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What to use for a vapor barrier above a cathedral ceiling? #9896 04/24/06 01:44 AM
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Thomas-in-Kentucky Offline OP
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I'm 90% done raising my frame and it is just about time to put a roof on it. Most of my roof is framed with principal rafters that accept 6x8 purlins (and a 8x8 ridge beam) roughly 4' on center.

Above this I plan to put
1. 7/8" oak tongue and groove, nailed to the top of the purlins.
2. a vapour barrier
3. 2x10's on edge, 24" apart, running from the ridge to the eaves. Two layers of 4" polyurethane (Firestone) insulation between the 2x10's.
4. 30lb felt paper
5. 1x4 oak battens running horizontally
6. slate

I have a pretty good handle on everything except the vapour barrier. Can anyone give me suggestions for what kind of material to use for the vapour barrier. I used some 6mil platic sheeting (commonly available at Lowes or Home Depot) under my concrete basement slab. I later tried to use this same stuff for covering my frame from the elements. (for that it failed miserably - the wind ripped it to shreds within two weeks - once torn, this stuff tends to rip easily). Is this the same material that everyone uses for the vapour barrier directly above the t&g ceiling or is there something more robust (e.g. rubber like?)? What kind of adhesive does one use to join the seams? Do you worry about the inevitable multiple nail punctures (vapour breaches) it will endure?

Also, does anyone have a suggestion for alternatives to the 30lb asphalt impregnated felt that I plan to use directly under the slate and battens? It seems like something with a little more vapour permeability would serve my roof better. Since the slate is not air tight, I'm thinking that the slate should be able to let the moist air that breaches my vapour barrier eventually escape to the outside... assuming the 30lb felt will let the vapour through. (is that a good or bad assumption?)

Comments and suggestions on the vapour barrier, felt, and the whole roof plan in general would be greatly appreciated.

-Thomas

Re: What to use for a vapor barrier above a cathedral ceiling? #9897 04/25/06 02:22 AM
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Emmett Greenleaf Offline
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T,
Some observations for your consideration.
Historical background: My home in the finger lakes in NY state is a classic timber frame with shipplank front, clapboard sides and a slate roof all circa 1804-1811.Standard center hall 4 over 4 with 11'ceilings and rooms roughtly 20 x 25.
the roof system is simplicity itself only complicated by the box gutters which need lotsa maintenance. The rafters are half rounds about 6-8" in diam spaced more or less at 18-20". The battens are randowm width pine and cedar planks just under 1" thick. the slate is laid directly on the wood.
In recent repairs we added ice damn on the lower 30" as we replaced slate and repaired the box gutters. At some point circa 1850 the center of the roof started to sag so a long purlin was added about mid rafter span on the underside and diagonally braced to the attic floor. The roof is still sound even with monstrous lake effect snow loads and weeks of sub freezing weather each winter. The house is unsulated by blowing in stuff between the ceiling joists and under the attic floor do a depth of about 10". Works like a champ over the lath and horsehair plaster ceilings below. Net effect is a cold roof over a slightly warmer attic space with no significant moisture problems.
What is your snow load and average winter temp ?
Keep in mind a vapor barrier is meant to separate a warm space from a cold space. In the case of a roof system the warm space is below the ceiling. You can stretch house wrap after you lay the t&g and before you set the 2x10's.
By insulating over the ceiling planking your only moisture in the roof is minor infiltration around the slate and mayhap on the eave ends if snow/ice buildup is routine.
The insulation needs to breathe via soffits and a ridge cap. Ice dam material on the lower 30" of the batten boards is climate driven and influenced by the roof pitch. The paper under the slate helps protect the slate from breaking under stress (including walking on it).
By all means check with your engineer for the monstrous weight of the slate.
good luck,
deralte

Re: What to use for a vapor barrier above a cathedral ceiling? #9898 04/25/06 02:13 PM
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Jim Rogers Online Confused
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6 mil plastic vapor barrier should be taped at the overlap seam.

If you're going to use 2x10's then they are 9 1/4" tall, and your putting in two 4" layers of foam, which equals 8".
How are you going to deal with the extra 1 1/4"?
You could leave it and add another layer of sheet goods, osb or plywood and then attach your felt and then your 1x4s that way the 1 1/4" gap could be used as a soffit to ridge vent and keep you having a cold roof.
The tar paper will catch any leak through and shed it down the roof, if your 1x4's have cut outs under them to allow the water to drain.

We heard a horror story at a conference about the 1x4's holding the water from draining and the roof decking rotted.

As far as the holes through the vapor barrier caused by attaching the 2by stock that's just something you'll have to live with and it isn't that much to worry about...

Jim Rogers


Whatever you do, have fun doing it!
Re: What to use for a vapor barrier above a cathedral ceiling? #9899 04/27/06 12:00 PM
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Thomas-in-Kentucky Offline OP
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Thanks Jim and Emmett for responding to my post.

6 mil poly - glad to hear that's what I should use for a vapor barrier, since I know where to find the stuff. That stuff comes in huge sheets, so fortunately there won't have to be many seams. Live with the nail holes... I hear you. What kind of tape do you suggest for the seams?

Air gap - yes I was wondering if leaving the gap above the insulation would be a good idea - thanks for confirming that. I will be sure to vent it at the eaves and ridge.

The slate I have ordered weighs 850lbs/square or 8.5lb/sq.ft. I used a snow load of 30 or 40 lb/sq.ft. when calculating/checking the timber sizes. (very very conservative for our area, but you never know what mother nature is going to dish out. Every 25 years or so we get a wet snow that collapses recently underbuilt barns and metal buildings). The timbers that support my roof (dead load) weigh something like 20lbs per sq.ft of roof area. So, if they support themselves without bowing, creaking, or moaning (which they have been doing nicely for several weeks!), they should support the slate and snow. I have done the calculations, but it's nice to see it standing upright without any visible deflection whatsoever. It looks like you could park a bull dozer on the roof. (Which is good, because I've ordered 40,000 lbs of slate! More weight than a bull dozer!)

I will try to make sure that any errant water will not get trapped within the roof system as you suggest. I am resigned to the fact that leaks are going to happen at some point, so the roof should be built to survive a leak that goes unoticed for a while. This is one reason that I do not plan to use plywood or OSB in the roof. It would be nice for the stiffening effects, but after reading a book on slate (and reflecting on several asphalt roofs that I know of that needed a sheet or two of plywood replaced when the 20yr. asphalt shingles were being replaced), I've come to my own conclusion that you can't beat solid oak boards under slate.

Thanks again guys. I'm looking for more suggestions if anyone has them... we set 6 valley purlins yesterday, and they fit great. (i.e. the measurements of the actual roof, confirmed that we should field cut the purlins to within 1/16th of their theoretical numbers!) Sighting up the valleys from the ground shows that all the purlins line up as they should. Hope to saw, notch and set 3 ridge beams today. Roofing is not far away!

-Thomas

Re: What to use for a vapor barrier above a cathedral ceiling? #9900 05/22/06 04:23 PM
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Mark Davidson Offline
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I use acoustic caulking for seams and holes in vapour barrier. If you know where you're going to place fasterners, you can put the caulking in place and fasten through it....

Re: What to use for a vapor barrier above a cathedral ceiling? #9901 05/22/06 11:14 PM
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Thomas-in-Kentucky Offline OP
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Mark,

Thanks for the tip. Do you have a brand or source of caulking that you could suggest?

We're putting down the tongue and groove right now while I wait for other materials to ship.

Thanks,
Thomas

Re: What to use for a vapor barrier above a cathedral ceiling? #9902 05/25/06 05:38 AM
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Thomas-in-Kentucky Offline OP
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Thanks Emmett, Jim, and Mark. We're installing that tongue and groove ceilings, so now is the natural time to put down the vapor barrier, it seems. I have a picture of the 6mil vapor barrier we installed today on part of our roof if anyone is interested... As it ended up, I was able to get a piece of plastic large enough (20'x40') so that there were no seems on this face of the roof.
blog entry with tongue and groove - covered with 6 mil vapor barrier and battens

Re: What to use for a vapor barrier above a cathedral ceiling? #9903 05/25/06 12:14 PM
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Raphael D. Swift Offline
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Looks good, I really like that herring bone pattern you've created on the ceiling.


Raphael D. Swift
DBA: DreamScapes
Re: What to use for a vapor barrier above a cathedral ceiling? #9904 06/17/06 03:53 AM
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Thomas-in-Kentucky Offline OP
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Thanks Raphael. The herringbone might be a little too busy for some tastes, but I've decided that I like it... if for no other reason than "form follows function" in this case.

Anyone have any more advice to give me on the roof system? I have a semi-tractor-trailer load of Firestone PolyIso insulation in my barn, twenty 20' long 2x10's, and I'm less than a week from starting on my roof using a built up system very similar to the one I described in my first post in this thread.

I'm toying with the idea of running my 2x10's 4ft apart rather than 2ft apart. To keep the 1x4 battens from bowing in the middles, I'll turn a 2x6 flat and run it parallel to the 2x10's. Basically, I'm taking my old design and replacing every other "2x10 on edge" with a "2x6 on flat". This lets me use a little less wood, lets me use the 4x8 polyiso slabs without ripping them to 2' wide, and most importantly will give me 1/2 the thermal breaks that I would otherwise have had. One concern is whether half as many 2x10s (anchored to purlins with Simpson L brackets) will be sufficient to keep the whole roof system from sliding off of the timber frame. (I think it will work, just running some numbers on the weight of the slate and the shear capabilities of the L brackets) Another concern is whether the foam insulation, over time, will not compress where the 2x6 bears on it. Any thoughts on this?

Perhaps I'll build a little section of the roof tomorrow and try to get a feel for how the materials will work together. Here's the same polyiso insulation where I've used it for infill walls...

timberframe infill using PolyIso insulation


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