Brad Steeg’s Seattle home was built in 1915, and from the description he provides in this post at GBA’s Q&A forum, it’s not hard to understand why Steeg is so uncomfortable during the winter: not much insulation, single-pane windows, and lots of air leaks.
“During the winter, my thermostat reads 70° but it still feel cold because the cold walls and ceiling suck the heat out of my body,” Steeg writes.
Exterior walls on the single-story, 900-square-foot house are framed with 2x4s. Steeg says some fiberglass batt insulation has been “stuffed in nooks and crannies” as previous owners opened up walls in the past. But the insulation is apparently spotty, and what’s there is dirty, indicating plenty of air gaps.
His plan is to add Tyvek housewrap, two layers of 2-inch-thick Roxul ComfortBoard 80 mineral wool insulation, 2×4 furring strips to create a rainscreen gap, and, finally, fiber-cement siding. Ceiling insulation will have to wait until an electrical upgrade in the future.
Steeg’s single goal is “comfort.” The question for this Q&A Spotlight is whether he’s on the right track.
Air-sealing before insulation
A continuous layer of insulation on the outside of the wall is typically a good idea, but adding insulation to a wall with lots of air leaks is putting the cart before the horse, suggests Dana Dorsett.
“Adding insulation over an air-leaky wall isn’t the best investment, since the air leaks can be anywhere,” Dorsett says. “Whether you add insulating sheathing or not, fixing the air leaks is the first order of business.”
One option would be to drill holes through the siding and sheathing (or removing a clapboard) and blowing in either cellulose or fiberglass from the outside of the house. That, Dorsett says, would “tighten things up a lot” and give Steeg a whole-wall R-value of about 10, even before any exterior insulation was added. Even a single 2-inch layer of ComfortBoard would cut the heat loss through the walls nearly in half.
“An old 102-year-old house like the one you describe is a good candidate for blower-door-directed air sealing,” adds GBA senior editor Martin Holladay. “Hire a home performance contractor or a weatherization contractor equipped with a blower door and focus on air sealing.”
Steeg plans on removing all of the existing siding, he replies, so it will be easy to drill holes through the sheathing and blow in some insulation. However, he wonders whether it would be safer to remove the drywall inside the house to insulate the wall cavities because in that case he would be able to check the wiring.
“That way I could remove all existing bat insulation (most likely incorrectly installed, just like it is incorrectly installed in the attic) and I can inspect the wiring,” Steeg writes. “With an old house like this I don’t want to do anything without seeing it first.”
He adds that when he removes the siding, he will be able to seal cracks and seams in the sheathing and install housewrap.
“I could place a membrane over the sheathing to completely air seal the walls,” he says, “but in an old house like this I’d be nervous. Rainscreen over 4 inches of Roxul over taped Tyvek over my sheathing seems like the least risky approach to insulating the walls of my home.”
New windows will make a big difference
One reason the house feels so cold may be the single-pane windows and clear glass storm windows, or clear double-pane windows, do not have a low-e coating, Dorsett says.
“Weatherstripping the windows and replacing the storms with tight, low-e storms can also make a huge difference in comfort,” he says. “The low-e storm on the exterior raises the temperature of the interior side glass, reducing the convection drafts and raising the radiant temperature slightly. That might make the most economic sense if you don’t have to pull the windows when adding the exterior insulation.
“If you’re pulling the windows and replacing them, a U-0.25-ish double-pane with low-e coatings on surface #2 (the usual low-e glass unit) and surface #4 (the inside facing surface glass) can make a major uptick in comfort,” Dorsett continues. “The low-e on #4 lowers the surface temperature of the window a bit, but it reflects body-heat and room heat back toward the source, improving the mean radiant temperature (the primary source of comfort) by quite a bit.”
In fact, he says, the comfort shortcomings in the house can best be explained by single-pane window sash.
“The U-factor of wood sash single-panes is usually about U-1.0, aluminum sash single panes run about U-1.2. That is about 10 times the heat loss of a cedar-clad 2×4/R-11 wall per square foot, about 4 times the heat loss of an uninsulated 2×4 wall, and the windows are likely to dominate the current heat load numbers, while dragging down the mean radiant temperatures.”
Unless Steeg is shooting for Passive House performance, triple-glazed windows would be overkill. Yet replacing older units with low-e glass would make a dramatic difference in the mean radiant temperature of the room, and thus just how comfortable the house will be.
What’s the best exterior insulation?
Steeg clearly has mineral wool in mind for exterior insulation, in part because of the horrific fire in London earlier this year at a high-rise apartment building clad in a product containing polyethylene foam insulation.
Holladay, however, tells him that a foam insulation exterior will not increase the risk of fire.
“But if rigid foam makes you nervous, you can certainly use semi-rigid mineral wool instead if that’s what you want to do,” he says. “Just be aware that installing semi-rigid mineral wool isn’t as easy as installing rigid foam, because mineral wool is squishier. That makes fastening the furring strips a little trickier — it’s harder to get the furring strips co-planar with mineral wool than it is with rigid foam.”
Yet there’s another reason Steeg might prefer mineral wool over rigid foam insulation — ants.
Carpenter ants are a particular problem in the Pacific Northwest, he says. Citing , Steeg notes that carpenter ants like wet wood because it’s easy to chew. And they like foam insulation.
Our expert’s opinion
Here’s what Peter Yost, GBA’s technical director, added:
Even if it’s all about thermal comfort, you still need to honor the physics embedded in hygrothermal management.
Here is a document I use with all my clients on what we have to manage and why in older buildings: Hygothermal Analysis Summary.pdf.
The order of priority is always the same:
(1) Bulk water management
(2) Air leakage (convective) control
(3) Addressing dedicated directional drying potential
(4) Thermal (conductive) control.
Can you detail housewrap as a key element of your continuous air barrier? Sure, just make sure that it connects/extends to overlap with below-grade wall and roof air barrier components. But where is the control of air leakage most important? Low and high (from stack effect in cold climates, Climate Zone 4 and above), so rim joists and attic should receive the greatest initial attention in terms of air sealing. You should strongly consider redoing your wiring before you address the #2 priority, air leakage.
Windows absolutely are key to thermal comfort, from both a convective and conductive perspective. You can keep your existing windows and increase thermal comfort, and if you choose that route try using to learn more about your options. If you are considering a window replacement, use for guidance.