Cost-Effective Wall Assembly
Hello,
I am a builder-owner at the design stage in zone 6, coastal Maine. I am considering options on my wall assembly. I am going for green-as-I-possibly-can on a budget. I will be providing most, if not all, of the labor but am also considering labor saving choices since building my house will be taking me away from my work as a contractor.
Here are the basics of the wall assembly I am considering. I am using dense pack cellulose. Its about r-38. Moving inside to out:
1. horizontal nickel gap ship lap
2. non structural 2×3 wall
3. 4 inch gap
4. 2×4 structural wall
5. structural sheathing
6. MRB
7. vertical strapping
8. horizontal strapping
9. board and batten
So, I can get locally sourced hemlock and pine for 60 cents a board foot. So, b and b and shiplap are looks I like and are attainable. I will prime the exterior boards all the way around before installation.
I am considering going with plywood and hydrogap MRB and saving on the double strapping listed above. If not, I would probably do a zip system and double strap it with a rainscreen. Would appreciate feedback on this cost saving choice.
Also, will I need a moisture variable wrap on the inside of the 2×3’s? I might latex prime and paint my nickel gap in some areas, or clear coat it with an alternative to poly.
Finally, does hydrogap require a mini-rainscreen at the bottom to allow moisture to get out? The website does not seem to mention it.
Thanks!
Stephen
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Replies
Hello Stephenr, I would stop just shot of emphatically saying “DONT DO IT”.
Given that you are in a coastal area, I expect that you have good amounts of rain and humidity.
According to the founder of building science corp Joe S., the double stud wall, in particular the one you described, is prone to failure for 2 main reasons.
Before you read the article I suggest, I point to a quote Joe S. in an article dated 2012:
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There are in fact some beautiful Passive Houses. But double walls are tricky to construct, increase the risk of moisture damage and in my view they don’t make a lot of sense. Give me a single advanced frame 2x6 wall with lots of insulating sheathing any day. I did the double wall thing 30 years ago before there ever was a Passive House. It was dumb then. It is dumb.
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If you insist on a double stud wall, you could look at this article to understand more about the challenges with double stud walls
https://www.greenbuildingadvisor.com/article/lstibureks-ideal-double-stud-wall-design
I would strongly recommend looking at the Building Science Corp “perfect wall”.
I am surprised that you didn’t get more input given how many GBA contributors are familiar with building in Maine.
This article has some pushback against Lstiburek's double-stud wall worries from some high performance builders in Maine that is worth reading: https://www.greenbuildingadvisor.com/article/the-mythical-threat-to-double-stud-walls
I've also occasionally tried to find built examples of Lstiburek's ideal double stud wall. And granted, my research consisted of a little googling and a question on GBA, but I've never seen documentation that the Lstiburek double stud wall actually exists in the wild, which is always a little concerning. Building Science Corporation is generally the gold standard for answering building science questions, but I feel like the double stud wall stuff is always a little weird.
I'm neither a double-stud wall expert, nor a coastal building expert, so I don't want to make any claims about what stephenr should or shouldn't do, I just want to point out that there are some differing opinions out there about double stud walls!
I really like the ideas of a 2x3 or 2x4 utility channel/ cavity for reason you are likely familiar with.
As an alternative to double stud wall, you could look at the building science" option that is very very simple and can be built in any climate with a 2x4 wall.
If you have not already, you can read about "the perfect wall" here.
https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall
You can see an example on this Youtube video (there is a series), but this uses some materials like ZIP and self adhering membrane that are not necessary for the perfect wall. You will see that the design of the house is super simple.
https://www.youtube.com/watch?v=hTwq-qUnr9I&list=PLDYh81z-RhxgCk1qJD6AJktVHZH7-BLSR
As you alluded to, a big part of what you want is an air-tight envelope and as someone else suggested you can address this with a blower door test mid-construction. You can choose to havea an exterior air barrier (including roof/ceiling) and have the blower door test once the sheeting is completed and windows are installed
I find Huber Zip incredibly resilient. I have had a half sheet 2’x8’ piece standing vertically outside unprotected attached to a post for 8 months in the snow, heavy spring rain, hot humid summer weather, fall rains and it looks like new. There is now à competing product from LP I believe.
While the perfect house uses foam that is more carbon intensive
Thanks Hugh,
I appreciate your reply and insight, as always.
A couple of questions concerning the perfect wall as it applies to my prospective build...
1. I am hoping to limit the amount of rigid foam above grade. In order to attain r-40 without using insulation in the wall cavity, I would need 10 inches of comfortboard or a similar product. That is both expensive and kind of ridiculous. Does anyone use cavity insulation in this type of perfect wall assembly?
2. Moving outward from the "sweater", what type of moisture barrier would you suggest in between the rockwool and the strapping for the rainscreen?
3. Would a single la.yer of rigid foam on the outside of all that comfortboard work as a rain barrier? or a more solid base for exterior strapping. I am thinking of going with b and b and may need two courses of strapping so i have horizontal nailer for my boards.
4. Finally, I am an "open the windows in the summer" kind of guy (coastal maine, no reason to air condition). I imagine this perfect wall construction would be amenable to higher humidity levels on the inside of the house in the summer when i turn the erv off and take in the breeze, right?
Thanks,
Stephen
Will reply early in the week.
We are currently building a remote wall (perfect wall) house. I'd be happy to share some of the wall schedules we have with you if you like.
Stephen, referencing your above questions:
1) I don't think anyone is suggesting that you use no cavity insulation, just that cavity-only insulation via a double stud dense packed with cellulose is a risky assembly in a coastal location. You can use 2x6 or even 2x8 framing filled with fluffy insulation, and then rigid foam, rockwool, or wood fiber insulation on the outside of your sheathing. You can still create a service cavity of any size you want by strapping on the inside. This wall should still have "smart" membrane on the primary structural wall that also acts to retain the fluffy insulation inside the wall.
2) You don't need a WRB between the exterior insulation and the strapping for siding. The WRB and air barrier layer are typically either inherent to the sheathing (like with ZIP sheathing) or are added as a membrane (Tyvek or other air/water barriers, self-adhesive products, etc.)
3) Comfort Board does not soak up water, but does allow water to drain through it. You can probably get away with just cross-strapping across the top of the Comfortboard if that is your choice for exterior insulation.
4)The interior of the walls will be no wetter in any season than the interior/exterior air when the windows are open. As long as mold isn't growing on your furniture, it won't grow inside the walls either.
Thanks Peter.
Regarding the riskiness of a double stud wall in a coastal location....Is it just a matter of doing a blower door test to make sure everything is as tight as possible or are there other measures that can be taken to mitigate the riskiness? Would you improve the double stud wall described on GBA by Martin and others in any specific way to make it less risky in a wet environment?
i am leaning towards a double stud wall with cellulose over a perfect wall for reasons of cost and because it seems a greener choice. I can get locally sourced 2x material for pretty cheap. Am i correct in assuming that dense pack is a cheaper choice than the rockwool and rigid that I would use in a perfect wall? Thanks.
We're about 15 miles from the Maine coast. We put an air barrier on the exterior side of our inner stud wall. Outboard of that, we used dense pack cellulose. Inside the interior stud wall we installed fiberglass batts after rough wiring and plumbing. House is pretty airtight, .59ach50. We also taped the Advantek sheathing. After 7 years, seems fine.
Stephen,
I have followed your build somewhat here on GBA. If you have time, what is the HDD for your location and the R-values for your building components. Do you track the heating season building performance for your house? For our MN winters, I like the double wall.
Thank you,
Doug
Doug. HDD averages around 7200.
R values:
walls: 42; underslab: 18; roof: 70;
We don't have any tracking per se. My best guess is we use around 2500-3000 kwh/year for heat.
The double stud walls were easy for our contractor to install, even though he'd never done a house with them.
Stephenr,
Double stud walls are considered risky in general because the sheathing is always cold in winter and well below the dewpoint of the interior air. If there are bypasses that allow interior air into the wall cavities, it can condense on the sheathing, causing problems. The risk is higher in a coastal location because it is so hard to make the exterior walls watertight in coastal conditions. If you have wetting from both the front and the back, the sheathing just won't last long. That said, there are plenty of people with double stud walls and cellulose in ME that have had no problems at all. To minimize the risk you must be aggressively diligent in detailing your exterior flashings, drainage cavity and air/water barrier layers. You should also be diligent in air-sealing the interior walls to minimize warm/moist air entry into the stud cavities. I notice from your OP that there is no air barrier on the inside of the wall. That nickel gap finish on the interior will have LOTS of air leaks. Use a smart membrane behind the nickel gap (or properly detailed gypsum drywall) to stop that air leakage and minimize moisture flow into the cavities.
Thanks Peter, That makes sense. Would it be wise to put a layer of rigid or comfortboard outboard of the sheathing to keep it from cooling and getting wet?
This is exactly what we're talking about when we discuss "continuous insulation" on either the interior or exterior. Continuous insulation also reduces the thermal shorts created by the studs, but staggered double stud walls do the same thing automatically. If you put enough continuous insulation on the exterior, the sheathing stays warm enough that condensation doesn't happen. In your climate zone, you need 30%050% of the total wall insulation value outside the sheathing.
But now we're talking about a very different wall system. The double stud system with cellulose counts on the hygric buffering of the cellulose to soak up modest amounts of moisture that might get in and then release this moisture when conditions permit. The moisture can dry to either the outside or inside with most of these systems. But double stud walls are relatively expensive due to the need to build two entirely separate stud walls, treat the utility openings properly, etc. Exterior insulation works to keep the sheathing above the dewpoint so that it can dry in either direction in almost any weather. But CI is expensive because of material costs and labor to install the insulation properly with extension jambs, window bucks and all of the other stuff that these walls require. You want to pick one or the other. Doing both would become cost prohibitive for moist owners.
Stephen Sheehy,
That's an interesting approach. Would you know, by chance, of someone who could do a blower door test in mid-coast Maine??
Try Evergreen Home Performance in Rockland.
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I am considering these two applications that I have seen done to lessen the risk of air leakage and wet sheathing.
1. Zip system sheathing applied in what I believe to be "passivhaus" style. That is, stand my walls and sheath them 80%, leaving the top 20% open. Lay my rafters (1/12 pitch, unvented) and leave no overhang, cutting the tails flush with the outside of the top plates. Block between the rafters with wood, rigid and spray foam. Sheath the remaining 20% of the wall. Then, after taping and air sealing this tight box, I would pre-build my overhangs, install them, and sheath my roof so that the sheathing carries and ties in my overhangs, and completes the air barrier (connecting the wall and roof sheathing). Everything spray foamed, caulked with big stretch and taped.
2. Build my rain screen in the following way... 3/4 inch strapping vertical on every stud. Horizontal 3/4 inch strapping every 2 feet. Apply board and batten and run my boards up into the soffit cavity. Run my soffit (roughly 2 ft. overhang) tight to my boards. Do a continuous "soffit" vent, which wouldn't vent my roof assembly at all but would rather vent my rain screen. Do a standard rain screen vent at the base of my b and b.
This is similar to the approach of the PERSIST system, but PERSIST puts all or nearly all of the insulation outboard of the sheathing/air/water barrier so the sheathing stays at or near the indoor air conditions. Your approach could be very water-tight from the exterior if executed properly, but the sheathing is still cold in winter and at some risk of condensation from the inside. As we've discussed previously, if you use a smart membrane on the interior and are diligent about air-sealing it, your wall could perform well. Similar wall systems are used around New England, even in coastal locations, with success. But it is still considered to be a somewhat risky system based on the physics of the system and modeling that shows the sheathing getting pretty wet by the end of the winter.
Stephenr
SSorry for the delay in responding.
>>Trust your gut
1. Do research and ask for advice but I hope at the end of the day that whatever decision you make that you stay true to your gut feeling and you what feels right. It is your house, not mine, not anyone else’s.
>>Very interesting case study by Building Science Corp
2. I read some more about the risk of condensation on the exterior sheeting. I found this article, along with the related comments, to be filled with very good information. I have read lots about this before but this article contains a case study by Building Science Corp on different insulation and the impact on sheeting.
https://www.greenbuildingadvisor.com/article/is-cold-sheathing-in-double-wall-construction-at-risk
>>ZIP as sheeting vs Plywood
3. This next article I found after you suggested using ZIP. I think ZIP is an amazing product; it is really tough/durable. I would not use it as exterior sheeting in heating dominated climate zone. ZIP in my opinion is too vapour impermeable. I would use 1/2" plywood with all the same taping details as ZIP. All my reading says that plywood is more permeable than ZIP or any other OSB. That plywood would help your wall cavities dry faster. In the south of the USA, ZIP on the outside makes total sense because the vapour retarder is on the outside of the envelope.
https://www.greenbuildingadvisor.com/article/working-with-zip-system-r-sheathing
Now, our own house has ZIP but it is only used as the structural sheeting and air barrier. It is 4 inches from the interior face of the wall, but 8 inches from the exterior face of the wall. It is far from the exterior. We have a "very" vapour open, pretty expensive exterior sheeting that is not structural. We then have an expensive European WRB (yes it is stapled in place... head shake.)
>>"Easy" air seal detail with standard roof overhang
4. I recently saw this Matt Risinger video about house to air seal around standard roof overhangs. I thought is made lots of sense.
https://youtu.be/fjwjeoLi6X0
Start the video at 5:30 to see how roof overhangs are air sealed with less effort.
To vent your rainscreen at the top, you could build a false soffit under the Air-tight sealed actual soffit. Your rainscreen could terminate in that narrow (thin) soffit space and be vented as you describe.
>> Exterior insulation, petrochemicals and carbon
5. I would have 2" of exterior insulation on the outside of the plywood sheeting myself to keep the plywood sheeting warm.
- I think that I would go with Rockwool as my first choice if I could afford it. It is very vapour open like you know and will allow the sheeting to dry.
- EPS is more vapour open than XPS but less than Rockwool from what I have understood and it has less of an impact on the environment. I would be taping all the seams on the EPS if I used this.
**********But foam shrinks - lots of GBA articles on this worth reading.****
- We discussed that insulation like XPS is really challenging from several environmental angles, and it is not very permeable compared to the other 2 options.
>> WRB
6. WRB - commercial grade Tyvek wrap
Without ZIP, we need a WRB. Unless I could afford the more expensive European WRB, my next choice would be the thicker, heavier, tougher version of Tyvek. I only found out about this product recently. If it is well installed, it can provide some very solid protection. I need to be honest that I don't know the actual cost.
https://www.dupont.com/products/tyvek-commercialwrap.html
>>Cheaper, local 2x4 product
7. Since you can get 2x4 locally at a good price, why not use that to create a 2x8 wall. A good friend has a house with two 2x4 walls back to back with studs offset to reduce thermal bridging at the studs (not top or bottom plate though).
I would put the vapour barrier between the 2 walls (does your building code and city inspector allow this?). With almost no services going through the vapour barrier, it would be easier to seal really well. Many people suggest using a smart vapour barrier that can dry to the inside when the levels of humidity in the house allow for this. You could then use the 2nd interior 2x4 wall to run electrical and other services as required.
Myself, I would avoid using 2x3's if I could. Our 2x3's up here twist unless fastened tightly to something that will keep it straight. Twisting 2x3's are not great for the drywall over the long term.
>> Selecting insulation for wall cavities
8. If I could be sure to get the density required, I would use cellulose in the wall cavities. This might require a different sequencing if you have a "smart vapour barrier" between the 2x4 walls. Maybe there would be some insulation netting in the center, along with the "smart vapour barrier." If I could not do cellulose, I would go with Rockwool batt. The cellulose holds water and allows the water vapours to be spread evenly throughout the assembly. Rockwool does not absorb water vapours so all the water vapours end up in the wood elements of the building structure - resulting in higher humidity in framing when using Rockwool or so I have read.