John Downie and his wife are planning to build a house in Ontario, and instead of choosing a conventional wall system they are considering poured concrete exterior walls that are insulated on the outside.
Downie likes the “solidity” of concrete and recalls a comment from former GBA editor Martin Holladay that a poured concrete wall is functionally the same as a wall built with insulating concrete forms (ICFs, pictured above).
Otherwise the house looks fairly straightforward: between 1200 and 1800 sq. ft. over a single story with polished concrete floors, high ceilings, and a gable roof. Heating and cooling would be with a minisplit heat pump.
“I bought a book on ICFs 20 years ago and am still intrigued,” Downie writes in a Q&A post. “I fear, however, that they are the 3D printer of the building world—the wall of the future and always will be…
“To make a short story long, I am wondering if a conventionally poured concrete wall with sufficient external [extruded polystyrene] XPS only would be functional, or would insulation be required on the inside as well.”
Our topic for today’s Q&A Spotlight: Is Downie onto something, or should he look for another way to build his house?
It will work, but…
Although Downie’s proposed wall will be “perfectly functional from an energy efficiency standpoint” without any interior insulation, Brendan Albano writes, it carries an environmental price tag.
“If you are concerned about your environmental impacts in addition to your energy use, it is best to minimize the use of concrete in our project to only the areas where there are no reasonable alternatives,” Albano says. “Concrete is great for foundations, retaining walls, etc., but unless you have very unusual requirements for your home, probably not the necessary for your walls.”
A better idea might be a 2×6 wood-framed wall with dense-pack cellulose cavity insulation and the right amount of exterior rigid-foam insulation for Downie’s climate zone 6 location, according to Albano.
It’s not as simple as you might think
Think the project all the way through, suggests Malcolm Taylor.
First, construction costs are likely to be much higher than they would be for a more conventional house. Downie might think about how how the foam would be attached to the outside of the walls, how the cladding would be attached to the house, where electrical and plumbing runs would be located, and how the door and window openings would be handled.
“Permanence may or may not be beneficial,” Taylor adds. “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.”
Finally, Downie might consider why there aren’t a lot of concrete houses located around him. “When a common material or technology fails to find a place in house construction, there is usually some reason for that,” he says.
Maybe so, Downie replies, but the houses he’s seen “seem to be made to the lowest possible standard, period.”
Is this a ‘perfect wall’?
Downie suggests his proposed wall is pretty close to the “Perfect Wall” described by building scientist Joseph Lstiburek—a wall in which the rainwater control layer, the air control layer, the vapor control layer, and the thermal control layer all are located on the outside of the structure.
It’s complex at the least, with a “dramatic” environmental impact and few benefits over a wood-framed wall, replies Rick Evans.
Evans also cites plumbing and wiring, wall penetrations, and the difficulty of future alterations as reasons to reconsider the idea. “The wall seems like a complicated built to me,” he says.
Would those same concerns apply if the wall material were concrete block instead of poured concrete? Downie asks.
CMU construction is common in Guatemala, where Evans says he spends time every year, but it’s “tedious, labor-intensive work” to run wires through the walls. Even the low labor costs there don’t make the approach very appealing.
“The big challenge is adding siding over the thick exterior foam,” Evans adds. “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.”
Boring into the environmental considerations
If the use of concrete is an environmental issue because of its embodied carbon, why not donate $1000 to a good environmental cause? asks Jon R. Then, the walls become an environmental benefit.
“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,” says Albano.
If you’re going to give what’s left over in the construction budget to an environmental cause, you’d do more good by building a wood house. “A wood house seems likely to both be better for the environment and leave more money to donate, right?”
Carbon choices are arbitrary at some point, Downie says. It also is possible to reduce the environmental cost of a house by making it smaller, putting in fewer windows, moving closer to town, or even choosing to live in a dense, urban environment.
“We all have our own thresholds,” he says.
Don’t count out ICFs
Despite Downie’s view that ICFs are the “3D printer of the building world,” he shouldn’t be so quick to dismiss them, Yupster writes.
“Our design firm is located near Peterborough, Ontario,” Yupster says. “We have designed a lot of houses built with ICFs, usually for the basement but occasionally for the whole house. The ICF folks seem to have gotten a good foothold around here … If you want to build your walls out of concrete, ICFs are a great way to do it and one that the builders will be familiar with.” Attaching 5 in. of foam insulation to the outside of a concrete wall will be difficult, and that means it will be expensive, Yupster adds. His suggestion is for Downie to consider Amvic’s R-30 ICF wall. That, he says, would be “a great option for your target.”
Our expert’s view
GBA technical director Peter Yost added this:
With Joe Lstiburek’s Perfect Wall, all of the insulation is placed on the exterior of the assembly so everything to the interior is close to experiencing interior conditions. The Perfect Wall is ideal for every climate from a hygrothermal perspective. What building materials don’t like steady temperatures and moisture content and no exposure to ultraviolet light?
Lstiburek does not call out concrete specifically in his Perfect Wall—he uses the more generic term, “structure.” As he adapts the wall to specific applications (institutional, commercial, residential), he does specify the structure as concrete, steel, or wood, respectively. And while Lstiburek does not elaborate on all of the reasons for switching structure materials, I am pretty sure he does so in terms of cost, code (particularly fire), and custom (customary structure for residential is wood-framed).
But he does start off by describing the “best of the best,” the “very special wall” with concrete block for structure. Not cast concrete, but concrete block and again, probably for cost consideration.
Where does all this leave John Downie? Here is my cut:
- Consider concrete block instead of cast concrete as a more efficient use of cement.
- Consider alternative concrete blocks that further reduce cement content, such as NexCem and Faswall.
- From an environmental and global-warming perspective, find a substitute for portland cement wherever you can. For example, in CMU grout.
- From an environmental and global-warming-potential perspective, value-engineer your block, using 8-in. where you can instead of 10-in., or 6-in. instead of 8-in.
- Don’t cheat on the concept of all insulation to the exterior. The insert insulation is just silly given the thermal bridging of webs in any type of block.
- An ICF does not cheat on the continuity of all of its insulation. Both the interior and exterior face insulation are continuous. However, the best place for all that structural mass is on the inside. The interior face of an ICF dampens the thermal mass connection to the building interior.
- For both energy efficiency and acoustics, focus with equal intensity on the thermal and airtightness of your windows as you do your walls.
- The performance of your ceiling or roof assembly is as important as your wall. Pay particular attention to the thermal bypass that is inherent to eaves, where wall and roof assemblies meet.
- In a modest home, design to carry all loads to your exterior walls, meaning all interior room configurations can be more easily changed over time. To build a 500-year structure, as Malcolm Taylor points out, you need interior flexibility.
- Whenever you can, build out trim such as baseboard as wiring chases with shallow electrical boxes. In my own SIP kitchen addition, I was able to avoid any panel routing by building out my baseboard trim.
- In our neck of the woods (Brattleboro, Vermont, which is not much different than Ontario climate-wise), the backup for mini-split heat pumps is a high-efficiency woodstove.
–Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.