with any sheet good that can't be bent I'd have to first install 1" nailers, which is a lot of added work.

Thanks for any ideas!

 
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06-13-16, 02:54 PM   #2  
I did just find this article which was incredibly helpful and intelligent: Site-Built Ventilation Baffles for Roofs | GreenBuildingAdvisor.com

It even mentions my concerns about permeability and even brings up the point about rafters allowing moisture through.

I'm leaning toward using 1/2" xps and using 1" strips as my blocks.

It still isn't clear to me if there's any benefit to making the air space more than 1".

 
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06-13-16, 04:10 PM   #3  
Hi chimpy, your last description sounds like the way to go. As for the 1" or more, the one inch gets close to being a narrow gap, for which the air flow is somewhat restricted. But it is acceptable and much better than exposed fiberglass. Plus, you are working with 5.5" of total space. The only difficulty comes up as to how thick will the fiberglass layer be. 1" gap plus 1/2" rigid leaves you with 4". No real benefit of stuffing that R-19 in there as it is the depth that determines the r-value and stuffing too much really makes the job more difficult. You can pull off some of the 6" but again, slow.

2" of rigid plus drywall over that should be good. Be sure to carefully mark where all of your rafters, studs, and joists are to be sure you hit them with the long screws.

Bud

 
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06-14-16, 06:45 AM   #4  
Thanks Bud!

I'm going to take a closer look at all the r-19 we took down. It's pretty old and may be lacking in thickness. Perhaps it's already near the 4" thick mark, in which case it'll fit in nicely. It's free to me vs hundreds of dollars for new fiberglass. I wonder if I should remove or do anything with the paper facing on the old fiberglass?

Actually, I've used R-19 in 3.5" spaces many times - the manufacturer states that the fiberglass will retain it's per inch value even when compressed to some degree, so doing so will revert the R-19 to maybe R-13. I also see the added benefit of more compression causing the space to be filled more completely, and unfaced holds itself in the wall better. I wouldn't normally pay the extra cost, but I've gotten some deals on R-19 that made it a lot cheaper than R-11/13/15. If memory serves, compressing any further will reduce the per-inch value.

And you're right, R-19 in 3.5" will puff out and make drywall install more difficult. I always use 5/8" drywall so it has no problem pressing it back.

I'm headed to the hware to play with the foamular and decide between 1/2" and 3/4" (whichever seems like it'll be easier to work with).

Oh, and concerning the air gap, it's my opinion that the air in that space only has to move up the bay at a snail's pace in order to be effective. Just the slightest circulation should get it done. 1" should be fine for that and articles advising 2" or more seem a bit odd to me. One thing I'm not sure about is how to treat the rafter bays that begin at a valley, so they don't connect to the soffit. Since the roof is sheathed with 1x8's with gaps, there is some bay to bay connection. I could also drill holes to connect these orphaned bays?

 
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06-14-16, 07:17 AM   #5  
There is a performance curve there where increasing the compression actually increases the r-values per inch (think high density) but then it goes the other way and starts to decrease in r-value as the insulation loses the spacing between the fibers. I've seen the curves but long ago and not sure if r-19 in a 3.5" space is past the peak. I do know it is a pain stuffing it in there and if yours has naturally shrunk, it is probably the older itchy stuff. IMO, this is not a job you will be revisiting any time soon so some actual 3.5" high density fiberglass (r-15) or Roxul (r-15) would perform better in the long run.

Dealing with orphaned bays is a problem. I can't recommend drilling holes but they would be better than no holes . That issue is best resolved when built, by installing the sheathing over strapping.

One of the reasons for 2" vs a 1" gap is some vaulted ceilings can have a point source of air leakage, like a recessed light where there is no ventilation help from all other sources. Thus each bay needs to have more. Given that many homes survive with far less, including those skimpy baffles, having a nice clear 1" channel is acceptable.

As for the " vs 3/4" rigid, remember you will be compressing that insulation and don't want to collapse the rigid.

Bud

 
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06-14-16, 10:37 AM   #6  
I was actually thinking of the 3/4" foam because it's stronger and less likely to collapse. Now that I've taken several test pieces of my old fiberglass it looks like it'll snug nicely in a 4" space so, pending the last minute change at the hardware store, I plan to go with 1/2" foamular. I'd like to reuse the old fiberglass. There's really nothing wrong with it and I don't see need to toss it and spend hundreds on new material. It's already up there waiting to be used!

The house actually has a T shape, and the minor section of the T has a much lower roof ridge (85.5" from subfloor to bottom of rafter at the ridge). For this area I don't think I can do the 2" rigid over the rafters, so I was thinking of doing rigid foam to fill the entire rafter space then maybe 3/4" then drywall. It'll probably end up being a walk-in closet given the height restrictions. To me, I think I should still have the vent space and ridge venting. There is some thinking that when using just foam you can simply fill the whole space, but I really see the sense in venting it. What do you think?

Last I just have to decide if I'm going full height with the main ceiling or having a horizontal portion.

 
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06-14-16, 11:38 AM   #7  
If a flat space is not going to be big enough to be accessible the only other reasons would be maybe better venting and keeping the inside air out of the peak. Yours won't be that big, but I've dealt with large cathedral ceilings and that is where the hot air accumulates, good or bad.

Be sure you meet all of the code requirements for electrical and get all of your inspections.

Bud

 
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06-16-16, 08:28 AM   #8  
Just an update: we removed all of the wood blocking along one side (blocking was between the rafters, preventing soffit air from coming in). That was a hassle since it was obviously nailed in place before the house had a roof. On the other walls I may just drill holes through the top of the blocking and/or cut the top 1" off.

We used an angle grinder to cut short any long nails but for the most part all the roofing nails were under 1". Then swept out each bay of any debris (old insulation & spiderwebs).

The 1/2" xps I chose is going in beautifully. The 1" strips are super quick to cut on a table saw and press right up against any intruding nails, then we tack them in with a 1.5" medium crown staples. The baffles are also quick to cut on the table saw - the 1/2" foam is forgiving enough that you can overcut and press fit. It works really well and aside from cutting a bit too narrow here and there, no fasteners are needed to hold it in place. We're caulking all the edges which will more permanently hold it in. Small blocks are cut to reblock at the bottom.

The only annoying thing is that the 1/2" Foamular (and only the 1/2" size) comes with a plastic film, I would guess to decrease the permeability and increase strength. The film isn't adhered well enough and I worry it'll peel over time and clog up my air chutes, so we're peeling all the film off of every piece.

Overall it's working great. I have a roofer coming today to look at removing the twirly vents and add ridge venting.

 
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06-16-16, 09:56 AM   #9  
Sounds good. Don't know if you also have gable end vents but if so they can stay. Roofers like to insist they be removed but not necessary.

Bud

 
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06-16-16, 10:13 PM   #10  
No end vents, just one electric roof fan exhaust and one passive fan wind powered fan, both of which are non working and I plan to have removed. I don't think active venting is a good idea thermally and it eventually ends up making noise. A well-vented attic should slowly circulate air completely passively.

I think I'll plan to add the ridge venting and take my insulation baffles all the way to the top, even though I'm not sure of my ceiling design yet. There are some soffit vents but I'll need to add more to get up to spec.

Will get some photos up soon.

 
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