CEI – combining different types of insulation
Hi—I need some direction on my wall assembly. I am building a Passiv type home in the mountains West of Denver – climate zone 5. The exterior wall is framed with 2 x 8’s and then from the inside to outside is 5/8″ drywall, 5 1/2″ of Rockwool batt, 2″ of spray foam, with 1/2″ OSB sheathing- with taped seams. For the exterior continuous insulation, I was originally going to use 6″ of Rockwool Comfortboard 80 with furring and rainscreen—but the rockwool is very pricey, so I was thinking of using 4″ thick polyiso and then 2″ of Rockwool Comfortboard on top of it to create the 6″ of CEI. The WRB is Commercial Tyvek-D applied to the OSB to prevent wicking and create a small drainage plane between the polyiso.——-one other detail–the Polyiso is a factory second and has the fiberglass paper on one side which I was planning to orient towards the outside of the wall. I have a few questions on this assembly:
1. Do you see any issues layering 2″ of Rockwool over 4″ of polyiso” –btw, I am using fiberglass girds so no issues on attaching the furring.
2. Do you see any issues for vapor drive or drying? I would think the 4″ polyiso kind of prevents the OSB from drying to the outside
3.For the 2″ of sprayfoam on the inside, would closed cell be a bad idea since it would prevent drying to the inside? and open cell would be a better choice?
4.Should I abandon this polyiso idea and just bite the bullet and use 6″ of Rockwool?
Thanks!!
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Replies
1- No, except for cost. I'd just use ALL polyiso, and no comfortboard. You'll get similar performance, maybe even a bit better, with all polyiso, but less cost. When you are using a layer of polyiso anyway, the higher vapor permeance of the Comfortboard is essentially cancelled out for the wall assembly, so you're not gaining anything by keeping some of the exterior insulation as mineral wool.
2- You will have limited drying to the outside with fiber faced polyiso, and minimal towards the interior due to the spray foam. I would skip the spray foam layer completely, since it's not really necassary, and air seal the exterior sheathing instead. Fill the entirety of the stud bays with mineral wool, or fiberglass batts if you want to save some money and/or have any issues getting thick enough mineral wool (and don't want to layer two sets of thinner batts to build up a thick layer). The spray foam layer here is probably for air sealing, but there are other ways to do that that are both cheaper and greener, by strategically using caulk and canned foam. I'm not a fan of thin layers of spray foam like this for two main reasons: first, the spray foam adds unnecassary cost to a project that could be better spent elsewhere (such as more exterior rigid foam), and two, the spray foam pretty much ALWAYS produces an uneven surface that the batts won't perfectly fill.
3- Open cell would be better for drying, but skipping the spray foam completely is really the best option in this assembly. There is NO NEED for spray foam here, it does nothing except add cost. Do air sealing the "old fashioned way", with a bead of sealant between the perimeter of the framing and the exterior sheathing, tape the seams in the sheathing, and use canned foam to seal any/all penetrations in the sheathing or framing.
4- Polyiso is fine. You're way beyond the minimum for your climate zone with 6" of rigid foam here, so your wall should be fine. If you want additional moisture protection, use a smart vapor retarder behind the interior drywall instead of using comfortboard on the exterior.
Bill
SkiHouse,
+1 on all Bill’s advice, especially skipping the spray foam, which adds nothing but risk.
Have you done any energy modelling of the walls? I would bet that In zone 5 the benefit of the extra 2” of exterior insulation will be next to nothing.
Thanks Bill and Malcolm - one slight correction - we are actually in a local Denver ski area which is climate zone 7. I did check the code tables - one strange thing, none of the tables include 2 x 8 walls - the thickest wall is 2 x 6. So I interpolated by adding 25% extra R value to their values - I am assuming this is ok? so per tables, it would be R23 + 15 for a 2 x 6 wall so I added 25% which gives me approximately R28 + 20. Which is why I went with 6" of CEI - Do you think I could get by with 4" of CEI? -------would be easier!
BTW, the reason I was using the 2" of Rockwool over the 4" of Polyiso was because we are in a high fire risk area, so I was hoping the 2" of Rockwool would help minimize the risk - is this valid vs using a full 6" of Rockwool?
One other complication - the available factory 2nd polyiso is only available as 4 x 4 sheets that are 4" thick - so I was planning on taping them and spray foam any obvious seams- is the 4 x 4 size a bad idea given I can't overlap the seams?
Lastly, I know some GBA experts sigh loudly when we propose new "perfect wall ideas" - but the 4 x 4 polyiso above sparked this idea - one obvious problem with polyiso is that it is not vapor open---what if I cut 2" thick Rockwool Comfortboard into 4" wide strips - then place them on their sides around the perimeter of the 4" thick polyiso - basically forming a "picture frame" of Rockwool around the polyiso which would then create breathing channels through the wall from the OSB to the rainscreen - not to difficult to do and creates a vapor-open wall assembly - sorry for causing any sigh's ------thanks again, Brian
The important thing with the exterior continuous insulation is that the RATIO of exterior to interior R values is correct for your climate zone. If you, for example, double the R value on the interior, you need to double the R value on the exterior too to maintain the same overall ratio of interior to exterior R values. As long as you extrapolated from the tables like that you should be ok. Refer to Martin’s classic article on “rethinking rigid foam” for more info.
Bill
Thanks Bill---so I also did the ratio as well - with 6" of CEI and 7" in my bay, I get 24/28 +24 = 0.46 which I believe is right in the range of the recommended 0.43 ratio. What did you think of the 4' x 4' x 4" thick polyiso surrounded by picture frame Rockwool idea? ---might be easier to just alternate rows of 4' x 4' polyiso with rows of 2" wide x 4" deep Rockwool above it. This would result in 4" thick wall of insulation, so I would then cover the whole thing with 2" of Rockwool for fire protection which gives me 6" of CEI - creating in theory, a somewhat vapor open
exterior wall. ---Brian
SkiHouse,
As Frank said, 4" of polyiso is enough on its own.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
You don't need to worry much about the seams between the foam. Tape will suffice. Introducing small areas of mineral wool between the sheets unfortunately doesn't do anything useful, as unlike air-barriers, vapour-barriers are effective proportionate to the area they cover. You would get no appreciable drying.
A 7 1/4" stud cavity filled with batts, with 4" continuous exterior insulation is a pretty good wall.
Thanks everyone----very helpful information ---I would like to go with a 4" polyiso CEI - much easier than 6"----my only remaining issue is the fire protection - we are in a high risk fire area - which is why I was hoping 2" of Rockwool on the outside of the polyiso CEI would help mitigate ---------assuming the consensus is that the rockwool would help, --should I use 4" polyiso with 2" of rockwool on top (6" total) or try to find some 2" polyiso and put the rockwool on top of that (4" total)---thanks in advance for all or your input----brian
Rockwool is more fire resistant (it's basically fireproof) than polyiso no question. Using fiber cement siding, or other non-combustible siding material, would be another way to get better fire resistance for the structure. You basically want the exterior materials to be non-combustible to reduce the chances of ignition, and to slow the rate of rise of the temperature of the more interior materials to keep them safe too.
Bill
This is an extrapolation, not interpolation and just be advised as you read other sections of code that this is generally not allowed.
You didn't mention any vapor retarder, so you are likely planning to use latex paint on the drywall for the required class III vapor retarder.
In zone 7, use of a class III vapor retarder is allowed with at least R-15 continuous insulation over a 2x6 wall, so extrapolating to R-20 over 2x8 is reasonable and 4" of polyiso will provide that.
Thanks Frank---yes---I was planning on just latex paint -----but now, based on some of the above info, I think I will change my wall assembly a bit-----for fire risk, I think it might be best to use just 4" of Rockwool Comfortboard 80 on the exterior wall. Since this will be vapor open, I should be safe--- I am also thinking for the vapor barrier to use 2" of closed cell spray foam on the inside bay wall and then fill the rest with rockwool comfortbatt. (I know Bill is not too fond of spray foam but it is done all the time up here and it really seals up the house super tight. ) --So this would now give me R16 on the outside and R37 on the inside ---total wall is now R53----ratio is now 0.3 ----(below the preferred 0.43)------but this apparently is not much of an issue since the Rockwool Comfortboard is vapor open -------but still theoretically could get some condensation??--Does everyone think this is a robust wall and that 4" of exterior rockwool is enough?-----one thing that I keep wondering - this house is in a Colorado ski resort - the air here is very cold and super dry! - so I don't see where the vapor/water would be coming from to condense against the OSB - (these ratio recommendations don't seem to take into account outdoor humidity) - especially with the spray foam on the inside which would block any inside vapor penetration- thanks everyone - this is kind of a big decision and is driving me crazy!---brian
Brian,
You measure the ratios from the interior face of the first condensing surface, which in your case is the 2" of closed cell spray foam, so you are safe.
Like Bill and Akos I don't see what benefit that layer adds that air-sealing the sheathing hasn't already given you, but that's your decision to make. It does no real harm.
Vapour drive in cold climates is towards the exterior. The moisture that moves through and can accumulate in walls is warm interior air, so the climate doesn't make that much difference, indoor humidity does. The main benefit of cold, dry climates is the ability of the outside layers - cladding, trim, etc. - to dry. That's a lot different than it was when we didn't adequately air-seal and insulate houses. Then houses in cold dry climates didn't have the same high indoor humidify in winter that houses elsewhere did.
SkiHouse, I'm probably THE, if not "one of" the most pro-spray foam people on GBA... And *I* am telling you it doesn't make sense here.
Many builders use spray foam as a sort of "wide brush" to do air sealing. Spray foam can cover up all kinds of sloppy issues that should have been addressed in other ways. If you're careful, and it's really not all that difficult, you can achieve equally good air sealing results using just tape, caulk, and canned foam. You end up using less materials, spending less money, and then you can put that money towards other uses that will get you more bang for your buck. An easy example I can think of in your situation would be to skip the spray foam, and use some of the money saved to put in an interior side smart vapor retarder. That would avoid a vapor barrier layer in the wall, and it would help keep moisture out of the wall while still allowing for drying towards the interior when conditions allow. Since you seem to be very concerned with drying, I really think that interior side vapor retarder would gain you a lot more than spray foam here, and it will do so at much less cost.
The other possibility would be dense pack cellulose so that you could take advantage of it's moisture buffering abilities. This would also be cheaper.
I really think you should reconsider your use of spray foam here. "Everyone else does it" isn't really a good argument in favor of spray foam here.
Bill
Thanks Bill - all of your insight has really helped with this. So now, the wall would be from inside to outside: 5/8 drywall, Smart Membrane, 7 1/2" of probably rockwool batt, over 1/2" OSB then going outside, taped OSB seams, Tyvek Commercial D housewrap, 4" of Comfortboard 80, Rainscreen.
Two questions:
1. Is the 4" of Rockwool comfortboard 80 adequate?
2. Any benefit from using the money saved from eliminating the spray foam and use Blueskin to cover the exterior of the OSB instead of just taping it? Seems that would create a bullet proof air and vapor barrier or is a waste of cash and better to allow the OSB to dry to both sides?
3. BTW, I have only framed the garage so I am thinking to switch to real plywood on the walls - money well spent or a waste?
thx!!---brian
Brian,
I'll let Bill answer, but one small point. If you are going using rockwool not foam, there is no benefit to the Tyvek Commercial D over regular Tyvek Commercial.
Based on your original plan for 5.5" of rockwool in the stud bays, that 4" of comfortboard still gets you around 40% of your total R value on the exterior, which should be safe. Note that removing R value on the exterior is riskier than removing R value on the interior in these type walls.
Blueskin probably would be better, but I doubt it would be enough better to justify the cost. All you really need here is a good air seal at the seams between panels. I suppose you could just use Blueskin in "one roller's width" over the seams though. I've always thought of it more as an "all or nothing" type product, so I've never really thought to try only using it in a few locations.
Personally, I prefer plywood. Plywood has better fastener holding power, and is more tolerant to getting wet, compared with OSB. OSB is cheaper again (yay!), and OSB tends to be very dimensionally predictable whereas plywood quality these days isn't what it used to be. I think you still come out ahead with plywood, but probably not by all that much.
Bill
If you get rid of the spray foam (which I agree is not providing a lot of value for the money here) and fill the stud bays with batts or cellulose, then R-16 on the exterior will fall a bit short for protecting the wall, so you would need a vapor retarder on the inside. Regular polyethylene plastic will work fine to block the moisture, which is coming from the interior in your climate. You could also use a variable product like CertainTeed MemBrain.
If you put a vapor barrier membrane on the OSB, then the interior vapor barrier will have to be a variable product to allow drying to the inside. But I think it is better to allow vapor to pass through the wall to the exterior. Otherwise, it will just accumulate in the OSB all winter long until the weather warms up enough that it can start drying back to the inside.
Frank,
The ratio may be a bit short, but if the exterior insulation is permeable does it matter? Once the spray foam is gone the sheathing is in much the same situation as if there was no exterior insulation in terms of drying ability to the outside, but does benefit from some warming, even if that warming is not sufficient to always keep it above the dew point.
With Denver's dry climate and the permeability of the Rockwool, it would likely be fine. But the IRC isn't that nuanced, and still requires either a class I or II vapor retarder.
Frank,
For sure. Go with the code!
This is what I would build:
http://effectiver.ca/calculator/wall.php?id=4272
The exterior insulation can be any type, MW or rigid foam work just as well (permeable is always more robust). Edmonton is zone 7. 2" rigid is pretty easy to deal with.
No need for any spray foam as long as you tape the seams on your sheathing. The center of cavity R value with spray foam looks impressive but because of the thermal bridging of the 2x8 studs, the assembly R value is about the same as cheap batts. You are wasting your $ plus you might end up with issues if the spray foam install is off.
thanks Akos----I like that calculator program! ----what do you mean by "issues if the spray foam install is off"-----do you mean if it is too thin or missing areas??? or improper mixture??
Bad mix, using the wrong temperature material, mixing precursor batches, installing onto sheathing that is too cold or too wet. You name it, unlikely to happen, but can happen, and it sucks if it happens to you.
Spray foam to me falls into that category to use it when there is no good alternative and no more.
Brian,
The problems occur relatively infrequently, but are often difficult or impossible to remediate. It's a rare month when we don't have a thread in the Q&A asking for help from someone who has had it happen.
I'd consider upgrading the air barrier from tyvek commercial D.
There's a cost but, vaproshield, Henry, tremco, sika among others make products more robust with better performance, better detailing and less leaking.
stamant1,
I think the primary air-barrier is intended to be the taped sheathing. The Tyvek is the WRB.
Hi all----all this decision making is kind of fun!---for me, this is my first home build - I am a retired doctor. Update on my air barrier - yes it is the tape - so I found this green tape called THICC on amazon - very inexpensive - applied it on a "warm" day - about 25-30 degrees and it wouldn't stick at all to the OSB - kind of annoyed that I cheaped-out on tape!! So I then took my propane weed torch and heated up the tape on the OSB - amazing - it nicely softened the glue on the tape and it is now permanently bonded to the wood - impossible to remove - so now I am happy with my cheap tape - but I don't trust my framers with my propane torch, so I ordered a heat gun from Amazon for next week. will update.
I believe that OP is considering tyvek directly over OSB with the tyvek as the WRB and the OSB as the air barrier.
One problem with this configuration is that it requires a lot of diligence by the installer to plug any errant fasteners. During install of siding or furring, it's pretty common to install temporary fasteners to hold up cleats or to pry against or any number of other reasons. These holes rarely get filled and create discontinuities in both the WRB and the air barrier.
Similarly any penetrations for MEP are often post installed to the tyvek. Most installers are not going to cut the tyvek back to detail the penetration to the OSB and then patch back the tyvek. It's enough to expect them to only put one wire or pipe per hole and tape them off to the tyvek.
on the other hand, the self-healing WRB/air barriers can handle small holes and have published warrantable details for addressing larger penetrations. this added cost is something to consider in the mix.
stamant1,
I don't think there is much to distinguish one type over the other. Are you sure any of the self-adhered WRBs you listed are self-healing. I understood only rubberized membranes were? As with any WRB you still need to diligently flash and seal penetrations. In the wall stack-up the OP is proposing I don't see they offer any real advantage over a robust conventional sheet WRB, which comes in much larger widths, relies on laps rather than adhesion for water tightness, and can be installed in any weather.
Hi all----here is my take-away from everyone's above brilliance and input (and a recent post from Martin on 9/6/24 titled "A Fresh Look at R value") - BTW, obsessing on wall assembly is a good thing because like cement, once it is done, it would be very difficult to go back and change it!----please provide feedback on any failures in my logic below.-----here is my proposed wall: 5/8" drywall, Membain VP, 4 1/2" rockwool batt, 2" CC spray foam, 1/2" plywood, 41/2" rockwool comfortboard 80, 3/4" rainscreen, cladding.
A. I am still leaning towards using 2" of closed cell spray foam. I know the spray foam and the flash/Batt decision is clearly debatable, complex and regional specific. Yes, the cost for SF is a bit more - but perhaps offset a bit by saving labor and material for the air barrier and perhaps even CEI thickness (assuming I am correct that the ratio is calculated from the inside surface of the 1st condensing surface--ie the foam). Here are some of my reasons for including spray foam:
1. I am trying to build a somewhat Passiv home so air tightness is super important- would like an ACH50 below 1.0 if possible. I still believe it is next to impossible to find every single air hole/gap in a wall - so the "Wide Brush" of spray foam makes some sense to me. Also, am I interpreting Martin's 9/6/24 comment correctly?----".....Because of these factors, a leaky wall assembly insulated with fiberglass batts often performs worse than a wall assembly insulated with spray foam having the same R-value as the batts. The performance differences are due to spray foam’s ability to reduce air leakage, not to any difference in R-value between the two materials. To obtain the best performance from fiberglass insulation, the Energy Star Homes program requires most fiberglass-insulated framing cavities to be enclosed by air barriers on all six sides. While the recommendation is sensible, it’s sometimes hard to achieve in the field. If such a six-sided air barrier can be created, however, fiberglass insulation will meet the performance expectation promised on the product’s R-value label. (- SEEMS TO ME THE SAME WOULD HOLD TRUE FOR ROCKWOOL BATTS and air-sealing all 6 sides sounds almost impossible!)-----sounds like Energy Star is almost encouraging foam.
2. WHERE IS THE WATER COMING FROM? - per above, "Vapour drive in cold climates is towards the exterior" - so to me, it almost seems that for the water accumulation problem we should be less concerned about the CEI but rather it is more important to catch the water at the source by putting in a Membrain under the drywall and making the wall air-tight- I guess however, given time, water could still accumulate so CEI is still important - even so, my wall easily dries to the outside
3. CONDENSING SURFACE- If the closed cell spray foam indeed moves the condensing surface to the inside layer of the foam, wouldn't this be a good thing because it gets the "condensing water" away from the OSB ? in my wall the condensing surface would now be between the inside surface of the SF the Rockwool batts and even if it got wet, would be able to dry towards the inside through the batts and Membrain--do I have this right?
4. CEI - do have this right? for calculating the ratio, the "exterior layers" would now include R12 from 2" of CC spray foam on the inside of the OSB plus R18 from 4 1/2" of Comfortbat (total R30) --so with a 2 x 8 wall filled with 2" SF and Rockwool batt, this gives me an R52 wall, with the ratio of 30/ 22 + 30 = 0.57----- did I do this right?
5. BTW, I am using 3 layers of 1 1/2" Comforttboard 80 because Lowes sells this at a very good price of $54/ bag -----any issues putting up 3 layers?
thanks, Brian
Reply to post #30
1- You can get air tightness done just fine by sealing the exterior sheathing. There is NO NEED for spray foam to do this. A sheet of plywood is an excellent air barrier, as is a sheet of drywall. If you want to build a tight home, seal the exterior sheathing as I've described previously, and ALSO detail the interior drywall air tight. Pay attention to transitions and corners and you'll have a very tight home with NO NEED for spray foam. Spray foam is just a crutch to cover for poor detailing in this case, and is completely unecassary if you're careful with your construction details.
Mineral wool is much denser than fiberglass, so air doesn't move through it as well, making the air barriers less important as far as keeping the batts performing well. You still want the air barriers for air tightness of the home though.
2- Moisture is sneaky and nearly always finds a way in. Your CEI will keep the interior side of the exterior sheathing warm enough to avoid condensation, and condensation is where the problem comes from, not the moisture itself. A smart membrane on the interior behind the drywall helps to limit how much moisture can get into the wall, but still allows for drying, which is like extra insurance against moisture and is also the way I like to build.
3- Since you plan to use CEI anyway, you're already dealing with the condensing surface issue so spray foam isn't helping you here. Without CEI, you can have the interior side get below the dew point and condense out moisture if your R value ratio is off. Moisture is going to migrate through the batts regardless, and it will condense out on the first surface it "sees" that is below the dew point of the moist air. With spray foam, that surface can be the surface of the spray foam, or, in the case of a thick layer of spray foam, somewhere inside the foam. With a wall that has enough CEI, the interior side of the exterior sheathing is kept above the dew point, so you accomplish the same thing, again, WITHOUT the need for spray foam. Spray foam is NOT a magical material here, it does nothing that other materials can't do UNLESS you NEED a fully adhered insulating layer for some reason (usually an unvented roof assembly).
4- Yes, the spray foam can count towards the exterior R value since it's essentially a vapor barrier in this case. I'd allow more around R10-R11 for 2" though, and I'd handicap that down to more around R9-R10 because it will be VERY difficult to keep that layer that even. There is no averaging here, any areas of the spray foam that are thin enough to get down below the dew point temperature are at risk of condensing out moisture from the air. This is one reason your spray foam idea can actually be more risking. If you use enough CEI to keep the interior side of the exterior sheathing above the dew point, you are ensured of a nice, flat layer that is ALL above the dew point temperature, so no moisture risk.
I'd put your wall more around R48 or so, to be conservative. All you do is add the R value up from all the layers though to arrive at the overall R value.
5- I see no issue with three layers of comfortboard aside from the labor of putting them all up.
I would seriously consider eliminating the spray foam layer from your assembly here. I really think it's only wasting money and not gaining you anything. You seem to be thinking that spray foam is the only way to get an air tight assembly, but that's not true.
Bill
Brian,
As I said earlier in the discussion: the spray foam doesn't really cause any problems, it just doesn't add anything to the walls. If you want to include it, your ratios are fine, but I think it's fair to say - as pretty much every commenter has - there really aren't any good building science reasons to do so.
1. Martin's quote, and the Energy Star recommendations, are in the context of leaky houses, not new, well air-sealed construction. If you look at the high performance houses being featured on GBA, there isn't any evidence that spray-foam is necessary to get low ACH numbers. As a strategy it's the outlier, not the norm.
Fiberglass batts are more susceptible to wind-washing than rockwool, although both only lose incidental R-value, as you see from tables 9 & 10 in this link: https://www.rdh.com/wp-content/uplads/2017/10/Van-Straaten-Windwashing.pdf
2, 3, & 4. When we talk about the "first condensing surface", the distinction is often made that it be "the first condensing surface of interest". In other-words, it's not necessarily a bad thing, it just one that needs attention. What matters is that the rate of drying exceeds the rate of wetting. With your exterior mineral wool, the sheathing will accumulate very little moisture, and that moisture can dry to the outside. It's when you interrupt the ability of the wall to dry to the outside, by adding an impermeable surface like the spray-foam or exterior foam that the ratios come into play. The sheathing on walls with deep cavities filled with permeable insulation (like double stud walls) - dry to the outside fine - and that's a without the exterior mineral wool which you will have to keep the sheathing warmer.
5. The only issue is that each layer will require the same amount of labour to install and fasten as one layer would have.
In case one more vote for no spray foam is needed to make your decision here’s mine. It makes absolutely no sense in this scenario. I would vote for a SA membrane like Henry with liquid flash or high quality tapes for your penetrations and plates. If you actually think about your air barrier layer and how things will detail out in that layer you will get a better seal then if you just throw spray foam at it. I think spray foam is going to be our generations asbestos with remediation and removal being a whole profession once all these houses need remodel.
Hi Bill/Malcolm/Freyr-------thanks for all your input!--I am not completely sold on the spray foam, I am not really trying to beat a dead horse here, but - are you thinking the air tightness has a point of diminishing returns? --ie. Bill says "you can air seal just fine" and Malcolm says there aren't any building science reasons to use spray foam, just curious if you have building science references supporting/clarifying what you are saying? The reason I ask is that this is a Passiv-ish home so air tightness matters----Matt Risinger posted a video where he did a super good job air sealing job on a new build house with zip, tapes, etc. then did a pre and post blower door test after adding spray foam ---the ACH50 dropped from 0.82 before down to 0.27 after - so he got 3x better air sealing after adding the spray foam to an already super tight 3000 sq ft house. ---of course, the pre-foam 0.82 is already pretty good---Are you thinking this is a meaningless improvement?--thx, brian
https://www.google.com/search?q=study+passiv+home++air+tightness+before+and+after+spray+foam&client=firefox-b-1-d&sca_esv=0abcab0091fd1269&ei=FtFtZ-XGHpvH0PEP4Jj80A8&ved=0ahUKEwjlqbbdtsaKAxWbIzQIHWAMH_oQ4dUDCBA&uact=5&oq=study+passiv+home++air+tightness+before+and+after+spray+foam&gs_lp=Egxnd3Mtd2l6LXNlcnAiPHN0dWR5IHBhc3NpdiBob21lICBhaXIgdGlnaHRuZXNzIGJlZm9yZSBhbmQgYWZ0ZXIgc3ByYXkgZm9hbTIFEAAY7wUyBRAAGO8FMggQABiABBiiBDIIEAAYogQYiQVI8UpQywlY7UZwAXgAkAEAmAF1oAHCEqoBBDIzLjS4AQPIAQD4AQGYAhqgAsERwgIKEAAYsAMY1gQYR8ICCBAhGKABGMMEwgIKECEYoAEYwwQYCpgDAIgGAZAGCJIHBDIzLjOgB4xy&sclient=gws-wiz-serp#fpstate=ive&vld=cid:eb664826,vid:QhWeixZY2xg,st:0
Brian,
When I say there aren't any good building science reasons to add the foam I'm referring to two things.
The first is that foam is not necessary to get down to very low air-tightness. That's incontrovertibly true. The tightest house ever measured relied on poly as its primary air barrier, and Michael Maines' projects regularly achieve sub 0.5 ACH50 relying on using the sheathing, to give two examples.
The second is whether adding the spray foam gives you a safer wall assembly. To me the foam is curing the risk that adding the foam caused, if you get my meaning. The wall works well without the foam. The foam means you then have to worry about having added an impermeable layer to an otherwise permeable wall designed to dry to the outside, and now you are into the complications of ratios, drying in two directions - with no appreciable benefit.
Diligent air sealing of the exterior sheathing should take you well into the area where air-leakage is inconsequential. There are a number of ways to further improve that ACH. You could (as you are suggesting) coat the whole enclosure in spray foam, you could use AeroBarrier, you could detail either the drywall or the interior membrane as a secondary air-barrier. I guess you could do all four. At some point these decisions need to include a weighing of effort and result. To me in your case those would be:
- Cost vs return. Is there an ROI on the foam, even if you factored in an unlikely increase in comfort - remembering that the most common sources of noticeable air leakage for occupants are penetrations like windows and doors, and ventilation systems, none of which will see any benefit from the spray foam.
- Introduction of risk. Spray foam is one of the very few building materials that can cause irremediable issues to a house's indoor air quality.
- Environmental impact. Not just the process itself, but as freyr-design alluded to it makes the materials - like studs and sheathing - un-salvageable and makes future work difficult.
Adding to Malcolm's last point, my parents have a 45 year old closed cell spray foam house. 20 years ago the original cedar shake roof began leaking enough to show up on the drywall of the cathedral ceiling. I wasn't around at that time, but I know some of the cdx roof sheathing was rotten and had to be replaced.
In the last few years, there has been a improperly flashed replacement window that rotted out the sheathing and framing below it.
The 2nd story vent stack had a trim nail in it that only recently showed up. Tearing into it the 2nd story rim joist, 1st story wall and sheathing, and 1st story rim joist were all rotted out.
Anything that requires drilling a hole in the exterior walls requires scraping out/chipping the foam to ensure you aren't hitting anything.
A small hole was drilled down through the kitchen sink cabinet to the unfinished basement below. The sink drain ran parallel to the rim joist and was encased in foam. Drain pipe had a hole punched in the top edge that wasn't noticed when hole was drilled. Pipe also had a bow in it that couldn't be seen because embedded in foam. So it would only leak when the sink was ran for a long time and then it was difficult to find the hole and the whole length of pipe had to be tore out and replaced.
The foam hides minor water issues until they become much larger issues.
Foam tends to hide minor water issues until they become much larger issues.
JustusM,
Our regional landfill and recycling depot accepts wood waste from construction or demolition, but not if it has foam adhered to it.
Thanks everyone - you have pretty much talked me out of the spray foam---at this point, I can frame the house and seal it as best I can then do a blower door test and see what the ACH50 is at that time. ----can always add SF if it is really leaky - but probably won't be necessary. The environmental and health issues are a bit disturbing.
One final question---I am currently using 3 layers of 1 1/2" rockwool Comfortboard 80 on the outside -----------just curious, what would you think of putting 1 1/2" of polyiso against the osb then put two layers (3") of the comfortboard over over that?----my thought is that using the 1 1/2" polyiso adds an additional R3 value to the exterior wall (which you had mentioned was a bit minimal) - and more importantly minimizes the possibility of condensation against the OSB . It also makes air sealing more robust. This obviously substantially reduces the exterior permeability - but the polyiso is still somewhat permeable with a perm rating of 5 per inch so the wall could still dry to the outside----it can also dry to the inside with no spray foam---just curious from a building science viewpoint - -to me, the issue is how fast does it need to dry to the outside? - I would think the polyiso would slow it down by a factor of 10 --but it also would slow down the migration of water towards the osb----so more complex than it seems - kind of a catch - 22 ------slowing or preventing condensation might be more important especially if it is ok for the wall to dry out more slowly --kind of confusing--- thanks---brian
Brian,
The alternatives are:
- All rockwool, preferably thick enough to completely eliminate moisture accumulation in the sheathing. Practically that isn't as important as it can readily dry to the outside., but as Frank pointed out your code may have something to say about that.
- Introducing a lower perm material to that exterior mix (like foam), which means you definitely need to meet the ratios. That foam can be all or part of the continuous layer.
I don't understand why substituting polyiso for rockwool against the sheathing "minimizes the possibility of condensation against the OSB" The moisture accumulation is from interior air, the foam is on the outside.
I don't have much to add with all the experienced folks replying here, but to the question of framing it out and running the blower door test, that is a good plan. However, if you are still above your desired ACH50 I'd either go back through with the caulking gun again, or consider AeroSeal acrylic spray, which seems like it would be ideal for a case where you know there aren't any big holes (you just built it, after all), but want to seal many tiny cracks. Cost could still be less than the spray foam at that point in the construction, and you wouldn't be creating this unmaintainable layer of foam on everything to cause future problems (less greenhouse gas impact, too). I see spray foam as a bit of a necessary evil in retrofitting, but when you have the ability to build new, then I'd do whatever I could to avoid it.
Be careful with polyiso perm numbers. The polyiso FOAM itself is permable, but the most common foil facer is essentiall 0 perms. If you want permeable polyiso, you need one of the fiber/kraft faced variants.
The polyiso would only slow drying here. Any condensing would be occuring on the INSIDE FACE of the exterior sheathing, as moist interior air would condense out there. Moist are would only be condensing on the exterior surface during summer if the interior was air conditioned enough that that sheathing got down below the dew point of the outdoor air.
Moisture drive during heating season is from the interior towards the exterior, as Malcolm mentioned earlier.
Bill
Just curious freyr_design - why?
I could certainly and probably will be wrong but even now in California we are seeing extremely stringent regulations of construction waste recycling and tracking. When we are talking about demo on one of these jobs it seems like it is going to be impossible to meet these requirements with all/ most of your materials coated in spray foam. That, coupled with new focus on microplastics and their effect on health and environment, I think there are going to be specific regulations put in place when dealing with these foams so that all those fine plastic dusts arent released into the surroundings. Similar to lead abatement. But again, who knows, just my two cents.
Freyr_design,
See my post #39 above. The presence of foam moves the construction waste into another category for disposal here.
Yes exactly, and here you have to report with specific receipts all of your waste, and only specific recycling and transfer stations are acceptable.
A disadvantage of the closed cell foam is that it will prevent drying to the interior. That may become an issue if you have a water leak somewhere repeatedly wetting the framing. Being able to dry in both directions versus only one will somewhat reduce the risk of rot.
In that Risinger video, he acknowledges that he wasn't particularly diligent about air-sealing the penetrations, and even missed some, because he was relying on the spray foam for that. If he had done a good job of air-sealing to begin with, and then used the blower door to identify and seal the remaining leaks, the difference wouldn't have been nearly as dramatic.
Wall is figured out---now designing the roof---I did review the "5 cathedral roofs that work" article-----Looking for some thoughts on modifications to make one of the 5 described roofs FIRE-RESISTANT. Recall that this house is in climate zone 7 in the mountains and I would like to make the roof fire-resistant, durable and ice dam resistant. I started with the idea of using 8" of exterior polyiso with the top layer being rockwool all above the roof deck but the roof became 22" thick and looked terrible. We have 11 7/8" I joists for the roof (some of this roof is on top of exposed timber frame trusses in the great room area)--I now believe the only way to build this without a super thick roof assembly is with a flash and Batt approach similar to roof #5 above. The problem is that #5 is not fire-resistant. So I was hoping to modify roof #5 with some of the ideas in Joe Lstiberec's 2013 article "ROCKS DON'T BURN" (https://buildingscience.com/documents/insights/bsi068_rocks_dont_burn) ------similar to his figure 2---hoping this community can help me create a fire-resistant R70 roof that is less than 16" thick ------here is my current plan from the inside to the outside:
5/8" drywall
11 7/8" I-Joists filled with 6" of Rockwool then 6" CC spray foam against the plywood sheathing
plywood sheathing
vapor open breathable peel and stick membrane (possibly titanium FR)
1 1/2" or 2" of Rockwool Comfortboard 70 or maybe Comfortboard 110??
Layer of Tyvek Commercial D over the Rockwool to protect it
2 x 4 strapping going vertical to create vent channel----this would help prevent ice dams (could be talked out of the vent channel which is a fire risk but I believe I can reduce the risk with fire-proof vents and eaves.
2nd layer of plywood
2nd layer of vapor open breathable peel and stick--fire rated Titanium FR (or possibly Polystick).
Metal roof in some areas over the shed roofs and Bravo fire-rated class A rubber shingles in other areas over the gable roof areas.
Will this work?---looking for ideas to improve this---I do like the idea of having two sheathing layers and two membrane layers which I would think would reduce the chances of a roof leak---yes, more expensive but cheaper than re-roofing if a leak occurs. Am I really gaining anything here or is this overkill and a waste of money as I could easily just put a fire resistant membrane over the one layer of plywood and be done with it.-----Thanks in advance----Brian