Climate Zone 5 insulation: unvented cathedral and band joists
Hi!
I really hope someone can help as I have been reading and reading and see many conflicting information.
We live in central ohio-climate zone 5 and have recently added an addition. We would like to use spray foam in the new space, walls and unvented cathedral ceiling and band joists. Our walls are 2×6 and the exterior is insulted crane board and batton. From what I have read we need to use closed cell in the cathedral. It also seems like we should use closed cell on the band joists. We could use closed cell and then add another type to increase R value? Can we use open cell and then apply a vapor barrier?
We have gotten 2 quotes and both have said they would use open cell in every place, should this make me nervous? I want something effective but also not break the bank.
In central ohio we do get very cold winters and very warm summers, from that I have read it has to do with vapor permeability and condensation. The old part of the house is brick and none of the band joists are below grade.
The unvented area has valleys on both sides and has a double ridge beam.
Please help!
Thank you
Sarah
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Replies
Hi Sarah.
Working only from the inside of an unvented roof assembly, closed-cell foam is a good choice (you do not want to use open-cell foam here), though you don't need to fill the entire rafter cavities with closed-cell foam, you just need enough to control the dew point temperature at the face of the foam, then you can use a more cost effective and environmentally friendly type of insulation to fill the rest of the cavity. Of course you have to be able to hit your target R-value, which may be up to the building official. Please read this: How to Build an Insulated Cathedral Ceiling
If you have insulated siding, a water-resistive barrier, and sheathing, you have many options for insulating your walls. Dense pack cellulose is a great option from a performance and environmental perspective. I don't know much about the siding type you mentioned, but you will need to determine what level of interior vapor retarder which will depend on the R-value and permeability of the siding. Variable perm vapor retarders like CertainTeed's Membrain and Intello are good options when the need for a class II vapor retarder is questionable. This article may be helpful when it comes to thinking about how to insulate your walls: Walls that Work.
It may make sense to have the rim joist insulated with closed-cell spray foam since the installers will be in site for the roof. There too, the right flash coat is all that you need, then you can add other insulation. One more article for you: Insulating Rim Joists.
Thank you Brian! That definitely helps!
So just to clarify: we have insulated siding, house wrap and then osb. We could use open cell there with a vapor barrier? Or would we be better to just use kraft faced fiberglass?
Thanks for the links, I had visited one of them but not the other two.
Either open-cell with a smart vapor retarder or kraft-faced batts should work (again, with the caveat that I am not familiar with your siding). I would avoid a class I vapor retarder like interior poly.
I'm sorry but one more question. I don't think our seams were tapped (on the outside) can I tape the inside? If so what would I use?
I'm going to look into our siding and the vapor barriers you suggested.
This was a big help and helped ease my worries.
Hi Sarah.
No need to apologize.
If you were to go with fibrous insulation, you would need to seal the seams and also around the perimeter of each stud bay (where the studs meet the sheathing and where the bottom and top plates meet the sheathing to do a good job of air sealing from inside (not just taping the seams since there are long vertical seams where the panels meet on studs that you don't see from inside). You could do this with air sealing tape, but caulk would probably be a faster and easier option. But I have never tried doing it with tape, so, I may be wrong about that.
All that said, if the sheathing wasn't detailed as an air barrier, open-cell spray foam seems like a pretty good option for your walls.
I don't believe the sheeting was detailed as an air barrier so it does sound like open cell would be a good idea, less work on my part! But we would still need a vapor barrier as you suggested above?
I also just spoke with one of the companies we got a quote from and asked about doing a couple inches of closed cell. He said their company won't "flash and bat" he said it is a bad idea and that no one does it. Hummmmm I know there is a lot of conflicting information but I have yet to read that it is bad and that no one does it! I know that being in Ohio could make it more or less effective.
Your thoughts?
FWIW, CraneBoard Board and Batten siding is an insulated vinyl siding product.
Sarah,
Flash and Batt works just fine in zone 5. You just have to make sure that there is enough spray foam that you don't get condensation on the surface. This is about 1" in 2x4 walls, and 1.5" in 2x6 walls. The remainder of the wall can be filled with open cell foam, fiberglass, cellulose or any other "fluffy" insulation.
The only downside is that you have to make two passes, with two different types of insulation. That often increases costs over a single pass with a single insulation type.
Peter, Thank you!
I just read something that said flash and bat was a waste that if there was an air seal there really wasn't a need for more insulation? However the same article said that open cell doesn't hold water and I believe it does, or can... maybe it only does when exposed for a long time, is that repeated cold days or the days of constant fluctuation? I know that closed cell is more expensive but is it worth it? i don't want to do something that isn't beneficial.
Sarah this is far from my area of expertise (spray foam) but I do know that if you used closed cell and didn't fill the bays, you are increasing the thermal bridging surface area (to the extent that the bay isn't filled).
It's true that air-sealing is higher on the list than thermal barriers, but this increased thermal bridging could be significant.
Peter (or anyone) how much time is usually needed between cc foam lifts? because it can't be done in one 'go' can it?
https://www.greenbuildingadvisor.com/article/is-using-closed-cell-foam-worth-the-trade-offs
^ I'll just leave this here in case this side of the issue hasn't been made known.
Tyler,
Thank you for adding that link, I had not read that yet. Ugh, What to do! There seems to be, as I've said before, so many conflicting opinions or "facts". I don't want anything toxic, I don't want my roof to rot and I don't want to spend a ton of money for problems....
Does anyone know if we can use a vapor barrier behind open cell foam?
thanks for all the input!
With the caveat that I'm not very familiar working with spray foams, my understanding is that you can use open cell foam as you would other 'fluffy' insulations. It has air sealing abilities that most of those don't have, but not the vapor impermeance of closed cell. So you just need to build your stack-up accordingly.
If you follow the instruction in the 'how to build an insulated cathedral ceiling' article linked above, you'll be fine. Read that thoroughly and maybe click on some links to related articles from there.
The short answer is that yes, depending on the assembly, you can use other products for vapor retardancy other than cc spray foam.
You might find this relevant: https://www.greenbuildingadvisor.com/article/smart-vapor-retarders-for-walls-and-roofs
edit: Actually this might be a better general intro/overview of smart vapor retarders: https://www.greenbuildingadvisor.com/article/smart-vapor-retarders
If you've worked yourself into a corner where you really need cc spray foam, and you're concerned about its potential impacts, perhaps consider minimizing it's use to accomplish the bare essential task (air-sealing and dew-point management). This might mean working with another contractor willing to 'flash' with foam. Unless that truly doesn't exist in your area.
You may need it in the ceiling but not walls... the ceiling is the tougher stack-up to get right.
Have fun!
For the roof what is the depth of your rafters? Plan on 3” of HFO blown ccSPF (R20-21) covered R30 of batts or open cell foam. Furring may be needed to fit the insulation. This can be foam strips on the framing or crosswise wood if using open cell. These approaches get insulation over the rafters, which has a big impact on the total r value.
Pretty much anything will work in the walls. There is some more sealing to do with batts. With open cell foam plan to still seal bottom and top plates, headers, and the at least the front third of the top and bottom of each joist.
For the HFO foam, find installers of one of the these products:
Demilac HFO high lift
Lapolla FOAM-LOK 2000 4G
GacoOnePass low GWP
Icynene ProSeal HFO
There may be others as more are becoming available. Ask for the spec sheet and ask to see the resin containers on the day of installation. To confirm they match.
Great info Matt! I appreciate your input! I will definitely be asking about that!
Thank you tyler!
In Zone 5 it takes a minimum of R7.5 on the exterior of the first condensing surface for dew point control on R20 of vapor &/or air permeable insulation. CraneBoard is only about R3-R4 and vapor-impermeable, with essentially zero drying capacity toward the exterior. So an all open-cell cavity fill is going to come with some risk, even with interior side vapor retarders.
A flash-inch of closed cell foam as the first condensing surface is sufficiently vapor permeable to provide a (slow) drying path for the sheathing toward the interior, while sufficiently vapor retardent to prevent loading up the sheathing with excessive moisture over the winter from interior side moisture drives. That would allow the use cheaper & greener fiber insulation (cellulose preferred) for the remaining 4.5" of cavity depth with only a tiny hit in whole-wall performance.
With band joists covered with brick veneer on the old part of the house it makes a difference if the brick veneer is vented, since the exterior moisture drives can be quite high. On band joists with CraneBoard it has the same issues as the studwalls- a flash inch of closed cell + R15 rock wool batt would hit code performance for bandjoists in basement & crawlspace foundations.
The environmental impact of insulation materials differ by quite a bit, and even HFO blown closed cell foam has a high global warming footprint compared to EPS or rock wool at any given R-value. But organic fibers such as cellulose/wool/scrap cotton are essential carbon-negative, since it becomes stored sequestered carbon, carbon that has been removed from the atmosphere since the beginning of the fossil-burning industrial age.
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
For completeness, not shown in the bar graph, open cell foam's footprint is comparable to that of fiberglass a bit less than EPS, 1lb density foil faced polyiso foam board's runs about half that of HFO blown closed cell foam.
While 4.5" (R16.5 - R17) of cellulose is about enough to offset the CO2e footprint of the R7 HFO blown inch of spray foam + R3/R4 CraneBoard. The unvented roof insulation would still be fairly CO2e heavy with an R21-ish closed cell + R30 rock wool.
How much rafter depth do we have to work with that cathedralized ceiling?