PERSIST with concrete wall?
Hello,
Longtime listener, first-time caller.
My wife and I are designing a house. We are north of Lake Ontario, climate zone 6, with a downward outlook. We want a simple, housey-house and have settled on a few details:
Slab on grade, FPSF
Polished concrete floors
Single storey, 12-14’ ceilings
Compact layout, 1,200 – 1,800 sq ft
Classic gable roof with healthy pitch
Probably a metal roof of some description, agricultural-looking is fine
Mini split heat pump(s) for heating and cooling with ceiling cassette(s)
Cedar siding?
I have read a lot on walls and am having trouble converging to a solution.
I want an R value in the cost-sensible zone and was thinking ~R25?
I bought a book on ICFs 20 years ago and am still intrigued. I fear, however, that they are the 3D printer of the building world – the wall of the future and always will be.
I like the solidity of the concrete, however, and read a thread where Martin, I believe, said there was little to no functional difference between an ICF wall and a poured concrete wall with insulation applied.
I appreciate the reuse of salvaged XPS (pet peeve – reuse is not the same as recycling).
To make a short story long, I am wondering if a conventionally-poured concrete wall with sufficient external XPS only, would be functional, or would insulation be required on the inside as well.
Thanks so much,
John
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Replies
A poured concrete wall with sufficient exterior rigid insulation can be perfectly functional from an energy efficiency standpoint, no need for interior insulation if you don't want it.
That said, if you are concerned about your environmental impacts in addition to your energy use, it is best to minimize the use of concrete in your project to only the areas where there are no reasonable alternatives (individual definitions of reasonable may vary). Concrete is great for foundations, retaining walls, etc., but unless you have very unusual requirements for your home, probably not necessary for your walls.
A 2x6 wood framed wall with dense-pack cellulose insulation in the cavity and an appropriate amount of exterior rigid foam for your climate zone/energy goals will be significantly lower impact than a concrete wall from a global warming perspective.
Are there specific reasons that you are drawn to a concrete wall?
Brendan,
Thank you for responding.
I like the simplicity and relative permanence of the design.
John,
A concrete house may make sense for you, but I wouldn't commit to it before considering all the implications, including the much higher cost of construction.
Think through the total wall assembly and sequence of construction to see if it is as simple as it seems. The attachment of the foam and cladding, where the services run, the window and door sections.
Permanence may or may not be beneficial. Longevity rarely depends on the robustness of the structure, but rather how well it can be adapted over time as the occupant's needs change. Concrete structures aren't great at adapting.
Also, consider why you don't see any concrete houses around you. When a common material or technology fails to find a place in house construction, there is usually some reason for that.
Excellent points.
I will, of course, price out the options, but wanted to check viability before doing a deeper thought experiment.
The houses that I observe being built seem to be made to the lowest possible standard, period.
John,
Yes, that is disheartening.
To more directly answer your question, leaving aside the things Brendan and I have raised, what you suggest is completely viable.
Thank you, Malcolm.
I suppose the design is reasonably close to the Lstiburek Perfect Wall.
John, the wall you describe adds complexities with little if any benefits compared to a wood framed wall. The environmental impact is dramatic.
Where will you run your plumbing and wiring? Some kind of chase/stud cavity will be needed. At that point you might as well just insulate the stud wall and nix the concrete.
Penetrations must be well thought out. Drilling holes in 10" of concrete for bath fans, ERV ports, mini split lines, power, water, etc can be a challenge. God forbid you ever have to alter/move a door or window...
The exterior foam will have to be at least 4 inches thick. How will it be attached? Concrete screws are treated and more expensive. They would need to be at least 6" long to penetrate the strapping, foam, and concrete. I suspect each screw would need to be pre drilled?
The wall seems like a complicated build to me...
Rick,
Would you have the same concerns with the Perfect Walls that use concrete blocks (aside from the difference in concrete used)?
John,
Are you referring to using cmu blocks instead of poured concrete? I don't know that that would solve any of the issues above. I spend time every year in Guatemala, where cmu blocks are the standard building material. They do manage to run wires through the blocks but it is tedious, labor intensive work. (Labor is cheap there so it is cost effective). I'd stick with poured concrete as it would be a better air/moisture barrier.
The big challenge is adding siding over the thick exterior foam. Consider something like the Nudura 'One' product. The insulation can be increased on the exterior and it has 'ribs' of hard plastic that can accommodate conventional wood screws. You can tack on your strapping to these ribs and then your siding.
https://www.nudura.com/divisions/nudura-products/one-series
In the photos, they have it backwards to what you want. So you would want to reverse the ICF product to achieve the "perfect wall".
Donate an additional $1000 to a good environmental cause and concrete walls become an environmental benefit. It's too bad that SCIPs (foam in the middle, structural sprayed concrete on both sides) aren't common. But you can put foam into the forms (either side) and later apply stucco to the exposed foam side.
Jon, I don't know that I love that accounting method, given that there's a high chance that building a concrete house is likely going to cost more than a wood house.
Say you have a fixed total budget to split between building a house and donating to environmental causes.
Building a wood house and donating the leftover budget to environmental causes seems like it's going to do more good for the environment than building a concrete house and donating the leftover budget to environmental causes. A wood house seems likely to both be better for the environment AND leave more money to donate, right?
With that unusual assumption, I could say the same about nice kitchen cabinets - put in the cheap ones and donate the savings. But I have few problems with luxuries that have a net positive environmental effect.
Looking only at the building is overly segmented accounting that leads to non-optimal macro results.
At some point, the carbon/energy choices we make are arbitrary.
You could argue that a building should be smaller, with fewer windows, or closer to town, or foregone altogether in favour of living in a high density structure full of very small units.
We all have our own thresholds.
Jon,
That's tricker than it seems. A study published in 2017 by the European Commission examining projects based on the Kyoto protocol's Clean development Mechanism found that 85% of its offset projects had failed to reduce emissions.
They seem to me a bit like recycling. An imperfect bandaid for a problem better dealt with at source.
An example of foam and poured concrete that leaves no exposed foam. May be less expensive than ICFs + covering on both sides.
https://basc.pnnl.gov/images/4-inch-layer-rigid-foam-insulation-r-20-will-be-sandwiched-between-two-layers-concrete-poured
Our design firm is located near Peterborough, Ontario. We have designed a lot of houses built with ICF, usually for the basement but occasionally for the whole house. The ICF folks seem to have gotten a good foothold around here. The environmental concerns have already been mentioned. If you want to build your walls out of concrete, ICF is a great way to do it and one that the builders will be familiar with. Trying to convince them to screw 5 inches of foam to the outside of a concrete wall will be difficult. If it's difficult/unusual, that usually means expensive.
From my experience, ICF is the way to go if you want concrete above grade walls. Amvic makes a "R30" ICF block that would be a great option for your target.
As another note, I would advise against a ductless minisplit solution in our climate with your specified level of insulation. I have a bungalow with reasonable airtightness and insulation levels in the realm you describe and while the comfort provided by the single ductless minisplit in the hallway is acceptable, I would not accept that level of comfort in a new house with a new system. Too much room to room variation. The amount of variation is highly dependent on the layout of your house. A ducted minisplit would likely be a much better solution.
Thank you for the insights- I will take a more fulsome look at ducted minisplits.
Do you have any insights into Amvic versus Nudura? The Nudura One product seems interesting, as noted above.
Nudura has a newish product, XR35, which has 4” of EPS on both sides.
It would spec out a little over R36, if the same assumptions are used that got to Amvic’s R30 number.
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