MOST efficient wall construction for cold climate?
Avasa
| Posted in Energy Efficiency and Durability on
Im a contractor currently designing my own dream house in Quebec. The idea is that the home will eventually be Net Zero, which means I’d like the house to be as energy efficient as possible.
so is there a consensus? my insulation guy is telling me that the best is a 2×4 construction with horizontal 2×3’s on edge, filled with 5″ of urethane. (VERY little thermal bridge, super tight, etc)… any thoughts please?
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My advice is to think less about "energy efficiency" and more about environmental impact and cost effectiveness. They are related, but can be quite different.
In Quebec, electricity is cheap, so for sure it isn't "cost effective" to spray urethane everywhere. I do however like the idea of building something durable, healthy, and efficient. the urethane is allegedly soy based, and using 2x4 instead of 2x6 covered with an inch of styrofoam has environmental arguments in its favor as well. I'm also considering double 2x4 walls filled with 10" of injected cellulose, but this has more steps and is more prone to failure over time...
I was watching a Youtube of a contractor that essentially reviews new home building technology and he showed a 2x6 which was insulated by using studs front and back separated by internal dowels and then sprayed with insulation in the void resulting in a structural 2x6 with an R-20 rating. Also there was a guy that had 10" rigid insulation. 5,000 foot home that only needs a tiny amount of energy to heat and cool. https://youtu.be/mxDSulcLpAE
The proposed 2x4 + horizontal 2x3 is effectively the same as (but somewhat lower performance as) Case #3 in this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf
If you look at Table 3 on p.9 (p13 in PDF pagination) even with 2x6 /24" on center studwall + 2x3 horizontal on the interior ( Case #3 ) this was not exactly a great performer compared to other walls. Details on the construction for Case #3 start on p35 (p39 in PDF pagination).
Case #4 is pretty simple to build, outperforms Case #3 by 50% (33% lower heat transfer) using about the same amount of board-feet of lumber, and much greener (cellulose) insulation.
"Soy based" polyurethane really has less than 20% of the total content from plant sources. It's a green-washing marketing term.
If you swap out the fiber insulation in Case #3 for closed cell polyurethane foam it adds a bit less than 10% of the "whole wall R" overall. Dropping to 2x4 16" o.c. and using closed cell polyurethane delivers a net result comparable to (perhaps R1 lower than) Case #3 with fiber insulation, as analyzed in the document. The reason you don't get much of a boost from the much higher-R insulation in the cavities is due to the thermal bridging of the framing. The potentially high performance of R6 to R7 per inch closed cell polyurethane is pretty much wasted when installed between R1.2 per inch framing.
For the math on that see:
https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste
Thanks for that info!... I will dive deeper into it later. 2 questions arise right away (as those are basically the 2 models I am most fond of): will injected cellulose (3psi) REALLY not settle over time? And does case 3 really mirror the qualities of urethane? (ie: no poly vapor barrier would be required therefore penetrations wouldn't be a concern. I just finished a construction for a client using injected cellulose between double 2x4 walls and the major source of leakage during the blower test was from the THOUSANDS of staple holes used to fasten the poly...
In a Quebec climate a wall needs to be dense packed to about 3.5-4lbs per cubic foot, which isn't as simple to achieve in a double-studwall as with a 2x4 wall.
Dense-packed cellulose in 2x4 construction is usually pretty tight even without poly sheeting, which makes me think the density of that installation is suspect, or that there was no caulking of the framing outside of the stud bays themselves (?).
Using a polyethylene vapor barrier as the primary air barrier is not a great idea for a high performance home. It's far more robust to detail the exterior sheathing as the primary air barrier, caulk framing to the sheathing on the perimeter of EACH stud bay, a bead of caulk under the bottom plate & subfloor, and between any doubled-up framing such as top plates, jack studs, window headers, etc.
Whether or not you use a poly vapor barrier in a double-studwall for moisture control is between you and the inspectors. Installing in on the exterior side of the interior studs where it wouldn't be damaged by electrical & plumbing penetrations would be fine, since there would still be cellulose between the cold exterior side studs and the vapor barrier. But it complicates the insulation process a bit. Using OSB on the exterior side of the interior studs in lieu of a poly vapor barrier will work too, since the vapor retardency of OSB will meet the IBC definition of "vapour barrier" when dry, and would be sufficiently toward the interior that it will stay dry in winter. But that approach may require some effort to educate the inspectors, and it's a lot more expensive than 6 mil poly.
MOST efficient wall in what way?
Smallest BTU loss per square foot regardless to its construction cost, functionality or appearance.
If that is your goal, find someone to build you a house size Thermos bottle with vacuum filled walls, most people would want more than one door, a flat floor, square walls and a few windows but to each his own.
Thick wall have an ongoing ever rising cost in the form of property taxes, in that the tax man measures your home on the outside so you pay taxes by the square foot even the ones can’t use because they are filled with insulation.
Foam has a very high R per inch but has a high dollar cost and is not environmentally friendly.
Another part of the equation for me is how many dollars it costs to get a BTU into my house. That’s about fuel choice, cost and equipment.
There are a lot of variables if your contractor does not like your choices they will put price tag on them that will change your mind.
Some of the choices are about dollars and others are more about preferences that each of us will feel differently about.
My most efficient wall turned out to be ½ drywall - 2x6 16 oc filled with wet spray cellulose –zip board – 1 inch foam – covered with vinyl siding.
Walt
Walter,
In Canada anyway, property taxes are based on the appraised value of the land and buildings. Small changes in the square footage don't appreciably affect taxes.
Often in urban regions the structures are shown as a small number compared to the ground. But that flip flops in rural areas
Property taxes for the structure are all about square feet times your neighborhoods multiplier. Weather you have improved your home or not you pay neighborhoods average level of improvements of home sold in your neighborhood.
If you pay taxes on a small 1000 SF home with 2 foot thick walls 24 x 41.6 home you get 852 SF inside so you tax increase for the same living space.
Walt
I guess it's different where you are. The quality of the house, its amenities and number of bed and bathrooms figures more into the assessment that straight square footage here. But even in your situation, how much does it really add? The choice isn't between a 2"x4" wall and one measuring two feet. No one except maybe in Alaska builds that way. The additional thickness is typically the increase between a 2"x6" wall and a 12" double wall. So in that 1000 sf house you end up with 65 sf less living space..
User-7107493,
I'm a little late to the party, but I'd like to join the conversation.
First, can you tell us your name? (I'm Martin.)
Several commenters have noted that walls aren't rated for "efficiency." We use the word "efficiency" to describe appliances that burn fuel, or to describe the amount of fuel used to keep the entire house operating. Walls, on the other hand, are rated by other metrics -- usually R-value and air leakage. A high R-value and a low rate of air leakage are good.
If you haven't seen it yet, you may want to read this article: "How to Design a Wall."
Q. "I just finished a construction for a client using injected cellulose between double 2x4 walls and the major source of leakage during the blower test was from the THOUSANDS of staple holes used to fasten the poly."
A. As Dana Dorsett correctly noted, it was a mistake to try to use polyethylene as an air barrier. For more information on this issue, see "Questions and Answers About Air Barriers." These days, most builders of energy-efficient homes put the primary air barrier at the exterior sheathing layer.
Hi Martin, I'm Avasa (I updated my profile to reflect that, but it hasn't seemed to kick in)
regarding the air barrier... the poly was the vapor barrier on the interior, NOT the air barrier on the exterior. the wall construction was as follows (please let me (and the architect!) how it could have been improved! From exterior: reclaimed cedar siding, 1x3 strapping, house wrap, osb, 2x4 wall (16 oc), 2" gap, 2x4 wall (24" oc) (that allowed for 9" of cellulose packed to 3psi). 6mil poly vapor barrier stapled to interior studs (with fancy cardboard straps to resist tearing of the poly during insulation), 1x3 horizontal strapping. drywall. (was there a misunderstanding about the wall design? 6 mil poly (interior) is part of the normal practice in these climes!
to clarify efficiency: I mean a combination of of high (whole wall) r-value AND low air leakage. I have seen some homes with theoretically high r-value walls that perform poorly because of air leakage, (and lower r-value homes that perform well because of tight installation). because the climate is so extreme here, I want to ensure that over time, I dont send moisture through puncture holes in the vapor retarder into a cellulose filled cavity, but this seems to be the type of construction you guys like the most.
what I love about the urethane (despite its less than green provenance) is that it takes away a lot of variables, mostly regarding air leakage and performance over time... do you factor in long term performance into your calculations?
thanks for all of this info!!!
Avasa,
What was missing from your wall assembly (unless you forgot to write about it) was high-quality tape to seal the OSB sheathing seams, as well as details at the plates (for example, Tremco acoustical sealant) to ensure continuity of the air barrier. Many builders also include a 3/4-inch plywood top plate that is wide enough to cantilever into the interior space -- allowing the ceiling air barrier to be sealed to the wall barrier.
If you saw air leakage through the wall assembly, there were clearly problems with your air barrier. You need an air barrier plan before construction begins, and each transition needs to be carefully thought out. Your blower door test was an eye-opener for you -- now you need to strive for better details on the next house you build.
Hi Avasa. I'm planning a build in Ottawa right now and have settled on a double stud wall with cellulose. In terms of value for money ($ per R per sqft) cellulose is unbeatable. The added labor and materials involved in building a double stud wall and blowing the cellulose in will counter the price advantage, but, you can build the wall as thick as you want and it's pretty much a fixed cost. Only the volume of cellulose goes up.
Applying tape down the studs after stapling and setting it with a rubber roller is one way you could seal up all those staple holes. Another would be to run a bead of acoustic caulk down the studs prior to installing the poly, rolling the poly into the sealant, then stapling. The caulking might work better, but the tape would surely be less messy.
Lance Peters
An R19 wall in your climate is not going to provide the kind of energy efficiency you're looking for. It's barely above code minimum, if I'm not mistaken.
I'm a fan of dense packed cellulose, like many others here. Cost effective and environmentally friendly. Air/vapor barriers/retarders can be implemented in a variety of ways.
I'm in the same climate zone as you are in. My home will be certified net-zero.
You don't need to go crazy to hit net-zero anymore. My home will be standard 2*6 construction with R22 batt insulation, and 2" of continuous insulation on the exterior. Windows are a decent triple pane. Basement is fully insulated, with the same R22 batt and 2" continuous insulation. There is also 2" of insulation below the basement floor.
Air sealing is a priority, and the target is to be under 1.5 ACH, which the builder is contractually required to hit, but the unofficial target is .7 ACH.
Get yourself a good ERV to go with this very air tight house.
Combine all of this with a decent cold climate air source heat pump, and a large DHW heat recovery system, and bob's your uncle.
My house will have a 10.8 kW DC / 10 kW AC solar array. It'll actually be significantly better than Net Zero (42 GJ consumed, 48 GJ produced), but the "extra" will be used by my Chevrolet Bolt EV.