Condensation Risk in Double-Stud Walls
Hi GBA community,
We live in south eastern Ontario, climate zone 6. We are (re)building an 800 sq. foot addition to our 1890s farmhouse, doing our best to build according to green building science and using environmentally-friendly products.
Our local building official has flagged a ‘possible condensation risk’ with our double-stud wall assembly. After much research and consultation, we have built an OSB free / sheathing-less wall, using metal bracing for sheer strength, in order to overcome the possible condensation issues that can happen with this type of wall assembly. Here is our wall from the inside out:
Gypsum board
6-ml poly barrier
2×3 non-load bearing stud wall, with R9 dense pack cellulose insulation
3 3/4″ space with R 14 continuous insulation – dense packed cellulose
2×4 load bearing stud wall, with R12 dense pack cellulose insulation
Diagonal metal bracing for sheer strength
Air barrier (Tyvek)
Horizontal 1X4″ strapping
1″ Comfortboard
Vertical 1×4″ strapping
Wood siding
Although we are awaiting his concerns in writing, our building official has said he is fine with the double stud wall, no OSB / metal bracing part of the assembly. But he is concerned that the exterior 1″ of Comfortboard and in particular the air-gap between the Tyvek and the Comfortboard (caused by the horizontal strapping) is a “possible condensation risk”.
Our understanding is that the warm inside air will hit the cold Tvyek which will be where the condensation risk is, but because there’s no OSB there, and because the wall is vapour open, designed to dry to the outside, we should be fine. We don’t understand why the building official is concerned that the added exterior Comfortboard (and that air gap) presents a new risk.
Are we missing something? Thank you for any and all advice and suggestions.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
cmylks,
It sounds like a really resilient double wall to me, with no risk at all. maybe he is thinking about it as though the comfort-board were foam?
Ideally you would not have an air gap(the 1x horizontal strapping) and keep the comfort board tight to the tyvek. Was that horizontal strapping just added to give a good surface for the comfort board to push up to?
Also, I would say ditch the 6 mil poly for something like Intello so the wall can more easily dry, but maybe you already have the drywall up?
sb1616ne,
What do you see as the problem with the air-gap outside the WRB?
Thanks for these comments about the air-gap - we too originally thought that the Comfortboard's insulation performance would be better if it was tight against the Tyvek, but as mentioned below, our contractor wanted the strapping for support for the dense pack & sheer strength.
With the building official's comments centering on that air gap as a condensation risk, we remain confused about this one. Since that air gap is outside the WRB, and since the comfortboard is vapour open, how would condensation be a risk in that air gap?
I'm not sure what the comfort board is doing in your assembly besides adding cost and complexity. If you want more R value, bump up the space between the double studs.
It is hard to get nice 2x3s, usually 2x4s are straighter. If you do go with 2x3, try to get finger jointed ones. 2x3 longer than 8' are also hard to find, usually end up ripping a 2x6 in half for long ones, adds extra work.
If you are worried about moisture performance, this is a great site that allows you to set up your assembly and see how it performs. It is a big of work to set it up and you have to do some imperial to metric conversions.
https://www.ubakus.com/en-ca/r-value-calculator/?
The winter outdoor temperature you use is the mean temperature over the three coldest months.
P.S. If the house wrap will be your air barrier, make sure to use one of the sturdier house wraps such ass Tyvec Commercial Wrap and nail it with cap staples.
Akos,
"I'm not sure what the comfort board is doing in your assembly besides adding cost and complexity. If you want more R value, bump up the space between the double studs."
That's a very good point - and one I missed, concentrating on whether it was a condensation problem or not. The whole assembly would be much simpler and less expensive without it.
I'm also curious what the comfortboard with an air gap is for? And is the wall getting dense packed, then having tyvek installed afterwards? Or is tyvek getting blown against? Not sure it can handle that.
I agree with everything
- comfortboard not adding value with strapping air gap
- using a more robust weather barrier
Maybe Tyvek commercial or something like one of the first four on this site:
https://www.dorken.com/en/residential/wall/air-water
- dense packed cellulose might not work with just Tyvek but check with a local dense pack installer.
Also consider the risk of getting the cellulose wet if you dense pack the exterior before having the siding installed. Do you know how long it will take to get siding on after insulating the 2x4 cavity? It usually takes more than just one day.
One vendor in Ontario who does prefab panel (passive house rated).
https://quantumpassivhaus.com/
Quantum also was putting only a weather barrier with no sheeting. They stopped using blow in insulation and started using batt insulation instead. My concern in Ontario with no sheeting is the risk of squirrels and mice getting into the assembly.
We chose an assembly that had a 3/4” vapour open wood fibre (rigid) panel under the weather barrier. All the seams we taped for extra “strong “ air barrier.
Be careful about horizontal strapping and diagonal strapping. They are not allowed right up against the weather barrier by Ontario building code. I can’t find the Ontario code section…sorry. Make sure you get the inspector to ok the horizontal strapping if you have it against the weather barrier.
Thank you very much for your comments, and your OBC / horizontal strapping comment may be at the heart of our building officials concern. If you do find the reference, thank you very much for sharing - we'll get online today and see what we can find.
Otherwise - interesting about Quantum opting for batts rather than blown-in - do you know their reason/s for this? Was it because that air barrier alone was not enough support for the dense pack? At this stage, we could still consider switching to batts if the sheathing-less wall risks too many 'blow out' concerns....
Otherwise - interesting about Quantum opting for batts rather than blown-in - do you know their reason/s for this?
If I recall correctly, I was told that they felt that there was a risk of the too much settlement with dense packed cellulose. I was in their factory, which was small at the time because they were just starting out (no impact on their quality). They did not have specific equipment to dense pack cellulose with prefab walls. I understood a bit more when I watched a video about Bensonwood in the New Hampshire. This video at 7 min 44 seconds shows you how they inject the cellulose with a special machine - for the prefab walls.
Cmylks
Vertical vs Horizontal stapping.
My apologies. I must have gotten this wrong because of what I see it in the British Columnbia code and in the National Building Code of Canada 2020, section 9.27.2.2
https://nrc-publications.canada.ca/eng/view/ft/?id=515340b5-f4e0-4798-be69-692e4ec423e8
https://free.bcpublications.ca/civix/document/id/public/bcbc2012/ep001029.27
I must have been reading too many different codes within Canada. The National Building Code of Canada is not applied, it is only a reference point for Provinces who develop and implement their own codes. My apologies.
No apology necessary at all! Your comments have all been very helpful. We see in the BC code that section "9.27.2.2. Minimum Protection from Precipitation Ingress" has more specifications than that section in Ontario's code. I guess they have much more rain out there than we do!
Thank you all very much for your comments, questions and suggestions.
To the point about the Comfortboard adding complexity and cost / why not just add more insulation in the double stud wall - valid point indeed. A bit more of the backstory - our wall assembly original design did not include the exterior comfortboard at all and with the double stud wall alone, we were happy with the Rvalue we were achieving. Our contractor recommended the exterior comfortboard with the horizontal strapping for two reasons - to add some rigidity to the Tyvek so that the blown-in insulation would have more support (the insulation contractor is aware of this and has not expressed concern about blowing in to a cavity that has no sheathing). Also, our contractor felt the additional layer of horizontal strapping along with the 2nd layer of strapping on the outside of the comfortboard (for the wood siding) would also support the sheer strength, in addition to the metal bracing. So this 'hashtag' effect of 2 layers of strapping was what he was going for, and the comfortboard was thought of as a 'bonus layer' adding both rigidity for the insulation and 4 more Rs.
Another point the insulation contractor recommended is for the wood siding to be installed before he does the blow-in, to ensure that we've got the rigidity needed to support the dense pack.
To the point about the risk of critters due to the lack of OSB - believe me, that has been a preoccupation of mine at every stage of this wall assembly design. In addition to mouse and squirrels, our region is host to the lovely and very long black rat snake. They had been coming into our old addition's attic for years (millennium?) to shed their skins in the spring. Our new addition is very tight in comparison to the old breezy drive shed that was there, but certainly we have been quite meticulously ensuring that the air barrier is well taped and secured, especially at the bottom and top of the assembly.
All these comments are helpful and relevant - but back to the condensation issue, which is the building officials concern. The comment made by mr_reference_Hugh about Ontario Building Code not allowing horizontal strapping against the air barrier may be at the heart of his concern. We will definitely do more research on this today!
Tyvek is vapor-open so it's not a condensing surface. It will block liquid water, but water vapor will pass through nearly unimpeded.
Comfortboard provides no shear strength. I don't understand what the horizontal strapping is doing for you, other than making installing the vertical strapping easier?
Thank you for these comments Michael. You highlight something that we had wrong in our understanding - we see your point that Tyvek is not our condensation surface, rather the Tyvek, being vapour open, will allow any vapour through where it will then meet the back of the horizontal strapping. So that 4" surface of the strapping is where condensation could form, yes?
Still - isn't that a much more minimal risk than if our walls were sheathed with OSB inside the air barrier? We were trying to do everything we can to mitigate the condensation risk (hence, no OSB) but I suppose by adding the strapping, there is a small surface area that could be affected by condensation. But how big a risk is this, given that that strapping is not structural, so rot on that surface will not lead to our house structure failing. Condensation on the strapping could lead to mold I suppose, but it's outside the air barrier so there would be little/no risk of poor air quality.
Maybe we're still missing something - thank you for helping us understand if there is some risk of condensation on the inner side of the strapping, what negative impact will this have on our wall assembly?
I don't know for sure but I would guess that a water vapor molecule that reaches the interior side of the Tyvek "sees" two options--go toward the 1x4, or toward open air. The open air most likely has lower vapor pressure than the 1x4 so it will be more inclined to go in that direction. If it does reach the 1x4, it can dry easily in multiple directions. In short, I wouldn't worry at all about the 1x4 in terms of condensation.
Thanks Michael, great point - the vapour is likely to 'run for the light' not head for the wall :)
Hello GBA friends, we finally received a response from our building official and it seems his concern has shifted. He is no longer talking about a condensation risk in the wall assembly. Instead, he has said that his issue with our wall assembly is that the compliance package we are governed by, under the OBC SB12, which requires R22 plus R10CI (Electric Heat). The building official says that the R22 MUST BE INBOUND of the R10 CI. In our case, we have an R14 CI that is in the middle of our double stud wall. Inbound of this is an ~R12 cavity created by the interior 2x4 double stud wall.
Has anyone out there, who knows the Ontario Building Code and SB12 in particular, ever come across this? We can't find anything in the SB12 that specifies the position of the R22. In fact, the SB12 outlines the following about CI:
Continuous insulation (ci) is intended to minimize the thermal bridges in an assembly. It is generally uninterrupted across all structural members. Exceptions to this include fasteners and service openings. Insulation may generally be installed on the interior or the exterior, or may be integral to any opaque surface of the building envelope. It may generally be made of various material such as board, blanket, sprayed or other types of insulation. Compressions such as blanket fasteners are permitted.
Thank you for any further support you can provide to help our building official understand and approve our wall!
I'm interested in following this discussion as I am a builder in South East On and having conversations with building officials about assemblies is getting a little more common. We have dense packed sheathing less walls recently and my recommendation regarding Comfordboard would be to install it over your air barrier/WRB and then install your furring. Tyvek may not be tough enough to withstand DP. Another option would be to simply make your double wall assembly deeper as others have suggested and use a product like Mento Plus https://foursevenfive.com/solitex-mento-plus/ this is a super robust WRB and can be used for sheathing less DP insulation when furred on the exterior.
Hi there - what township/s are you building in? We're in Leeds and Thousands Islands and our building department is not known for being very aware of green building techniques :). To your point about installing the CF directly on the air barrier - with no strapping in between - curious to hear more. Are you thinking of the OBC issue that someone else has mentioned about horizontal strapping not being allowed against the air barrier? Or are you thinking about the performance of the CF from an insulating point of view? Thanks for sharing more :)
And as for making our wall assembly deeper and/or using a more robust WRB - in retrospect, yes! But here we sit with our walls already built, siding already on - we've hit this snag at a rather advanced stage of the project :(.
We are located in Ashfield Twp (Huron County, Goderch area). Our building depts aren’t all that familiar with green / high performing building techniques either.
My thought about placing 1x4 furring between the Comfortboard and WRB is more from an insulation point of view and that it is really not necessary since you have a furring layer outboard of the CB as your rainscreen. Attaching the CB directly to the WRB will act a little like sheathing, not the sense of shear strength or anything but it will help prevent the DP insulation from bulging the WRB.
I have never heard that horizontal furring directly on the WRB is a no no from a building dept but it makes sense as it would cut off drainage/drying potential behind your siding.
Hi J_Sommer - we've had a response in writing from our building official now. His concern has shifted - he is no longer mentioning a condensation risk. He says that the SB12 compliance package specifies that the R22 must be INBOUND of the R10 continuous insulation (how would anyone ever build a double stud wall in Ontario if that's the case?!). Have you ever come up against a building official with this understanding of the SB12? Thanks so much.
I read through the SB12 and the only other mention of Ci other then the sentence you provided that I saw was
2.1.1.4. Elements Acting as a Thermal Bridge
(1) Except for a foundation wall, the insulated portion of a wall that incorporates wood stud framing elements that have a thermal resistance of less than RSI 0.90 shall be insulated to restrict heat flow through the studs by a material providing a thermal resistance at least equal to 25% of the thermal resistance required for the insulated portion of the assembly
No mention of location for the Ci just that it is required to reduce TB.
We are about to submit a double wall design for permitting so I’m hopeful we don’t run into any of these issues with our inspector. I have sent him preliminary sections for review and so far so good. Our DW is a staggered stud (2x8 R27 + 2x4 R14) with plywood sheathing on the exterior and Intello on the inside face of the 2x8 wall acting as a mid wall air/vapour control.
cmylks,
A few things around the strapping have become conflated.
- Forget about code concerns. There is nothing in the Ontario Building Code that limits the direction of strapping, as there is no rain-screen requirement in that code.
- Having the strapping behind the comfort board means its direction doesn't matter anyway.
Taking into account all the comments you have received, I would:
- Omit the horizontal strapping and comfort board, substitute a stronger WRB, and if you are worried about the bellying blocking the exterior rain-screen, to use 2x" vertical strapping to create the cavity.
- Use a continuous 1 1/2" wide U-shaped perforated metal flashing at the base of the walls. This will keep out pests and provides a base to slip the 2x" strapping into.
Edit:
" But here we sit with our walls already built, siding already on - we've hit this snag at a rather advanced stage of the project"
Just saw this - so we are back to the original question you asked - and I don't see anyone commenting that what you built poses any moisture risks.
Thank you Malcolm. Believe me, if we could roll back the clock, we wish we'd done a few things differently (no exterior CF, better WRB for example). But sadly, we sit here currently with walls already built and sided, with a project stalled while we wait for our building department to tell us more about their concern about a 'possible' condensation risk, created because of that additional CF & strapping.
The responses in this thread has helped reinforce our belief that our wall has not got a critical moisture issue. And it's also helped us better respond if/when the official provides a building code reference for what we have done wrong. Otherwise, we may just need to seek an additional engineer stamp on this additional feature of our wall assembly (original drawings had no exterior CF).
Thank you so much all!
cmylks,
I wouldn't look at it that way. What you have ended up with is a wall which will perform very well. No regrets, carry on and good luck with the rest of your build.
cmylks, you said "back to the condensation issue, which is the building officials concern."
I am copying and pasting text that talks about condensation issues in double stud wall. It comes from a BS&Beer (indirectly connected to GBA) competition on wall assemblies in 2021. If the building officials ever explains their concerns, this critique of the double stud wall may help you counter arguments.
Also see web links below.
This was posted in 3 separate pieces so you see CW1 - CW2 - Cw3.
CW 1 : "Double walls" (usually two exterior walls that are both cavity insulated using air permeable insulation) are great because they allow us to use insulations that are moisture sensitive-- we tend to prefer these insulations because they can be less expensive AND they're made of stuff that's better for the environment The difficulty is that we need to protect them from getting wet from rain AND we need to make sure we keep interior air inside to reduce the risk of wintertime condensation (because we're using insulations that are permeable to air, warm interior air wafts through the assembly and when it reaches the cold exterior, we'll get condensation). The way we deal with these two problems is: (1) we have an exterior water control membrane that keeps the insulation dry (which is included here) and (2) we have an interior air and vapor control membrane that keeps interior air out of the wall assembly (this is also included here). However, there are two practical problems (both fixable) with the wall assembly shown here ...
(continued) ...
CW 2: ... continued ...
The first issue is that the water control membrane over the wood fiber insulation is unsupported. The manufacturer no doubt tells you that this is okay ... but in my experience it's very, very difficult to detail these types of sheet membranes to be watertight unless they're fully supported by something rigid. In your assembly I'd recommend including gypsum sheathing over the fiber insulation to support the water control membrane. This makes detailing MUCH easier and more reliable. This is extremely important because this wall is not like a normal wall. This wall has a LOT of insulation. This is good! But it also means that if there is a defect in detailing (and there are always defects) the wall does not have a very good ability to dry. This means you have to be BETTER at detailing this wall than a standard wall. And that's why you'd want the gypsum. (Btw: gypsum is a good choice because it's rigid so it will give you the support you need, but it's vapor open (unlike OSB, for example) so you still get your exterior drying).
The second (less important) issue relates to keeping interior air out of the wall assembly to prevent wintertime condensation. Most of the time we just use a sheet membrane on the inside or in the middle of the wall, just as you show. But this isn't a normal wall! It's a super insulated wall, which again, means less drying. And this means that we have to be BETTER at making that interior or mid-wall air and vapor control material truly continuous than we would if it were a standard wall. Your mid-wall membrane is probably pretty good in that it's reasonably easy to detail this to be airtight (there aren't going to be many penetrations through it), but, again, it tends to be better if we can control air using a material that is rigid (or supported by something rigid). It would be less risky to substitute the membrane you have for a plywood or gypsum sheathing with the joints taped.
CW 3:
To put this all in perspective, I would consider the first recommendation related to exterior water control to be mandatory, and the second to just be optional (I really only mentioned it here because this is an academic exercise and I thought it would be helpful).
One final comment: the two weaknesses I identified (in exterior water management and interior air and vapor control) are *practical* considerations, NOT the result of a poor understanding of the principles of building science. This distinction is very important. If you were to run a hygrothermal simulation of this wall in WUFI, it would work perfectly. But as currently detailed, it's risky for the real world. This is a very ambitious entry and I applaud you!
*********Note*********
Examples of gypsum sheathing
https://buildgp.com/product/densglass-gypsum-wall-sheathing/
https://www.usg.com/content/usgcom/en_CA_east/products/roofing-building-envelope/sheathing/gyplap-treated-core-gypsum-sheathing.html
****Source of information****
Source document: https://miro.com/app/board/o9J_lMm4B7I=/
The image attached shows you where to find the comments if you open this link.
Original webpage : https://www.wallassembly.com/projects-3
This explains the context of why this critique of wall assemblies was taking place. Optional reading.
Thank you so much Hugh! Lots to read later this evening - we haven't fully decided on our interior wall membrane so this is super helpful.
Hopefully it makes up for my misleading info on horizontal strapping.
I thought I would share another great article that talks about the risks associated with double stud wall and the optimal design. This is from Allison A. Bailes III, PhD who references Joe Lstiburek from Building Science Corp (BSC) and the research by BSC. More help when talking to building officials.
https://www.greenbuildingadvisor.com/article/lstibureks-ideal-double-stud-wall-design
The article mentions using OSB in the middle. They also refer in more general terms to "sheet goods". Your sheets of insulation in my view qualify as sheet goods.
This article also suggests sheeting on the exterior, include the option for a gypsum product which is more vapour open than other options.