Confused on insulating converted covered deck
Hi all. I am hoping for guidance on the best way to insulate a converted covered deck. My understanding is that this deck was built with the intention of it being converted to conditioned living space at a future date. I am now in the process of doing said conversion.
Details of the structure:
A) I am a few miles south of Charlotte, NC in zone 3A.
B) The structure was built on a vented crawl space. I am not sure the technical term, but it is cinderblock on the inside with brick on the outside (no cinderblock is visible outside).
C) There is a vapor barrier on the dirt floor of the crawl space. There is approximately 3ft of height from the dirt floor to the underside of the 2×10 floor joists.
D) The walls of the structure are 2×6, although ~50% of the wall space are windows.
E) The roof is about a 2.5/12 slope “shed style” and unvented, with a metal roof. The rafters are 2×8 and there are 3 approximately 2×6 sky lights.
F) I intend for this area to be heated and cooled by a mini split.
Specific Questions:
1) For the ceiling, because it is a “hot roof” with no vent, I think I need to do everything that I can do to air and vapor from entering this area? Adding a vent would have been ideal, but it is not possible unfortunately. I had considered a closed cell foam, but am now leaning toward rockwool. If I go the rockwool route, would I need a vapor retarder on the room side of the rockwool?
2) For the walls, likewise, I am thinking rockwool would work here. Would I also need a vapor retarder on the room side of the rockwool? The outside of the walls are plywood/osb with a tyvek wrap, with HardiePlank cement siding.
3) For the crawlspace, encapsulating it sounds like it would have been ideal, but given that it will be conditioned with a mini split, this is not a feasible option. It is a small enough space that I am planning to insulate with batts between the 2x10s. From my research, it looks like air sealing the rim joist will be very important, but I am confused on the usage of the rockwool. Because they are 2x10s, there is approximately 9.5″ of space that I can fill. When I called the manufacturer of Rockwool, they said that I could use the rockwool that is made for 2x8s and leave a air gap between the top of the rockwool and the bottom of the subfloor. Meaning, the rockwool would be flush with the bottom edge of the 2×10. Friction fit to temporary hold and then put a layer of foam board over the entire bottom, being sure to tape the seams, to hold it all in and provide a vapor barrier. Does this actually make sense? I am concerned the air gap my allow critters easier nesting space, if any get in some how. Also, does it make sense to have the foam board act as a vapor barrier? They make a rockwool made for 2×10, but of course it is not available. I could try to combine some other smaller sizes, potentially compressing some of the rockwool to fit. Energy wise, this is overkill for my climate area (3a), from an R value perspective, but it is a small enough area that the cost will be marginal.
4) On the inside, the area will likely be used as any conditioned part of the home, most of the time, but given all of the windows, there will be periods of time where all of the windows are left open and the humidity in the area may rise. In a bad case scenario, I could see someone forgetting to shut the window and rain coming in. I am thinking of using purple board on the walls and ceiling, to help mitigate any of this possibly happening. Other than costing a little more, is there any downside to this approach?
I have been driving myself crazy trying to figure this all out. If you could lend any guidance, I would be so very thankful.
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Replies
2021 IECC: https://codes.iccsafe.org/content/IECC2021P1/chapter-4-re-residential-energy-efficiency#IECC2021P1_RE_Ch04_SecR402.1
1. I don't believe your concept will work for your hot roof. You might want to review these concepts to see if one will work for you instead: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work Flash and batt?
2. Mineral wool is good. Painted drywall should act as a vapor retarder.
3. With a vented crawlspace you do not want a vapor barrier beneath the insulation. You want a vapor barrier at the floor level and at the ground level. According to the IECC you need a minimum of R-19 for the floor so 5.5" thick mineral wool would work in this scenario and since you're using it for the walls you could possibly buy in bulk and just use the same insulation.
4. No downside that I am aware of other than a higher cost.
Good luck.
Hmm, I left a pretty lengthy reply to you, but it seems to be missing. I will give it a little while and retype it, if it doesn’t show up.
It's strange, I got an email saying you had left a response "flash and batt could be an option...." but now I don't see that response here?
——-removing duplicate, see below———
I am so sorry! I do not know what I am doing wrong. I tried to submit the reply again and it seems to also have gone to the void. Something in my message must be being flagged for moderation or something. I do not think I am saying anything spammy etc. I took a picture of what I am trying to say and attached it to this post. If there is a better way to send it, please let me know.
1. I would not use open cell spray foam on the roof decking. This could cause moisture accumulation and rotting. You could do full depth spray foam but I would only use closed cell for the first R-5 similar to the flash and batt detail. Spray foam is higher cost, can off gas, and is less environmentally friendly than other insulation types.
2. Open cell spray foam would work for the walls but it will cost more, off gas, and is less environmentally friendly than other insulation types.
3. You could do a similar flash and batt with mineral wool where you spray foam the first R-5 and then use batting below the spray foam to reach the R-19 total value. This could also be accomplished with R-5 XPS blocks cut and fit into place with spray foam used to seal the gaps. Blocking or strapping could be used to hold the mineral wool in place. I would not use open cell spray foam on the bottom of the subfloor. It could retain moisture and cause water damage. This is another situation where closed cell spray foam should also be used to act as a vapor barrier.
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Again, thank you so much!
1) I will need to make an account to go back to your link (it wont let me see the article again without one), but I though it said to do the first 4 inches as closed cell and then the rest with a batt to get to your desired r value. If I am just doing the first 5 r vale, are you saying to just do about an inch of closed cell foam (google says closed cell is about 6-7 r per inch)?
3) In the case of closed cell, google seems to think it is 6-7r value per inch. So I would need an inch or so on the subfloor, followed by a thick enough batt of rockwool to get my r value. If I am using blocking or strapping, I am picturing that the batt would be flush with the bottom of the 2x10 face. Will having that air gap between the batt and the spray foam be ok ?
1. The amount of closed cell spray foam you need is dependent upon the climate zone you are in as well as the ratio of closed cell spray foam to batt/open cell foam. If you look at the article on the fine home building site for the Flash-and-Batt it says for climate zones 1-3 you only need R-5 to keep the dew point located within the closed cell spray foam but (I assume) this requirement assumes you are only installing the code required amount of insulation. If you install more than the code minimum insulation you will need more closed cell spray foam in order to keep the dew point contained within the closed cell spray foam. It looks like for your region you want to keep the ratio at a maximum of 5:1 for example if your spray foam is R-5 you don't want your batt insulation to be greater than R-25 but if you want to increase to R-30 batts you would need to increase your spray foam to R-6 and conversely if you wanted to use R-19 batts you could reduce the spray foam to R-4 (but I would not go less than 5 just to be on the safe side).
"If you want to use this approach, the entire assembly must have a high enough R-value to meet minimum code requirements. In addition, building codes require the spray foam layer to meet the minimum R-value needed in your particular climate zone to avoid condensation problems. Here are the required minimum R-values for the spray foam layer: R-5 in Zone 1, 2, or 3; R-10 in Marine Zone 4; R-15 in Zones 4A or 4B; R-20 in Zone 5; R-25 in Zone 6; R-30 in Zone 7; and R-35 in Zone 8. The R-value of the total roof assembly will be higher than these numbers, of course, because the fluffy insulation also provides significant R-value."
3. You'll want it flush with the bottom of the spray foam. You want to avoid the air gap as it could be a potential location for condensation. I was imagining blocking between the joists. It might be a bit tricky to install this assembly but it should work regardless. You could always just add extra insulation and put blocking on the bottom of the joists but you would then need deeper closed cell spray foam to keep the dew point location within the closed cell spray foam. There are wire hangers, netting, and all sorts of different ways to support the insulation.
Thank you for the reply!
I found this, “ IRC R702.7.1 allows closed-cell spray foam to be its own vapor retarder at ≥1.5” thickness”. In my zone 3, as you pointed out, a r5 is all that is needed (with the standard code min insulation) which is somewhere just under an inch. It sounds like I would be missing out on the vapor barrier element of the closed cell spray foam, if I kept it to r5 depth only?
P.s. It looks like all of my other replies from yesterday showed up as well. I’ll delete them, as they are duplicates.
I don't see the 1.5" requirement. https://codes.iccsafe.org/content/IRC2021P2/chapter-7-wall-covering#IRC2021P2_Pt03_Ch07_SecR702.7.1
Thank you again.
In the link you sent, it talks about the perms of various vapor retarders. Class II being less than 1.0 perms (but greater than .1).
From what I can tell, google seems to indicate that most closed cell foams are less than 1.0 perms at 1.5 inches. Which would make closed cell foams a Class II vapor retarder at 1.5+ inches.
Also from the link you sent (at the bottom), I see this.
For purposes of compliance with Tables R702.7(3) and R702.7(4), spray foam with a maximum permeance of 1.5 perms at the installed thickness applied to the interior side of wood structural panels, fiberboard, insulating sheathing or gypsum shall be deemed to meet the continuous insulation moisture control requirement in accordance with one of the following conditions:
1.The spray foam R-value is equal to or greater than the specified continuous insulation R-value.
2.The combined R-value of the spray foam and continuous insulation is equal to or greater than the specified continuous insulation R-value."
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Am I off base? (I very well may be, ha)
I see. Chapter 7 is dealing with the walls. According to R702.7, Exception 4, a vapor retarder is not required for zone 3.
If you use closed cell spray foam in your walls you will not want to fill the entire cavity but only the first inch to ensure the perm remains above a 1 as per note c in table R702.7(2).
As the thickness of closed cell spray foam goes up, the permeance goes down since it allows less water vapor through the insulation. Once you get over 3 or 4 inches it will act like a Class I vapor barrier which is not permitted in climate zone 3 walls.
Personally, I'd just go with mineral wool or fiberglass batts and no foam for the walls since they're easy, inexpensive, and fit the requirements.
Section 806.5 of the IRC also has some good information regarding the ceiling/roof.
https://codes.iccsafe.org/content/IRC2021P2/chapter-7-wall-covering#IRC2021P2_Pt03_Ch07_SecR702.7.1
Thank you again!
Totally agree on the walls. No need to spray foam the full thing. The only area that I am considering is a perimeter 4x12 LVL that sits above the walls. I am attaching a picture, but there is a ~2 inch gap between the LVL and where the finished drywall will be (since the walls are 2x6). Given I am now doing some closed cell on the project, I was thinking I could just fill the gap with closed cell as well. Originally I was going to do rigid board. Any opinion on the matter?
I don't think it would make much difference either way. Whichever is cheaper/easier is what I'd do.
Looks good. Lots of windows.
Hi again! I hope you are well. I have been working with these local spray foam companies and it has been really pulling teeth to get decent quotes for CCSF for the ceiling. The primary issue seems to be job size, but they also are complaining about 12"OC ceiling spacing and height of the ceilings, among other things.
I am still trying to accomplish #5 from the original article you linked, "Flash and Batt". Something dawned on me, but I was hoping you might thought check me. #3 in that article essentially had rigid foam applied to the top side of the deck and then some form of fluffy insulation applied to the underside.
1) Could I just put the rigid foam on the underside in place of the CCSF? I would then put the rockwool below it as planned.
2) If so, I would imagine I would have to use some great stuff or similar can spray foam to seal the edges? This sounds labor intensive, but it is not a huge area and I think I could do it in a day or 2.
Thank you!