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Battman328 09-20-2007 11:34 AM

Newbie Insulation Question: Cathedral Ceiling
Hello All,

I'm having an addition put on my house and am about to insulate the cathedral ceiling. It's 2x12 rafters on 16" centers. As it's a sunroom/4 season room, the building inspector said I could go with R30, but I'd like to get R38 if possible (I live in WI). I have Durovent baffles running between each rafter from the soffit area to the cont. roof vent. My questions:

Which provides better insulation: a 12" R38 batt (compressed 1.5" around the baffle and by the fact this it's 12" thick in an 11.5" space), or an R30 batt that will fit with no compression? The guys at home depot argued with eachother about this for awhile last night and in the end 2 of them disagreed.

2nd: If I don't use faced batts, I need a vapor barrier, correct?

3rd: Do the Durovent baffles need to be spaced 1" apart? (meaning installed end to end with a 1" gap between them? I get different opinions on this one too)

4th: I've seen some of the High Density R38 insulation online...but it seems to be many times the this the only safe way to get higher R values?

Thanks for the comments! Battman

Ron6519 09-20-2007 06:25 PM

Compressing fiberglass reduces R-value. Put in as much as you can without compressing it. If you want to increase the R value to the roof, apply foamboards to the joists.
Unfaced batts need a vapor barrier.

Clutchcargo 09-20-2007 08:16 PM

63 Attachment(s)
Owens Corning makes R38 cathedral insulation that fits in a 2x12 rafter bay.

If you use an r30 insulation you get about 3.5R/inch. if you compress it, you still will get about 3.5R/inch. You don't gain anything by using thicker insulation unless you have the room for it. I would use the R38C insulation, the added cost is only a 1 time expense.

crecore 09-20-2007 09:55 PM

exactly, 38c, I agree

white29 09-23-2007 06:10 AM

I've had good success insulating cathedral ceilings by using foamboard as a baffle to get increased R value. Simply add a wood spacer in the corner where your rafters meet the roof sheathing to maintain the air space and attach the foamboard to that.The foamboard becomes your baffle.You can "kill two birds with one stone" using this method.:jester: :eek:

Wellcraft19 01-24-2008 05:37 PM

Maybe too late, but I would STAY AWAY from the Durovent baffles as the plague. If you put them in - especially along the entire run of the rafter, you will lock in any moisture (and there is always moisture) in the insulation since it will not be able to pass by the (styrofoam) Durovent baffles and out into the "air stream".

Hence, I would look for other baffles, baffles that actually allows mositure to pass through. Back in native Sweden, we have baffles running the entire lenght of the rafter, but these baffles are made out of perforated masonite board. Stable/steady but with a large number of small holes per square inch. And, make sure whatever baffles you use, do match the holes inthe birds blocks. The Durovent baffles are NOT designed to be used with standrad "round" birdblocks and need to be "shimmed" to proper height - unless your objective is to actually block the three holes in the BBs.

Might be a bit overboard, but a vaulted/cathedral ceiling is a construction detail you will only be able to reach once - during construction - do it right the first time. That said, as long as you can guarantee the air space on top of the baffles, it'll likely be better with short baffles to make sure you have free airflow from the (many times incorrectly positioned) holes in the bird blocks. At least that will not block humidity inside the insulation and allow it to evaporate upwards and hopefully be "whisked" away be the airflow between the deck and the top of insulation.

DO NOT trust the kraft paper on the insulation as a vapor barrier. You will need a continouos layer of a plastic film on the inside, before you put up drywall or other covering. ESPECIALLY in a cathedral ceiling in colder climates.

Why do I say all this?
Why are two peope in a early 1960s house i nthe Puget Sound Region. Rainy and mild climate but actually pretty low humidty. We are in a reasonably big house for two, do not take long showers, do not cook, and have NEVER had condensation problems before (older poorly insulated house).

We built a new addition in the fall, some 200 sff with a 16' high cathedral ceiling out bone dry building material, and raincovered before the fall rains started. Left it sitting to ventilate for a while and the in early December, I started insulation in order to turn on the heat inside (we keep it relatively cool at 60 at night and 64 daytime).

Put in Doruvent baffles and R-30 between 12" joists, spaced at 24" CC.
Mistake 1: Trusted the Durovent baffles.
Mistake 2: Trusted kraft faced paper
Mistake 3: Turned on the heat (should have had this area blockled off and worked in the cold - no humid hot air that can condensate on exposed surfaces).

The other day, when I was to start drywalling the walls, I noticed a small puddle on the floor. It had a been a very cold night (for here) with temps around 25F. Traced the source, and it turns out the ENTIRE north facing side of the roof deck is moist. The water I saw dripping down, came off the Durovent baffles since no air could pass through them, only up along them. There was water droplets that have formed on the insulation side of the Durovent baffles. One cavity between ceiling joist even have some severe traces of mold.

If it would not have been an unseasonably cold night, I would not have detected this and put up woodpaneling in the ceiling next week, hiding this "problem" for ever (until I liked would have had to remove the roof deck due to rot years down the road). Now I've taken all the batts down, removed all the baffles and is drying the deck (dry to the touch after less than one day but will leave exposed for at least another week with fans and dehumidifyer). The insulation seem to be "OK" since the only traces were water droplets on its surface (no soaking). It has been laid out aroud the house to "ventilate" and air dry.

Lessons learned - and this in a climate where the real need for a vapor barrier is "less" than in a cold are like WI:
- You can never have enough vapor barrier (Kraft faced is NOT enough)
- You can never have enough ventilation
- Do not - ever- trust the guys at HomeDepot, Lowe's, or contractors that come up with new ideas (foam insulation w/o any ventilation, Durovent baffles and other "cheap and ready made" solutions, etc).
- Traditional methods works best and allows houses to last for "eons".
- Stick to your 6th sense and do "the right thing", even if it takes time, cost money.

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