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Community and Q&A

Unvented Attic Cathedral Ceiling Insulation Recommendations for New ICF High Performance Home

New_Green_Build | Posted in Energy Efficiency and Durability on

Hi GBA Community,

We’re starting our ICF build next week and in the process of getting insulation quotes, I’ve started to rethink my roof insulation plan and would love some advice. 

Background Info:
The house will be a ranch with finished basement, insulated slab and ICF walls from footer to roof truss.  We will have a single sloped (shed) standing seam metal roof, with full ice and water shield underlayment coverage applied over zip sheathing, with closed cell spray foam directly on the underside of the zip sheathing.  The house will be in Climate Zone 5.  Code for roof insulation in my area is minimum R-49, however, I believe they will allow R-38 if you’re using closed cell spray foam and it’s a continuous transition over the top plate.  My building plans call for 28″ tall raised heel roof trusses, so we have plenty of room for whatever type of insulation we want to use.  We will be installing a wood plank ceiling (without drywall or any other air barrier) to the bottom of the trusses and will need to leave a little room for fire sprinklers, can lights and other wiring between the insulation and the ceiling.  

We had been planning to use a hybrid flash and fill with about 3.5″-4″ of closed cell spray foam on the back of the sheathing, followed by blown cellulose (or possibly open cell spf) to reach the R-49, however, the insulation contractors are telling me that I won’t really save any significant amount of money over just spraying the whole thing in closed cell because of the extra trip/labor to apply different materials, rather than just spraying it all ina could passes of closed cell at one time.  I’ve been told that by every contractor I’ve spoken with (and I’m building in a somewhat remote area so I don’t have a ton of options), so let’s just assume that’s true for purposes of this discussion.

I am trying to make this home “green” by making it as energy efficient as possible and by using sustainable materials where I can (and in the future I plan to add solar), however, with the cost increases I’ve seen over the past 18 months we’re way over our budget, so my main goals here are to make the house as energy efficient as possible without spending more.

My two main questions are:
1) How thick does the closed cell spray foam need to be?  I’ve also had several (but not all) insulation contractors tell me that closed cell spray foam is very high performing and that after about 3″ I’d see very diminishing returns on my investment.  They’re also telling me that they have performance data that they can share with the building officials and are confident that the officials would allow us to use only 3″ upon review of that data.  I’ve read all of the articles on GBA and five homebuilding and the DER paper, etc., and regardless of thermal performance, I’d be very hesitant to go less than 3.5″-4″ (to get to real world center R-20) just because of moisture and condensing surface concerns, so 3″ seems like a bad idea to me, but given the very significant cost savings (being a spread out rancher, we have a lot of roof deck surface to insulate), I didn’t want to just dismiss the idea out of hand.  I’d love to go to R-60, but I can’t afford it.  I’m leaning toward just going to R-38, but I wanted to get some GBA Community opinions on the issue.  

2) The second related question has to do with the roof truss/rafter depth and thermal bridging.  As mentioned above, we had originally planned to use a flash and fill (or batt, etc.) approach, which is why the trusses were designed to be so tall.  Our plans have already been approved by the county, permits have been issued, trusses have been ordered, and I can’t afford to pay my architect to make changes again, so we’re going to keep them as is.  A picture of the truss detail is attached.  Given what we have to work with, what would be the best way to insulate this roof?  If we end up going with R-38 closed cell spray foam, that’ll be about 5.5″-6″ of spf, so the entire exposed portion of the top chord of the trusses will be encapsulated, but there’s no thermal break between the top of the top chord and the sheathing and the other 22″ of the trusses will be exposed thermal bridges, transferring heat up to the roof in winter (we get 100″ of snow annually in this area) and transferring heat into the living space in summer (less of a concern, but still…).  I’d like to use R-Zip sheathing for the roof as a thermal break, but it’s too expensive. I’ve thought about adding rigid foam strips to the top of the trusses but not sure if that’s structurally sound?  I’ve also thought about coating the exposed portion of the trusses with a couple inches of spray foam (probably won’t be cheap, but it’s a smallish surface area so maybe won’t be terrible) and maybe doing rigid foam strips on the bottom of the truss chords so that the ceiling can still be attached cleanly while fully “encapsulating” the trusses to mitigate any thermal bridging, or possibly fully wrapping the trusses in rigid foam board (28″ strips on both sides and skinny strips on the bottom) and spraying/taping the seams as a possibly cheaper way of “encapsulating”  the trusses?  I’m not trying to build a passive house and it will have a standing seam roof so risks of damage from ice damming from the thermal bridging is somewhat reduced – does anybody think I’d be fine just using closed cell to R-38 (or maybe a little higher) and not worrying about the thermal bridging from the exposed rafters? 

Thank you all in advance for your thoughts and suggestions.


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