Using an “Arctic wall” design for a cathedral ceiling in Climate Zone 6
Wondering if the arctic wall design would be appropriate to use as a roof/cathedral ceiling here in CZ6 upstate NY. There is a comment about it on Martin’s blog, but I didn’t see any discussion : https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
from inside out it would be:
GWB,
2×4 service cavity,
1/2″ CDX taped and sealed,
parallel chord trusses 12-12 pitch dense packed with cellulose,
vapor open WRB (suggestions for best product welcome),
2×4 furring,
5/8 sheathing,
felt,
asphalt shingles.
I’ve done a lot of reading on the stackup being used as a wall in fairbanks, and there was some concern that it wouldn’t fair too well in a wetter climate.
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Replies
Trevor,
Your proposed assembly sounds just fine from a building science perspective.
The only two remaining questions are (1) airtightness and (2) buildability.
This assembly can be built in an airtight manner, of course -- the best place for the air barrier is probably the 1/2-inch plywood -- but it's important to pay attention to airtightness during construction.
The buildability question is more problematic. It's hard to install a fabric membrane (you describe it as a "vapor-open WRB") on the exterior side of the roof trusses on a 12-in-12 roof -- especially when there is no roof sheathing.
Here's a house with this design:
http://www.homepower.com/articles/home-efficiency/design-construction/extreme-efficiency/page/0/1
In the following video, the builder describes Henry Blueskin as an appropriate vapor open WRB. He used Grace Ice & Water Shield (Low Perm) without any moisture problems.
https://www.youtube.com/watch?v=rJsBXT1_f-A
This design moves the CDX layer to the outside of the insulation (which solves the problem Martin mentions). Dense packed cellulose apparently stops air movement and redistributes moisture well enough to make a theoretically risky assembly very robust.
A few of the things I like about it:
1. It puts the thermal barrier up near the roof plane, which is helpful if the summers get hot.
2. The self adhesive WRB doubles as the air barrier. (saves material and labor)
3. It lines up the WRB on the roof with the WRB on the wall, eliminating tricky details at the eaves because there are no eaves until the actual roof plane is built.
4. Flat 2x4 furring makes the roof plane easy to build and generously vented.
5. Roofing nails will never penetrate the WRB/air barrier
Here's the arctic wall: http://www.reina-llc.com/files/9913/2357/1044/Diffusion_open_walls.pdf
Trevor,
I believe Thorsten has indicated that the design has been adopted (for clients) to a cathedral ceiling. If you must have a cathedral ceiling your implementation seems like a good approach. Martin is right to warn about buildability, though.
Kevin,
The link I have been referring to is found is here: http://www.cchrc.org/sites/default/files/docs/Thorsten_walls_report.pdf
There are some differences, notably the detail at the slab/wall interface where the outer balloon frame wall is dropped - presumably to address the thermal bridge of the base plate. It appears that the foundation type changed too at some point, from a slab to a monolithic slab/footing. There are other, subtle (or perhaps not) differences, but in the context of this post, they do not seem critical.
The PDF of his presentation is to hazy (for me) to read. I would speculate that the file hosted on CCHRC is the most current iteration.
cheers
Martin, my reading of the arctic wall shows using the inner layer of cdx as the air barrier by taping the seams. The buildability question is one I was wrestling with. I was thinking we would be able to stand on the plywood between the trusses while installing the fabric, but the insulation guys wouldn't have much to work from.
Kevin, I don't know as much about building science as most of the guys on here, but that setup seems like it goes against everything I've read, but since it seems like the easiest way to go I have some research to do.
Trevor,
Assuming you move forward with the assembly you've described, have you considered omitting asphalt shingles in favour of steel panels?
Steel panels will fasten down just fine on the 2x4 strapping which may allow you to omit the upper sheathing layer.
In this case, instead of using a simple WRB over the trusses, consider a more robust membrane (like Solitex Mento Plus).
My only experience with steel roof on a residential building is a lady around here that had a standing seam roof installed by a local contractor.
The noise was so bad she had it removed, not only in the rain, but on windy days, and even on sunny days it would "oil can".
Trevor,
A steel roof can be a little noisy where there is little or nothing underneath (like in my barn), but over top of a thickly insulated roof assembly I wouldn't worry about it.
And you might never need another roof in your lifetime.
I'll look into it, but not really sold on steel roofing for anything other than a shop or barn. If i was to go with a steel roof and I run the strapping (always called them purlins) the lenght of the building instead of from the eave to the peak, would there be enough airflow for venting? Its my understanding that the steel would sweat quite a bit.
Trevor,
You need to run the purlins both ways (cross-hatched). First parallel to the rafters, then perpendicular. This way there is much better venting airflow, and drips can run down the slope. You are right to expect the steel to sweat, especially when it chills under the night sky.
Trevor,
My experience is different from TJ Elder's. Under metal roofing I always install a single layer of purlins, running rake to rake, on top of the roofing underlayment. This provides enough of an air space to allow any condensation to evaporate. The normal pumping action created by changes in temperature between day and night allows the condensation to evaporate before it causes any problems. There is no need for liquid drainage.
Martin,
What is your experience with the noise from those roofs?
Also I've read you burn wood, I am looking at a small woodstove to help with heating on really cold days not as a primary source of heat.(mostly because I like to sit by a fire, and have plenty of free wood) I was going to use decorative metal flue pipe exposed all the way to the ceiling/roof. transitioning to stainless triple wall.
Is there a better way?
Will I find off the shelf fittings to go through the proposed roof, or will ti take specialty/custom work?
Trevor,
Attaching a few photo's that show that installing SOLITEX MENTO Plus is pretty straight forward. This is of a project in NY, climate zone 6, 14" I-jiost at a 7:12 pitch - our http://www.foursevenfive.com/blog will feature this project soon with details etc. FYI - We are the US distribitor of the ProClima SOLITEX membrane.
You start at the eave on a scaffold or ladder/pick and roll one piece of 60" Solitex roof underlayment out and staple it to your trusses. After installing 48"vertical battens on rafters and two 24" o.c purling (or plywood for your asphalt roof) - you have just build yourself you own ladder and can work your way up from there. Steep pitches can be easy to walk on, especially with closer spaced purlins - which are best practice in combination with heavier gauge metal roofs - both of which contract noise and oil canning.
Trevor,
You won't hear your roof if there is an unconditioned vented attic between you and the roof.
If you sleep in a room with an insulated sloped ceiling and metal roofing, you will probably be able to hear rain on your roof -- a sound that many people find romantic and delightful -- and you will also be able to hear creaks and pops when the sun makes the metal expand.
If you prefer asphalt shingles, install asphalt shingles.
These days, most people who install a wood stove specify a metal chimney as you propose. Metal chimney manufacturers include roof penetration kits and flashing kits, and also include installation instructions.
It's also possible to build a brick or stone chimney lined with clay flue tile. My house has two stone chimneys.
For more information on this topic, see All About Wood Stoves.
Floris, how close to the proposed stack up I have, is the one you are using?
I am looking forward to reading your blog on this.
Martin, I would love to be able to do a masonry chimney, or a masonry heater, but the budget isn't there for it.
FWIW I'm going to stick to what I know and like, and put on asphalt shingles