Recommendations for insulating unconditioned 2nd level space
I am in the process of building a 1.5 story showroom (garage extension) for an appliance business. And I’m trying to figure out how I want to insulate it. I’m in zone 8a.
Some details to help frame my situation:
The showroom will be a large extension of a detached garage (where I do all the appliance repair work). That space is sometimes conditioned with a mini-split. I mostly run AC in the summer while I’m working, and on a very rare occasion I’ll run heat in the winter. This new extension will be about 1100 sq ft on the main level, and probably about 800 on the upper level. The upper level will be unfinished, and used for storage at this point. To store all my appliance parts. But its being built sturdy enough that I could finish it out at some point and have more living space up there if i ever wanted to. (Mother in Law Suite)
I dont plan to condition any of this new space at the moment. Typically my customers spend about 5 minutes choosing an appliance to buy. It rarely gets lower than 45 in my existing detached garage in the winter time, even if its 20 outside. It can get warm in the summer, but i dont plan to spend lots of time in there. And neither will my customers. So i dont see a strong reason to spend the money installing or running an AC in there. (Or maybe i should put a mini split in there just in case? hah).
On to my main question: I plan to insulate the 2×6 walls downstairs with batt insulation. And I suppose batt insulation for the ceiling of the main floor/floor of the upper level. And I’m trying to figure out what else I want to do upstairs. I’m using it for storage, but I dont want it to be 140 degrees up there in the middle of the summer, or I’m going to absolutely hate going up there. Plus it could potentially do some damage to some types of appliance parts. The attic in my house gets unbearable. I dont want that. I did some research and it looks like i could install foil backed roof sheathing that claims to help by like 20-30 degrees. Or they even sell rolls of foil that you can staple to the ceiling trusses that basically reflects the heat back towards the roof and out the ridge vents. But i was also curious about spray foam (closed cell) or even batt insulation on the roof. Is that a good idea, considering both the downstairs and upstairs will be unconditioned at the moment? Or could that be a bad idea with moisture and such? I assume if I did spray foam, that means it would be tightly sealed up there. And without it being conditioned, maybe thats bad. Or maybe i should spray foam and just put a mini split up there and set the AC temp to something high, like 84 degrees or something, so that it runs enough to take the edge off, but isnt running all the time and costing me a ton.
Any thoughts are greatly appreciated! I know this was long, but its hard to make it short and explain the situation in enough detail.
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Replies
This is not a simple question.
If the space will ever be occupied, insulate it now. Insulated spaces are constructed differently from uninsulated spaces.
If it's the top floor and the underside of the roof is the ceiling, you have what's called a cathedral ceiling. It is very, very common for these to be insulated improperly, which can lead to mold and rot which can cause the roof to fail. This article shows five techniques for doing it properly:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Thanks for the reply. Unfortunately I dont have access to the full article. Only have access to part of method #1. Is there a way i can access the whole thing without a paid membership?
I dont know if the space will ever be occupied. Its possible, and its being built with framing and floors strong enough such that it could be. But I dont know if it will be.
I've attached an image of the truss plan, to give you an idea of the space on that level. the usable space is 15x40 in the main room, and there a side room as well. Again, I dont have current plans of HVAC on the bottom level or this upper level. My main goal is for the upper level to be usable as storage and not 140 degrees in the NC summers. Ideally stay under 100 degrees.
In modern construction there really is any concept of semi-conditioned space, space is either inside the building envelope or outside of it. With that design I see three choices for how the building envelope runs:
1. Along the rafters from the top of the wall to the peak.
2. Horizontal from the top of the wall to the half wall, up the half wall, along the rafters, across the ceiling and down the other side the same way.
3. Horizontal along the floor from wall to wall.
#1 and #2 put the attic inside the building envelope, #3 puts it outside. #3 is the cheapest and simplest.
For any of these designs you have to have ventilation to allow moisture to escape from the attic. The ventilation probably won't do much to keep it cool in summer. With #3, the ventilation can just be vents at the ridge of the roof. For #1 and #2 you have insulation against the underside of the roof, this is where it gets tricky. You need to either insert air channels to allow air to circulate, or you need to seal up the underside of the roof with spray foam so that moisture can't reach it.
Let me ask you two questions: First, are you planning on having any HVAC equipment, including ductwork, in the attic? Because in that case it should be inside the building envelope. Second, what is your access to the attic? Unless it's an airtight and insulated door, the attic should be inside the building envelope.
The five assemblies in the article are:
Assembly #1: Vented assembly with fiberglass or mineral wool insulation
Assembly #2: Vented assembly with fiberglass or mineral wool batts and interior rigid foam
Assembly #3: Unvented assembly with continuous rigid foam above the roof sheathing and fiberglass batts, mineral wool batts, or cellulose under the roof sheathing
Assembly #4: Unvented assembly with closed-cell spray foam
Assembly #5: Unvented flash-and-batt assembly -- a layer of closed-cell spray foam with a layer of fluffy insulation (fiberglass, mineral wool, or cellulose)
Thanks for the response, I can visualize the insulation options that you're talking about. And really those are the options I'm trying to decide between.
As for HVAC, this entire structures purpose at the moment is for appliance parts storage in the attic and appliance showroom on the main floor. And I don't have a strong reason to have an HVAC in any of it. Customers will be in and out of the space frequently but they don't need to feel super comfortable while they're there, since they're usually only there for 5 minutes . Long term if I decide to get out of the appliance business, It will be a super nice and big garage/workshop. Or possibly can even be a mother-in-law suite. I could see at some point in the future wanting HVAC, and maybe I should just bite the bullet and put some mini splits in there now Just in case I want to use them at times . But I don't plan on having any type of ducted HVAC. Probably just mini splits if I did anything at all.
As far access, There's going to be a full size staircase from the main floor to the attic. I haven't really thought too deep into whether there would be a door or not, it could go either way.
But yeah the big thing I'm trying to figure out is if I want to bring the attic space into the building envelope like you're saying, with option one or two.. or do the cheaper option 3 and have the attic space outside of the envelope. But I really want the attic space to be a more comfortable temperature and not 140° in the summer so that I don't get angry every time I go up there, haha.
Let's say I don't do an HVAC on the main floor or in the attic. If I did spray foam using option one for example, do you think that would be enough to keep the attic at it more comfortable temperature than the other insulation options? And are you talking about closed cell or open cell? Assuming they spray foam the entire roof decking and seal it, as I your option 1, does that keep it from having any moisture problems like you're saying? I definitely don't want to deal with any type of mold or rot issues.. and I guess I don't really understand what happens in the case of a roof leak with spray foam. That's my biggest worry with spray foam.
Is there code enforcement where you are? HVAC is required by code in new buildings.
They are requiring some type of heat for the half bathroom (like a bathroom fan with heat strips)..But the rest of the building is being treated as a garage... Essentially this new building is extending an existing detached garage and attaching it to my house. My existing detached garage is where I plan to do all of my repair work and is already conditioned. This new structure is really for storage and for a showroom. Again, I guess I'm not 100% against putting some type of HVAC like a mini split in this new space, I just don't think that I'll use it very often.
I talked with an insulation company and they're recommending Open cell spray foam to the roof deck. Basically making it code in case I ever wanted to finish it out. They're saying that open cell is better than closed cell in this situation so that if there was ever any leak then we would be able to see it and it wouldn't rot out the roof deck. They're basically saying that with the spray foam, It will get a little hotter than it is outside up there but it shouldn't get significantly hotter, and that's what I'm trying to avoid. Does this sound like a good plan?
The whole "open cell so you can see leaks" has not been something the spray foam association has promoted in the last ten years. Some installers and salespeople still say it, but it's not accurate--leaks may or may not show up, and if they do, it may or may not be near the source of the leak. (Check out the spray foam alliance director's comments here: https://vtdigger.org/2023/05/22/i-wanted-to-cry-devastating-risks-of-spray-foam-insulation-hidden-from-vermont-homeowners/.)
Where are you located? Climate zone 8a on the DOE map would be Alaska (https://basc.pnnl.gov/images/iecc-climate-zone-map). I'm guessing that you're either in California or Canada, which have their own climate zone designations. Or maybe you're going by plant hardiness zones?
Yeah sorry I was looking at the NC climate zone map. I'm in zone 3. My county is in 3 which is right on the line of zone 3 and 4. This article kinda scares me away from the idea of spray foam, hah. Seems its a lot of risk.
So after reading about the horror stories with spray foam, i think im against using it. So heres an idea. See my picture. I think I want to add a radiant foil barrier on top of the roof trusses (See the yellow lines), and below the roof decking. Something like the rolls of stuff on this site: https://www.radiantguard.com/. I saw an installation method where you dip the radiant barrier down a bit between trusses to create an air gap. (see 2nd picture). Its supposed to be more effective this way as compared to having the radiant barrier flush with the roof decking.
And then, use batt insulation as indicated by the red lines in my picture, and go ahead and drywall the space as well (wasnt originally planning to).
And then just see how it goes with the temp up there in the summer time. And if it is too hot to bear, then open the stairs door that goes to the attic from the lower level, and install a fan on the opposite wall as the stairs, blowing the attic air outside. Such that it would suck cooler air up the stairs from the lower level and pull it across the finished/drywalled attic space, cooling off the attic. Something like this thermostat controlled 14" fan in the picture. So basically have it turn the fan on if it gets to 100 degrees or something.
Thoughts on my plan? Good plan? Bad plan?
And actually now I'm reading that open cell spray foam is more a concern with colder climates. Im in NC, zone 3, on the border with zone 4. I'm kinda lost now. Any advice is appreciated! I keep waffling back and forth between my idea above with using batt insulation, and just doing spray foam on the whole roof deck and sealing the space. Keep in mind that I dont at least at the moment have plans of HVAC in the attic or the main floor. Is spray foam a good idea or bad idea in my situation? My biggest fears are the horror stories with roof deck rot, etc
The open cell has to be thick enough to form a barrier that keeps water vapor from contacting the underside of the sheathing. Experts disagree on how thick that is, but 5" is a good middle-of-the-road guess. Where you are I think you need R38 on the roof, which would be 9-1/2" of foam so it's kind of moot if you go with full thickness. Be warned that foam guys are famous (notorious?) for saying that foam has a higher "effective" r-value and trying to get away with a couple of inches, that's not true.
Where it might be an issue is if you go with "flash and batt," where you spray a thinner layer of foam to get the air sealing and then fill the rest of the space with cheaper batt insulation. Closed cell foam is highly impermeable, so all you need is a thin layer and the rest can be fiberglass. Open cell you'd probably have to do at least half foam.
Thanks. Yes, we were looking at possibly R-30. The space is not going to have HVAC, so it shouldnt matter. So it sounds like R30 will be thick enough to have an air barrier. I guess the last concern i have is what happens when there is a roof leak. Which is bound to happen at some point. I've patched 5-6 holes in the roof on our house. It still seems like spray foam would be a disaster with a small roof leak. Water would get trapped in the foam right? Leading to roof deck rot.