Choosing Insulation
Colleena
| Posted in Energy Efficiency and Durability on
building a New home and doing research on insulation options. There is so many options on exterior insulation my head Is spinning. We are leaning on this idea and would like to know if it is a good idea or not?
2×4 walls sprayed with 3 inches closed cell foam
Osb
tyvek drainwrap
1 1/2 inch xps
We live in Michigan zone 5
Will this work or is a moisture issue? I’ve read articles with different opinions it’s all confusing.
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XPS and closed cell foam both have high global warming contributions.
For exterior rigid foam, EPS or Polyiso are more environmentally friendly.
In the stud cavities, cellulose, fiberglass, or open cell foam are more environmentally friendly and cheaper. Although they have lower R-values per inch than closed cell foam, the impact on whole wall equivalent R-value is pretty small because of the thermal bridging of the studs.
Whether the Tyvek goes under the foam or over the foam depends on how you decide to mount the windows. It can be done either way as long as the Tyvek and window flashing work together properly.
Where in Michigan are you building? I live in Linden (near Flint).
Are you doing the work yourself or hiring someone?
We have been looking into roxul too. But I feel we would need to go with a 2x6 wall. And when you add eveything else the wall would be 8 inches thick. I'm not sure that's route we want to go. From what I've been reading polyiso doesn't perform as well in the cold. We live near port Huron. We will be our own contractors for the house. I haven't found any local builders that specialize in energy efficiency. From anyone I've talked too so far not many are interested in doing above and beyond code and seem to think its a waste of time and money. I don't think so.
Colleen,
Installing a layer of rigid foam on the exterior side of a 2x6 wall is a good idea. This will increase the R-value of the wall, reduce thermal bridging, and reduce air leakage.
If you install exterior rigid foam, then installing spray polyurethane foam between the studs is a waste of spray foam. The spray foam is expensive, not particularly green (because most types of closed-cell spray foam are manufactured with a blowing agent that has a high global warming potential), and the spray foam won't perform much better than dense-packed cellulose.
For more information, see these articles:
Calculating the Minimum Thickness of Rigid Foam Sheathing
How to Design a Wall
Choosing Rigid Foam
Would you still stick with the 2x4 wall if using the cellulose? How do you feel about roxul instead of cellulose? we have cellulose now. Not convinced it's the best and I don't care for loose insulation.
Colleen,
The article I linked to (How to Design a Wall) explains my philosophy. In your climate zone, a 2x6 wall with an adequate thickness of exterior rigid foam makes much more sense than a 2x4 wall.
I agree that "loose insulation" isn't good. If you are having cellulose insulation installed, it needs to be installed using the dense-pack method. If you can't find a local contractor with experience using the dense-pack method, then mineral wool insulation, carefully installed, is certainly an option.
Builders that are eager to go beyond code are hard to find in this area. Custom builders may be willing to go beyond code if you are willing to pay the extra cost. The bids will probably be padded a bit due to the uncertainty of doing things differently. We were fortunate to find a builder that was willing to be a partner on designing the house. He was open to doing things differently and brought a lot of knowledge to the table that I wouldn't have gained from reading GBA. I think he learned a lot from my house and I hope he applies it to future projects. I doubt if he would be interested in going as far as Port Huron, but I would be happy to ask him if you would like.
Is there a reason that you are concerned about the wall being 8" thick?
You are correct that there is a concern about cold weather performance of Polyiso. In exterior walls, we ended up choosing graphite infused EPS which has the same R-value per inch as XPS, is cheaper (at least for new stock), is stable over time, and is environmentally better. We used regular EPS where we could increase the thickness to get the R-value we needed.
A few friends that have built recently in this area all have been recommended to do 2x4 walls with spray foam by their builders and the insulator we contacted said that's what he would do. We were shocked cause when we built this house 18 years ago. 2x6 was the way to go and I've always thought that. I like the interior look of a house with 2x6 better with the thicker window sills. I guess my concerns for the 8 " thick wall is windows and doors. Doesn't this create a problem? And more money? Our blueprints are still be worked on and currently are at 2x6 which he said we could change to 2x4. That's why I feel the pressure to know exactly what we should do before final print.
Colleen,
A 2x4 wall with spray foam insulation is a pretty lousy wall. There's lots of thermal bridging through the studs, and you have only 3.5 inches to work with.
if we went the 2x6 with cellulose would the steps be same
Tyvek drain wrap
1-1/2 rigid
And for Windows we want them to be out, not in Windows
My understanding is you need the drain wrap between osb and rigid to direct any moisture out
So do we install regular tyvek over foam also to tie in with Windows?
Colleen,
Your approach will only work if the rigid foam has an R-value of at least R-7.5. This is explained in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing. So either polyiso or XPS would work -- but not EPS. Note that XPS is not very "green."
I'm not sure what you mean by "drain wrap." Every wall needs a water-resistant barrier (WRB). If you install housewrap as your WRB, it can be installed on the interior side of the rigid foam or the exterior side of the rigid foam. For more information, see Where Does the Housewrap Go?
DuPont has a product called tyvek drain wrap. It's kind of crinkly looking. It's the product they use in there how to video for installing exterior rigid foam From what I gather it would direct any water moisture down out of wall between osb and foam
Colleen,
Tyvek Drain Wrap will work fine. Remember that wherever you choose to put your WRB, it has to be integrated with the window flashing and the door flashing.
Foam cavity fill isn't necessarily wrong, but the type of foam is. The thermal bridging discount is huge. A 2x4 wall with 3" /R20 of 2lb closed cell foam has almost exactly the same thermal performance of a 2x4 wall with 3..5"/R13 half pound open cell foam, despite the higher center-cavity R, due to the thermal bridging issue. And the 3" of closed cell costs more than twice as much as 3.5" of open cell.) Save the foam budget for the exterior.
Furthermore, open cell foam @ 3.5" has a vapor permeance about 3x that of standard interior latex paint, which allows the sheathing to dry toward the interior at a reasonable rate, whereas 3" of closed cell foam restricts the drying rate down to only ~10% of what it would be with latex paint for the vapor retarder. (What takes a month to leave through the latex paint takes about a year with 3" of closed cell foam.)
The air sealing of 3.5" of open cell foam is as good or better than 3" of closed cell foam. Open cell foam uses half the polymer per R as closed cell foam, and is blown with water instead of climate-damaging HFC245fa.
XPS is blown with a mixture of climate damageing HFCs, which give it the higher labeled R/inch. But that performance boost over EPS of similar density is temporary, and does not last the full lifecycle of the house. In 50 years that R7.5 for 1.5" decays logarithmically to about R6.3, the same as 1.5lbs density. EPS is blown with comparatively low impact pentane, a low environmental impact gas, and it is small molecule that escapes very rapidly (most of it escapes and is recaptured before it leaves the manufacturer.) It's R-value is stable over time.
Polyisocyanurate is also blown with pentane and a mixture of other (generally low impact) gases. It's labeled R6/inch is higher than it's actual performance when the mean temp through the foam is under 40F, so in zone 5 location when used on the exterior of R13, derate it to R5 inch or so. It's usually cheaper per labeled-R than XPS too.
Bottom line, save the high performance foam budget for the exterior. Installing 2" of polyiso on the exterior and using 3.5" of open cell foam is the same thickness wall as a standard 2x6 wall, and it outperforms the proposed 3" closed cell foam + 1.5" XPS. AND it's more resilient, since it gives the structural sheathing a good drying path, AND it beats code minimum by about R5 (whole wall performance, all thermal bridging accounted for), a ~25-30% reduction in heat transfer compared to a code-min R13 + 5 c.i. wall, compare to only a 10-15% improvement with the closed cell + XPS solution.
Bumping it up to 2 x 6/R20 open cell (or cellulose) + 2" of polyiso would add another ~R4 to the whole-wall performance number, and it would still have sufficient exterior-R for dew point control at the sheating, though bumping the polyiso to 2.5" or 3" for additional margin would not be insane, and would still likely have a lifecycle cost rationale on just the energy savings.
Colleen, I used 2x6 walls with 2 1/2" exterior foam with 3/4" boards to hold the foam and frame the window openings. I ordered Marvin windows and just gave them the wall width which adds up to 9 3/4", and they built the windows and sliding glass door jambs to match. I ended up with a 6 1/4" jamb from the inside of the window to the face of the sheet rock. I was very pleased with this approach.
Steve what was your process in layers?
2x6 walls
Osb
Did you use a barrier between osb and rigid foam?
Steve. Looks very nice by the way!
So we have for sure decided to stay with 2x6 walls
The idea of open cell foam is interesting we are looking into it
I guess I thought the area of special concern is where osb and exterior foam meet. As I understood there needs to be some kind of air space so in case of water it could be directed away and that it just needs to breath. . That's why I thought drain wrap in that location. Now I'm thinking since we want out set Windows to do
Open cell foam in 2x6
Osb
1-1/2 inch to 2 inch exterior foam. Grooved backed for the breathing/air?
Then house wrap and Windows
Will this configuration be better than the first? I hope so! Lol,
Colleen,
The concern about having a tiny gap between OSB sheathing and exterior rigid foam only arises when a builder fails to choose insulation materials that allow inward drying. The standard advice hear at GBA is that exterior rigid foam is an excellent way to go, because it addresses thermal bridging, reduces air leakage, improves the R-value of the wall, and keeps the sheathing warm and dry all winter long. Once you decide to adopt this approach, though, it's best to choose vapor-permeable insulation materials for use between the studs.
I like dense-packed cellulose between the studs, but other materials -- blown-in fiberglass, mineral wool batts, or even fiberglass batts -- can work if they are installed conscientiously.
This approach also requires that there be no interior polyethylene.
Once you follow this advice, you don't need any crinkly housewrap between the OSB and the rigid foam. The OSB will stay remarkably dry all winter long -- dryer than if the wall had no rigid foam.
By the way, a wall doesn't need to breathe. It just needs to be designed well.
Colleen, It sounds like you are getting close to selecting wall layers that will work. For 2x6 walls, you would need to have R7.5 of exterior foam for moisture control as described in Martin's article https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing. If you go with 1 1/2 inches of foam, that dictates that you need one of the higher R per inch types. If you go with 2 inches of foam, regular EPS would barely get you there. Like you, I live near the northern edge of zone 5, so I didn't feel comfortable barely satisfying the rule. I went with 2 inches of graphite infused EPS to get about R10.
You stopped listing layers at the Tyvek. What do you have in mind for cladding?
Once you get the layers figured out, you need to draw cross sections that show how to handle transitions at windows, the foundation, and the roof. (My builder used exterior foam once before my house and the experienced turned him away from it for awhile. They spent time debating these details at the job site with a crew sitting around. Having good drawings ahead of time prevents that.) Drawings of how I handled these details are at http://lindenairporthouse.blogspot.com/2015/12/walls-and-insulation.html and http://lindenairporthouse.blogspot.com/2016/03/windows.html. You will find other examples on GBA and other building science sites.
The cladding is probably siding and a stone siding around bottom that I found. I'm still looking at that and have to see it in person. I am still so unsure of what to do about insulation. Not having something between the osb and foam just doesn't seem like a good idea. Too many conflicting opinions. This is my forever house and making it right is most important
As to using spray foam in walls. We did thermal imaging today. My dense pack cellulose far outperformed the places where we used open cell foam. Those were corners and the band between first and second floor. Something to bear in mind. Dana was correct on dense pack cellulose.
Decisions on insulation, decisions on cladding, and other decisions can be related, so there may be some iteration in your planning process. Don't regard any decisions as firm until you have made the other decisions and determined that they are compatible. That is why it is so important to do good planning before you start to build. Once you lay the foundation of the building, some of your options start to come off the table.
Siding (either vinyl or fiber cement) are easy to do with exterior foam at the thicknesses you are considering. Brick presents some different challenges. The weight of brick is supported by the foundation and tied to the wall for stability, so you need to ensure that the details at the top of foundation / bottom of wall work out appropriately. There are a lot of different cladding products designed to look like stone. Some of them are much heavier than siding but are still designed to be supported from the wall. Do some research on the product you are considering to ensure that it will work with the wall structure you are considering.
Old thread, but similar question. I'm also in MI and we landed on a 2x4 wall with 1" GPS (R 5.0 ProBoard Red Label with a perm of 2.4) exterior foam and Benjamin Obdyke Drainwrap behind LP Smartside.
Our GC and insulation company are recommending 2" closed cell spray foam, but everything I read in GBA says that you want open cell, dense cellulose, or something that can dry to the interior in the cavity. Can someone please confirm that the closed cell spray would really be a bad idea and/or reference a good article on the vapor/moisture trapping issues? Thanks in advance!
>"Our GC and insulation company are recommending 2" closed cell spray foam, but everything I read in GBA says that you want open cell, dense cellulose, or something that can dry to the interior in the cavity. Can someone please confirm that the closed cell spray would really be a bad idea and/or reference a good article on the vapor/moisture trapping issues?"
At just 2" most closed cell foam is still north of 0.5 perms- a Class II vapor retarder- not a huge moisture trap.
But it's also not a good investment if thermally bridged by framing, which robs it of it's potential performance. Do the math:
https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
A 2" flash-fill in a 2x4 or 2x6 cavity (R12-R14) underperforms R13 cellulose or open cell foam in a 2x4 framing due to the severe 2" thermal bridge, and uses more polymer than a full-fill of half pound foam in a 2x8 cavity ! Unless a lower vapor permeance is required in the stackup (not the case with yours), it's on the less-green & more-expensive end of the spectrum, even when using low global warming impact HFO blowing agents rather than the US standard (but banned under the Kigali Amendment to the Montreal Protocol) HFC245fa. This isn't a close call. At any given R value even HFO blown foam has about twice the greenhouse gas footprint of mid-density fiberglass (like R13 batts), whereas cellulose is greenhouse-gas negative (since it's sequestered carbon.) See:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
With a relatively vapor open R5 on the exterior having a vapor permeable cavity fill in a 2x4 wall is safe in IECC zone 5 or lower, and even pretty safe in zone 6 if the siding is back ventilated &/or a "smart" vapor retarder is used on the interior side. Obdyke DrainWrap isn't anwhere near as good as Rainslicker from an outward drying point of view, but it offers at least some capillary break. If you're north of zone 5 (northern MI & da Yoop: https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTdlfwxGoQLIU1oJYZxiKqupQV87ZFF5DrWQ2YIo-5xeOeHZq0&s ) going with cellulose (which shares the moisture burden without damage or loss of function) and 2 mil nylon (Certainteed MemBrain) under the wallboard as a vapor retarder is cheap insurance.
Nate,
I think these sorts of questions are a little tricky to answer. Is the closed cell an up-charge? Does the closed cell use water as the blowing agent? If the answers are "no" and "yes," I don't think you have an issue.
Is the GC planning on closed cell in the attic? If so, that may explain why they are pushing this option. It would be easier for the sub to foam everything in one go and then move on to the next home.
If they want to charge more for CC in the walls or are using an older foam formulation, I would opt for a more standard assembly (exterior rigid plus air permeable). But one of the experts might have other ideas or suggestions for you.
I don't think it's an upcharge for the CC, and there are some places where CC makes sense. Either way, I'm not focused on the cost, more the health of the wall assembly. The proposed closed cell is HEATLOK HFO Pro, which has a GWP of 1.
With your wall assembly, closed cell doesn't really hurt you because your wall will be fairly vapor open to the exterior, which will allow it to dry to the exterior. You'll be even better if you have a rainscreen gap in there.
I would not used closed cell spray foam in the wall though, because you won't get a full fill -- usually only about 3" at best in a 2x4 wall -- and between that and the thermal bridging of the studs, closed cell spray foam offers no performance advantage over other types of insulation. You'd be much better off with open cell spray foam if you want to go with spray foam, since open cell is overfilled and then trimmed, ensuring you get a full fill in the stud cavities.
Personally, I'd go with mineral wool or dense pack cellulose in the wall over any type of spray foam, unless you are getting some kind of smokin' good deal on the spray foam installation.
Bill
Thanks for the insight Bill. Yeah...in Zone 5, we're 13+5 and 3" of closed cell would be too much from what I understand, so 2" is spec'd to get R14. I would prefer to fill the cavity AND get better sound proofing than the CC enables. The structural and sealing benefits of the closed cell are enticing, but I'm liking the other solutions better. But it's good to hear that you think the foam sandwich isn't a huge concern.
I wouldn't trust 2" to get R14. R12-13 is probably more realistic over the long term, especially considering that it's extremely unlikely you'll get an even layer 2" thick. Chances are you'll have spots that are thicker, and spots that are thinner too, and it doesn't quite average out (Allison Bailes has a good article as to why this is the case). I don't really thing the structural benefits amount to much in a wall myself.
A full fill of open cell spray foam will do a good job of air sealing too, and is probably even less likely to have missed spots since it will be a full fill. Mineral wool doesn't do much for air sealing, but is easy to install well. Dense pack cellulose is better for air sealing, but much trickier to install correctly. Personally, I'd do a conventional air sealing job on the exterior prior to insulating the wall regardless of which type of insulation you ultimately decide to use.
BTW, I have one wall of one room of my home insulated with closed cell spray foam, and a few other "walls" between some sliding glass doors that are only a stud bay or two wide too. The only reason I did that with closed cell spray foam is because the total length of "wall" was maybe 20 feet, and it cost me something like $100-150 extra to have it done since I was already doing the cathedral ceiling where I had no choice but to use closed cell spray foam. I requested that the installer "fill the walls", which got me about 3" or so, and they did have to trim down a few high spots with a sawzall. This was with a GOOD installer too -- it takes some skill to get an even layer of foam.
Bill
Thanks Bill. I've been telling the builder since day 1 that I wanted 1" exterior rigid and open cell full cavity in the interior. Having both the builder and the insulation guy push the closed cell got me questioning that approach, but this conversation is reinforcing that the better solution IS really the open cell or dense pack cellulose. Now I just need to convince them and make sure I'm not causing issues in other areas. More to come in a new comment : )
>"... 3" of closed cell would be too much from what I understand, so 2" is spec'd to get R14. "
R14 HFO blown foam in a framing cavity would underperform an R13 cellulose or R13 open cell full fill due the higher thermal bridging of the framing. There is no GOOD reason to go with closed cell foam here. There are much cheaper ways to air seal a wall than closed cell foam (which doesn't air seal the doubled up framing at top plates, jack studs, etc anyway- those details still matter.)
NateWil To my mind the large scale use of spray foam insulation on new construction plans is a huge red flag pointing to poor planning in an effort to quickly solve a problem with zero regard to the costs in dollars or to the environment. Spray foam is the most expensive ungreen way to get an R value of insulation in your home and best reserved for the few spots where there is no room for anything else.
Consider reading this article and making a different choice.
https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
Walta
Thanks Walta. I've read that article...multiple times, and agree. Since that article was written, I think the CC foam has gotten greener, and the quote from the installer is really pretty reasonable. Cost isn't as much of an issue as performance and wall health...and wanting to be green.
>" Since that article was written, I think the CC foam has gotten greener, and the quote from the installer is really pretty reasonable. Cost isn't as much of an issue as performance and wall health...and wanting to be green."
Then cellulose it is! See:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
Thanks Dana! Cellulose is moving to the top of the list. Nice graphic.
Thanks all for the responses to the thread / Q&A hi-jacking. I have another dilemma where the lap or b&b siding meet stone veneer. On 3 sides of the house, I'm either ending the foam and siding at the top of rim joist or top of foundation and transitioning to stone veneer without foam (don't want the stone to be proud of the siding). The rim joist pockets will be 2" or 3" closed cell depending on whether I run the exterior foam over the rim joist or not. The back side of the house is the big challenge where we have 2x6 walls in the basement walk-out area, and want to have stone part way up the walls. My current thought is to end the foam at the same location as I do on the other 3 walls, insulate the 2x6 walls to R21, and run 1" furring strips over the parts of the 2x6 walls where there will be siding. The other option would be to keep the exterior foam over the parts with lap siding, but to avoid an insulation imbalance, that forces R13 in the 2x6 wall and we would then only have R13 in any part of the wall with the veneer (which realistically, isn't "that" much wall). Any thoughts? Attached is my PowerPoint Engineering rendition of the system and question. And yes, I'm open to the idea that both options are terrible and there is a better way to do this...
It's better to have continuous exterior foam behind both the b & b siding AND the stone veneer, even if it means dropping back to a 2x4 wall to make space. A 2x4/R13 + R5 c.i. wall has pretty much the same performance as a 2x6/R21 wall, and having the foam between the wooden studs and the exterior keeps the studs warmer & drier in winter, and protects it from peak moisture drives in summer.
Stone veneers and other masonry siding have a much higher moisture drive due to the fact that they absorb & retain dew/rain wetting moisture. Sun exposed portions of masonry cladding deliver extreme bursts of moisture drive toward the interior whenever the sun heats up the cladding. A gao behind the cladding acts as both a capillary break against liquid water, and if vented to the exterior both top & bottom will remove a large portion of the intense water vapor when ever the cladding is heated by the sun, creating a convective drive purging the saturated air in the gap, replacing it with drier outdoor air.
The Z-flashing at the transition is fine (you may want use EPDM flashing tape rather than metal flashing to avoid the thermal bridge)
Thanks again Dana for taking the time to respond.
The walls are already built, so dropping back to a 2x4 in that area isn't really an option.
And I understand...getting the external foam everywhere and adding air gaps would be a much better solution. The stone veneer is also going to extend down onto the foundation. Ideally, we'd have a clean break...siding on the wood frame and stone on the concrete, but the contractor stepped the foundation in a way that we have to have mixed materials (siding over the concrete w/furring strips in some places, stone over the wood frame in others). So that brings me back to the original question...is it better to do the inside walls at R13/R14 and have the R5 external insulation only under the siding? Or...do the inside at R21 and furring strip the siding out from the wall?
I guess we could consider putting foam under ALL the stone all the way around the house instead of directly to the concrete, but then we end up with the stone sticking out proud of the siding by the thickness of the foam.
As far as far the barrier behind the stone, I was thinking of FlatWrap, then HydroGap, then another FlatWrap layer prior to lathe, but could do something like Slicker or similar to provide better drainage if this would be a big help and not be a real visible difference in thickness from the other stone applications.