Modern, efficient green housing design – insulation and condensation
I have grown to respect the advice of the many bloggers and commenters on this website and so am looking for some assistance. I am currently designing my off the grid, modern cottage want it to be as efficient as possible.
The cottage will be a single story, flat roof structure (basement TBD). Climate zone 5. I plan on using a variation of the ARTIC WALL proposed by Thorsten Chlupp, ie: 9″ walls filled with cellulose on the north side of the house, and east/west will be partially covered. For the ceiling, since I am going modern will have LVLs spaced 24″OC and would like the beams to be partially visible inside and continue outside to provide summer shading on the mostly-glass south wall. The LVLs will be supported on 2×4 or 2×6 24″OC as well (I beleive 2×4 is not permitted for 24OC construction on a single story building). 2×4 would be acceptable from an insulation standpoint since they will be free of insulation (all insulation being in the Larsen trusses outside).
My question is mainly regarding the roof structure. The LVLs will be appx. 24″ high (to be sized appropriately soon) and so I could keep 6″ exposed on the inside of the ceiling and still have 18″ of insulation (cellulose again) which would be retained with fabric and wood slats (slats nailed into 2×2 furring strips). This approach would also allow me to retain the modern aesthetic with pot light which I know are frowned upon here due to normal ceiling penetration issues. On the top side of the beams, I plan on going plywood/tyvek/4″ polyiso/EPDM membrane. I need foam on top to protect the Tyvek and the thickness is based on Martin’s guide of appx. 3.5″ required in zone 5 in order to protect against condensation. Since I will have so much cellulose insulation on the inside side of the vapor barrier, will I risk moving the condensation point inside?
Anything else I should look for or adress in a semi-warm roof with insulation on both sides of the barrier?
thanks
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Replies
By the way I took a look at
https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
but it seems having so much interior insulation is not recommended (violates the ratio I should be respecting).
Is there another solution, such as having a minimum-thickenss (1/2" or 1") vapor-permeable foam just to keep the EPDM off the roof sheathing and have all the "real effective" insulation on the inside?
Jonathan,
If you are building a roof with this type of "hybrid" insulation system (one with exterior rigid foam on the exterior side of the roof sheathing, and air-permeable insulation on the interior side of the roof sheathing) in Climate Zone 5, then the exterior rigid foam needs to make up at least 40% of the total R-value of all of the installed insulation.
Four inches of polyiso has a nominal R-value of abour R-24, but it's safer to call it R-20 in a cold climate (because the performance of the polyiso is degraded by cold temperatures). So as long as the total amount of insulation is no more than R-49, you should be OK.
By the way, it's a really bad idea to cantilever your rafters beyond your exterior walls. You can't really create an air barrier with that detail. You are setting yourself up for an impossible task of air sealing.
Thanks Martin for your response. I'm not too worried by the penetrations, I've seen some Passivhaus details on remodels in which old post/beams with similar penetrations still pass. R-49 is quite a bit lower than I'd like, 18" of cellulose should be good for more than that on its own.
Any other ideas for keeping the aesthetic but improving the insulation?
Thanks!
Jonathan,
A couple of suggestions that may or may not work depending on the aesthetic you are aiming for:
Rather than expose part of the deep LVLs every 24", why not increase their spacing and drop them below the ceiling? One every four to six feet means a lot less penetrations of the exterior wall to worry about and no thermal bridging in the roof structure.
You might want to look at recessed LED lights for the ceiling. They look a lot like potlights, but the size of the penetration and ease of air-sealing go a long way to mitigating the concerns people here might have about them.
Jonathan,
There are lots of ways to build a roof assembly. If you want a certain "exposed rafter" look, you can design exactly the look you want and build it. As Malcolm suggests, your ceiling should go above the exposed rafters. You don't want to try to create an air barrier halfway up the rafters anyway.
If your ceiling is above the exposed rafters, you can design any type of insulation details above the ceiling that you want, using cellulose and a vent space or a very thick layer of rigid foam.
I think I found what I need to design with - at BS:
http://buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation
I'll use Figure 11 but with a solid (LVL) joist instead of a i-joist. I'll leave the bottom of them exposed and backfill the cavity between the spray foam and the ceiling material with cellulose.
I should get around R-70/75 which is not as high as I wanted but acceptable...
thanks for your help!
.
Jonathan,
The problem with your plan is similar to the problem that occurs when you cantilever rafters over your exterior walls: Your proposed method eliminates the continuous air barrier under the air-permeable insulation. That continuous air barrier is an integral part of the BSC detail.
Of course, you can fuss with narrow rectangles of gypsum drywall inserted between the rafters -- but you won't end up with a good air barrier that way.
In this case the air barrier is at the high density spray foam, not the gypsum though, no?
thanks again...
Jonathan,
Just be careful that there aren't any leaks at the rafter cantilevers -- or else indoor air will escape through cracks in the narrow strips of drywall ceiling, and out the cracks at the cantilevers.