I am about 1 month away from starting construction on our new house. I'm looking for advice on the best way to insulate my 2X12 cathedral ceiling. Vented or un-vented are both options. Obviously I want as much R value as possible. I don't have it in the budget to use closed cell spray foam for the entire thickness. I am going to look at the added cost of going to a 2X14 to gain some additional thickness as well.

Location in North of Seattle

Thanks in advance for your input.

Drew

I went unvented.

2" of closed cell polyurethane spray foam and then blown-in blanket to fill the rest of the cavity. I'm just having the spray foam done now and it is really the cat's meow.

What is the span for those 2 X 12's?

Span is about 20' +or-

I would like to do a green living roof. My concern with the CC foam on the underside of the sheeting is I will lock in moisture to the sheeting. I'm considering this, from the inside up
unvented
5/8 DW
R38 batt
5/8 T&G sheeting
tar paper
2" rigid foam
60 mil EPDM
2-3" extensive lightweight living roof/ moss as I'm in the Pacific Northwest

Thoughts?

Thanks

Drew

I don't know anything about living roofs. I chose metal roofing in order to be able to collect rainwater.
20' is a stretch for 2 X 12 dimensional lumber on 24" centers. I would use TJI rafters if I had to do it again. What happens to living roofs when there is a snow load? Do you have to water it during dry spells? Is there soil up there or some other synthetic growth matrix?

Underneath my metal roofing is peel-and-stick underlayment which is essentially impermeable. Any water that does get through has to dry to the inside. It can do that even through a couple inches of closed cell spray foam. The spray foam keeps the interior vapor from coming up and condensing on a cold surface. near the sheathing.

I'm 16" on center and its been engineered. No issue on weight.

So you have sprayed CC on the underside of the roof sheeting and peel and stick ice shield on the the other side of the roof sheeting? 1 month ago I would have done the same but now paranoia has set in after reading all this "interior drying  vs  exterior drying and proper venting of roof structure. I'm more confused then ever.

Just want to do the right thing

Drew

I'm 16" on center and its been engineered. No issue on weight.

I was engineered as well. In fact, I am an engineer and have followed through all the work. We were trying to keep the members out at 24" in order to maximize the insulation efficiency. I have solar panels up there and a low-slope roof that has the potential to catch some real snowfall.

If I had seen the 1/2" ply sheathing (7/16" nominal) before it went on, I would have sent it back in favor of 5/8". As it was, the guys on site were primed to examine the sheathing for poor quality, but it looked fine to them. Once it was on, we started finding numbers of sheets that had major voids. I think we tore all those off, but it gives you the creeps about the remaining ones.

If I had to do it again, I'd probably use some TJI560s.

I feel good about moisture issues because of the metal roof. I've observed it now through all kinds of weather and it seems to dry pretty quick. Having something moist up there all the time, might be a different thing, however.......

Posted By Drew Reed on 19 Jun 2012 12:12 AM
Span is about 20' +or-

I would like to do a green living roof. My concern with the CC foam on the underside of the sheeting is I will lock in moisture to the sheeting. I'm considering this, from the inside up
unvented
5/8 DW
R38 batt
5/8 T&G sheeting
tar paper
2" rigid foam
60 mil EPDM
2-3" extensive lightweight living roof/ moss as I'm in the Pacific Northwest

Thoughts?

Thanks

Drew

If the rigid foam is XPS or iso that stackup would comply with IRC2012, but not if EPS.  R10 is the absolute minimum exterior R for an unvented attic in US climate zone 4C, but R15 would give you more margin.  Under the thermal cycling of a sun exposed roof XPS is likely to shrink over time- double-layering 1" sheathing with seams staggered would be preferable to a single layer of 2" ship-lap or T&G.  http://energycodes-new.pnl.gov/site...mblies.pdf

Which layer is part of the defined continuous (over all 6 sides of the cube) primary air barrier?

CC foam will not lock in moisture. Even at 2" thickness it has higher vapor permeance than kraft facers on batts.  A 1" flash-shot for air sealing and providing a non-wicking condensing surface may be useful, and is typically about ~1-1.5 perms (3-4x as vapor open as kraft facers). See: http://www.buildingscience.com/docu...of-systems   Air sealing is an order of magnitude more important than vapor-retardency for protecting the sheathing from wintertime moisture drives, and flash-foam provides both.

Dana,

Thanks for chiming in.

"Which layer is part of the defined continuous (over all 6 sides of the cube) primary air barrier?"

I need some guidance on this, not sure what the "continuous primary air barrier" is.

Am I going to experience the same type of heat cycling with a 2-3" green roof?

Drew

When desiging & building new construction the best bang/buck on energy performance you'll ever get is from defining an air barrier that extends from the sub-slab vapor retarder up the walls/windows, over the roof(or ceiling), and back down that is continuous- it doesn't jump from one layer to another at the edges where say slab meets foundation wall, foundation meets upper wall, walls seal with windows etc etc. If the defined air barrier is on say the interior of at the walls, but at the EPDM membrane at the roof, getting an air-tight transition from interior to exterior where walls meet roof gets tricky. Ideally these details get worked out ahead of time and marked on the builder's documentation, and critical junctures get inspected before they get covered up, or you'll end up with leak points that are vague, hard to locate and remediate.

The regulatory folks revising Calfornia's Title 24 energy efficiency code estimate that defining, inspecting, and blower-door verifying a typical 2200' house to a max of 3ACH/50 would add about $750 to the total upfront cost, but in the cooler parts of the state would have a lifecycle savings of a few thousand dollars.

See pages 18 & 19 of this set of slides:

http://www.energy.ca.gov/title24/2013standards/prerulemaking/documents/2011-05-31_workshop/presentations/Res_Infiltration_and_Testing-053111.pdf

Even without a pre-defined air barrier layer it's actually pretty easy to hit 3ACH/50 simply by being careful and air-sealing what/where you can as you go. But WITH a defined primary air barrier getting under 2ACH/50 (or even 1 ACH/50) isn't out of the question- it's done every day with Canadian R-2000 program houses. Air seal every layer that's easy, but pay particular care & attention to detailing and inspecting the primary air barrier.

With XPS and EPS just assume it'll shrink. How much will vary, and the shrinkage issues with XPS are much improved since the early 1980s. I'm not sure if or how much polyisocyanurate changes dimension over time, but it handles much higher temps, and uses pentane as the blowing agent (as does EPS), which is 1000x more benign from a greenhouse gas point of view than the HFC245fa used for blowing XPS. But XPS is more rugged, takes a bigger beating without damage on job sites, and at 2" still has a rationale for having a lifecycle greenhouse benefit on lowered heating/cooling loads, depending on the source fuels used for heat or electricity. (At 4", maybe not.)

Posted By Dana1 on 19 Jun 2012 11:01 AM

Posted By Drew Reed on 19 Jun 2012 12:12 AM
Span is about 20' +or-

I would like to do a green living roof. My concern with the CC foam on the underside of the sheeting is I will lock in moisture to the sheeting. I'm considering this, from the inside up
unvented
5/8 DW
R38 batt
5/8 T&G sheeting
tar paper
2" rigid foam
60 mil EPDM
2-3" extensive lightweight living roof/ moss as I'm in the Pacific Northwest

Thoughts?

Thanks

Drew

If the rigid foam is XPS or iso that stackup would comply with IRC2012, but not if EPS.  R10 is the absolute minimum exterior R for an unvented attic in US climate zone 4C, but R15 would give you more margin.  Under the thermal cycling of a sun exposed roof XPS is likely to shrink over time- double-layering 1" sheathing with seams staggered would be preferable to a single layer of 2" ship-lap or T&G.  http://energycodes-new.pnl.gov/site...mblies.pdf

Which layer is part of the defined continuous (over all 6 sides of the cube) primary air barrier?

CC foam will not lock in moisture. Even at 2" thickness it has higher vapor permeance than kraft facers on batts.  A 1" flash-shot for air sealing and providing a non-wicking condensing surface may be useful, and is typically about ~1-1.5 perms (3-4x as vapor open as kraft facers). See: http://www.buildingscience.com/docu...of-systems   Air sealing is an order of magnitude more important than vapor-retardency for protecting the sheathing from wintertime moisture drives, and flash-foam provides both.

Dana, can you define "flash shot" please

Flash-foam/flash-shot== a 1" nominal (but non-critical thickness) application of closed cell foam, primarily for air-sealing and moisture control rather than raw insulation R-value. Google the phrases "flash and batt"."flash & batt", or "flash & fill", etc. Closed cell foam is an excellent air-sealing material, and semi-permeable to water vapor in thinner applications. The permeance drops with thickness, making it a useful material for setting a particular vapor permeance range, adjusting the moisture transfer rates in either direction.

BTW: In any new construction it's useful to model the energy use and cost-optimize any improvements or modifications for energy efficiency. The newest version of BeOpt is now downloadable (a very good DOE freebie tool):

https://beopt.nrel.gov/download

In cold climate: What worked for me with 2x12@2' centers is (from the ceiling)
2x12
1/2 plywood
30lb underlayment
6" poyiso (might have sprayed)
2x4 sleepers perpendicular and lagged into 2x12's with timberlocks.
Building paper
corrugated metal

In the construction, I ran the plywood sheeting up high to the level of the roof and routed off the ply even with the last sleeper closest to the eve. Trim and drip edge were fastened onto the top edge.

What I have then is a compact insulation layer with a cold roof.

If you do not have lots if insulation in the budget, keep any insulation that can get wet and will stay wet, out of the roof insulation layer and just use what you consider a minimum r value in your lid, perhaps less than the some standards; ie, Rigid foam and not fiberglass. EPDM seems like a poor choice for the layer in your assembly that is under your moss; no good adhesion, expensive. You said cathedral, rt?

Thanks for all the input. I am still having a hard time with this decision. Lots of choices along with variation of cost and payback.

Lets try this, I have not started construction yet. Foundation is excavated waiting for permit. Whats written in stone is;

Climate zone 4C San Juan Islands Washington state
Due South exposure unobstructed
2100 sq. ft single story

slab on grade
2X6 @16" centers walls with 1/2" OSB sheeting
2X12 @16" centers cathedral ceiling Entire structure
Either metal or living roof TBD
Siding -combo of wood and metal siding

The rest is up for grabs. Cost is an issue to consider as we do have a budget.

What would you do?

Windows?
roof insulation method/type?
wall insulation method/type?

Thanks in advance for your input

Drew

Even if your framing is set in stone, I would consider 'Advanced Framing Techniques'.

In some climates, placing the insulation between the framing members is a mistake.