Insulation for detached garage with trusses, Climate Zone 4C
I’m looking for some suggestions/comments for insulating a detached garage with a modified parallel chord truss roof, as shown in the attached picture.
The garage will be fully drywalled, and will have a minisplit system. I’d like to get the right amount of insulation to help the HVAC, without wasting money (we’re in a mild climate 4C, Portland).
The builder has this insulation approach in the proposal:
-R-19 Unfaced 24×96 990730 PPS (B) Ceiling Insulation
-R-38 Unfaced 24×48 990425 PPS (T) Attic
– R-21 Unfaced 15.25×93 991185 PPS (T) 2×6 Exterior Walls
Options:
– R-23 INSULSAFE R MAX BIBS 2×6 651587 (T) 2×6 Exterior Walls
I’m thinking of accepting the R-23 option for the walls (+$895), which as I understand is blown in insulation, instead of batts.
But I’m not sure the ceiling/attic insulation portion makes sense. I understand that we need to insulate the area right above the ceiling. But since we’re not planning to use the “attic” space formed between the roof and the ceiling, do I need to insulate the “attic” (which I assume means the bottom of the roof)? I think we can just leave that part vented, with soffit and ridge vents?
The picture shown in the attachment has insulation only along the bottom of the roof (none at the drywall ceiling), which I don’t think is a good approach(?).
Looking forward to some helpful advice and guidance – thank you!
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Replies
This can be thought of as a cathedral ceiling with an attic at the ridge, and it doesn't really differ from any other vented roof. There isn't any reason you can't put the insulation against the ceiling planes in a traditional manner, leaving that wedge of attic empty above the insulation depth. The room I'm sitting in now is like that, but formed with rafters and collar ties instead of parallel chord trusses. I'm going to infer that it was drawn at the roofline in that way because it was easy.
BUT, insulating a truss with batt insulation is rather difficult; the batts will not fill the cavities between the chords, webs, etc., so you would either need to cut small bits of batt to fill all those voids, or just insulate the ceiling with loose fill insulation that will fill those cavities.
If you only have R19 in the ceiling as a cost saving measure, the loss from the voids may not be a big priority though....?
I'm actually planning a detached garage build at the moment, and that's one of the two truss styles I'm considering to maximize ceiling height (the other is a raised heel scissor).
You'll have to confirm with the builder, but my guess is that the plan is for a vented roof. The ceiling insulation likely just refers to batts against the sloped ceiling at the sides, while the attic insulation is for batts on the flat part of the ceiling.
It does look like there is room for a lot more than R-19 at the sides though. There would be less heat loss with a consistent R-28 across the entire ceiling. But as Chris says, the insulation value will be undermined by the gaps between the truss members. Filling those gaps with pieces of 1-1/2" EPS would be a good idea, but likely not included in your builder's quote.
Thanks Chris and Frank, that makes a lot of sense. So perhaps they plan to use R19 in the sloped part and R38 in the flat part of the ceiling, which they called "attic". But you are right, in either case we'd have the truss open webs which ideally would be filled with EPS, which is probably not in their plan.
A couple of questions - is there a significant energy loss if they do not fill the open triangles with EPS?
Also, is it possible to use damp sprayed fiberglass to fill the sloped portions (leaving a vent gap under the decking), to avoid cutting the triangles for the web?
Yes, some type of loose fill insulation is usually used with trusses instead of batt insulation, for that reason.
A full length vent chute can be added along the roof deck between each truss to provide the ventilation gap and keep the loose insulation out of the gap. This is a standard detail for a vented cathedral ceiling. No need to continue all the way to the ridge with the vent chutes, because that little attic wedge won't be full of insulation.
A good place to look for advice on insulation types is discussions on insulating scissor trusses, which are virtually the same as yours in context of insulation settling along a slope, applicable types of insulation, etc. Or cathedral ceilings, for the same reasons.
There is at least one good discussion here at GBA on this topic for scissor trusses from what I remember, and a few on cathedral ceilings too.
Picking an R value that will save enough energy to recover its costs will be no small feat.
The number of variables is mind boggling.
How many years will you own the property?
How many hours a year will the space be conditioned and at what temps.?
If you want a real number then you will need to build a computer model with your variables in BEopt.
https://www.nrel.gov/buildings/beopt.html
If you are the average home owner that moves every 7 years and you heat the garage 6 weekends a year the answer is likely about R4 to 6.
If you’re going to be spending 10 hours a day for the next 60 years with it conditioned the same as your bedroom then R23 wall and an R 60 attic seem reasonable.
Please find the time to read this article.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
Thanks Walta, these are good thoughts and considerations. I was in my previous home for 27 years, and now that I'm retired, my plan is to stay in this home for a similar length of time, knock on the wood :). I don't think I'll be in the garage more than a couple of weekends a month, but I do want it to be conditioned to a "tolerable" level even while I'm not there (but our cars are), so say 55F in the winter and maybe 85F in the summer?
My primary goal is comfort, second is getting decent return on my investment (even if I don't actually ever get it all back), and I really do not want to go to extreme measures to squeeze out the last bit of efficiency. After all, this is a garage, not my primary residence, and I know that even with the middle of the road approach, it'll be more efficient than our house is.
I read in the past the article you linked, and I think the first approach ("Vented assembly with fiberglass or mineral wool insulation") would work for me - the trusses are close to 20" deep, so I should be able to get R49 insulation (~14") plus a vent channel above, if my assumptions are correct. I'm just wondering how to deal with the open webs that create the air pockets, as others have mentioned. That article doesn't really address that issue, though I'm reading some other articles about scissor trusses that do.
I like that plan and the fact that you have 20 inches is great and you can avoid the expensive ungreen foam insulation.
When you are shopping for a heat pump make sure to check its min temp limit not all of them will go as low is you are planning.
Consider adding a resistance heater that will allow you to quickly get the space up to a comfortable temp with a remote controlled thermostat. As the HP may take hours to get from 55 to 68.
Walta
Thanks Walta, these are great tips about the min temp of the heat pump and adding a resistance heater.