Which Comes First: Air-Sealing and Insulation or New Minisplits?
Building science experts weigh in on an important question about best practices for installing a minisplit.
Slateandall (let’s call him Slate) has got his hands full. Writing from southern Connecticut, Slate describes his 1910s Tudor-style home as more or less of a train wreck from an energy performance point of view.
The house, according to Slate’s Q&A forum post, has no insulation to speak of and an aging gas boiler. Indoor air is high in humidity in the summer with heating costs running between $4000 and $5000 a year.
“The first thing we did energy-wise was install a minisplit HVAC system for the southern half of the house (all three levels) so we can survive the summer while figuring out what else to do,” Slate writes. “It makes the whole house livable, with the help of a couple window units.”
Then Slate brought in an energy consultant who developed a comprehensive plan. It included insulating and air-sealing the attic with R-49 loose fill on the attic floor, air-sealing the entire house, installing a “big dehumidifier” in the basement, and installing a minisplit to heat and cool the rest of the house.
The estimated payback for this work is 15 years, but that doesn’t include the additional minisplits.
“I have so many questions I don’t know where to begin,” Slate says. Should he skip the insulation and just install the minisplits? Will an insulated attic be too hot in the summer? Should he put a fan in the attic to keep it cool? Does it matter if the basement is humid? How to insulate the original slate roof is another issue.
In other words, what now? That’s where we begin this Q&A Spotlight.
Insulate and seal the leaks
Michael Maines, both a builder and a residential designer, recommends that Slate make sure the building enclosure is well insulated and air-sealed before he pulls the old gas boiler. If he skips the insulation and goes to the minisplit installation for heating and cooling, there will be problems.
“They will have to run too much, leading to frosted coils, high energy bills, and reduced comfort due to lukewarm air blowing continuously,” Maines writes.
Likewise, ignoring the humidity problem in the basement would be unwise.
“Your basement is connected to the rest of the house,” Maines adds, “and damp air tends to rise. So as your house gets tighter you could see moisture problems, usually near the ridge and sometimes at windows and other condensation points.”
As to the slate roof, Maines does not think it’s a good idea to apply spray foam to the underside of the slate roof in the space behind knee walls. Old slate roofs need to dry on the back side, he says.
But insulation will be expensive
Slate recognizes there are no easy answers here, but he worries that insulation will be costly, while air-sealing carries risks of its own.
“It seems to me that the cost of insulation vs. the price of energy at the moment makes it a nice-to-have luxury that can be kicked down the road rather than a must,” Slate writes. “And the thought that the sealing will make the basement more humid than it already is kind of scares me. I am used to air circulating more or less freely throughout the house.”
If, however, he could be assured of getting 80% of the benefit by spending 25% of the total cost, he’d go for it. “As with many things in life,” Slate says, “the first 80% is the easiest by far.”
“As to the windows,” he continues, “the energy consultants I’ve spoken to have universally said the windows aren’t a big issue (they will seal them with the usual weather sealants). I suspect a window contractor would stress all of the benefits of new windows (which are real, just expensive) and unsightly (to me).”
He adds that a blower-door test found that air leakage is more than four times what would be typical in a house of this size, and that it could be reduced to 2.8 times the standard if their recommendations were followed.
Don’t worry about the basement
Don Miller has a different point of view. First, he’d hold off on adding any insulation until there’s a final HVAC plan in hand. “Your energy consultant should be at least partially justifying the work based on payback,” Miller says.
As long as the boiler is less than 25 years old, Miller doubts there would be much savings in replacing it with heat pumps. (But he’s basing that on an incorrect assumption that Slate’s heating bills are not much more than $1000 a year.)
Further, Miller says there’s no need to worry about humidity in the basement (just get a $300 dehumidifier, he advises), and forget about the air-sealing.
“No reason to worry about sealing the house when the goal is 2.8 times standard,” Miller says. “Don’t forget about winter humidification. In my heavily restored/remodeled 1920s Tudor I need a steam humidifier to reach decent levels due to air leakage (3000 square feet and a yearly heating bill of about $700).”
That’s nuts, replies Maines.
“I strongly disagree with Don about the basement,” he says. “Every weatherization contractor knows that after fixing a leaky roof, the next most important task is to get the basement dry and air-sealed, not just for comfort and energy but to reduce the risk of mold and rot taking hold. A ‘disposable’ $300 dehumidifier is a good start, but putting a modicum of effort into reducing the source of water, moisture, and air will likely have a relatively short payback period.
“A better way to think of your investment in your home than payback period is the return on investment,” Maines continues. “A 15-year payback is comparable to a 7% simple return on investment. That may not be as good as the stock market average, but not too far off either, and about as safe as investments get.”
Is there too much ‘conflicting science’?
For Slate, the conflicting advice is difficult to process.
“I care a lot about these things, probably more than I care about the heating bill,” Slate says. “But there’s too much conflicting ‘science.'”
It’s not science, replies Miller, adding that it’s not possible to make a pre-war house perform like a new building without a gut renovation. “If you want to spend well over $500K to gut renovate your house in a historically sensitive manner to save a couple hundred a month on HVAC, that is a choice,” Miller adds. “But no one has ever taken a house like yours and for a few tens of thousands of dollars turned it into a house that needs an ERV to balance humidity in winter.”
Maines isn’t buying it.
“Slateandall, I’m not sure what Don Miller is doing on a green building website or what his background is, but sealing and insulating basements is done all the time and rarely doesn’t make sense,” Maine says. “There is not conflicting science; there are well-established techniques to do it safely and reasonably economically . . . I agree that it would not be financially wise to invest in a deep-energy retrofit, but shallow-energy retrofits are often good investments, for the reasons I noted above.”
Why not install minisplits now?
Natch Greyes wonders why, if minisplits are so much more efficient than Slate’s gas boiler, shouldn’t he just go ahead with that part of the project now.
The reason, says Peter Engle, is that if Slate waits until the house has been insulated and air-sealed, the equipment needed to heat and cool it will be much smaller. It will cost less to install, and less to run.
“It is very important that HVAC equipment be properly sized for your loads,” Engle says. “If you reduce your heating and cooling loads by half (not at all impossible in a house like that), then the equipment capacity would also be reduced by half. If you installed equipment that meets your current loads, it would be dramatically oversized for the house once you make your energy improvements. Oversized equipment costs more, takes up more space, wastes energy, and often provides poorer comfort performance.”
But, Slate says, what if “life is too short to worry about perfection and [I] am willing to tolerate oversized minisplits for the next 30 years?” He argues that he will have accomplished his goals of cooling the house in summer, helping the boiler in winter, continue to have zoned heat, and lengthen the boiler’s life.
“When the gas boiler does go kapputz,” Slate says, “replacing it won’t be an emergency and maybe then I will consider insulation.”
Our expert weighs in
Kohta Ueno had this to add:
When GBA first asked me to contribute and I read through the comments, my reaction was, “What do you need me for? Michael Maines is on the case.” Reading through his posts made me think, “Yep . . . yeah . . . uh-huh . . . agreed.”
To reiterate what several posters said: Put the effort into the enclosure first . . . and no, I don’t mean “achieve perfection.” I mean Slate should address the catastrophic air leakage that is likely occurring. In short, stop the spurting arterial bleeding before you start working on blood transfusions. As others have noted, an improved enclosure will allow for smaller (and more economical) mechanical systems, not to mention energy, comfort, and humidity control improvements.
In addition, we often point out that enclosure upgrades have a much longer lifetime than mechanical systems. Typical mechanicals have expected lifetimes of ~20 years (much less for water heaters). Yes, I know many of us see much older systems limping along. But enclosure upgrades have much longer lifespans, particularly in the opaque shell—easily 75 years or longer.
As a side note, I suspect that one of the big contributors to air leakage (only stated as “four times typical,” but no ACH50 number) is the roof design, with attics and knee walls. Martin Holladay did a great job of explaining how to go about retrofitting that condition in this article.
On the slate roof: I agree with Mike. I would be a little bit wary about shooting foam right against the underside of the roof boards. We’re dealing with a 110-year-old waterproofing system that likely relies on that downward drying to survive incidental rain penetration. Short of tearing off the roof and redoing it, a more conservative compromise is to install a ventilated air space (or drained and semi-vented, if you can’t get a ridge vent in) in those rafter bays. That is the approach we took in a deep-energy retrofit we did in Jamaica Plain, Mass., that had an existing slate roof.
On the basement: Yep, I’d definitely want to keep an eye on the humidity conditions. Of course, we all know that air leakage in summer makes basements wetter, air leakage in winter dries basements out, and that if water is running down the walls, that’s not helpful at all. Addressing the basement liquid bulk water issues is a step zero item. From there, Slate can probably incrementally work at the problem, including adding a dehumidifier . . . 60% RH is a decent first-cut set point for the unit.
There’s more on keeping liquid water out of basements here and here.
A strange aspect of damp basements is that the moisture emissions vary seasonally, due to the temperature of the foundation. In winter, if the foundation surface is cold, the surfaces might actually be absorbing moisture from the air, rather than evaporating moisture into the air.
On windows: Replacing windows saves energy, but not enough energy to make financial sense in terms of payback, given how much replacement windows cost. Check out Comment #19 in this Martin Holladay post.
In terms of energy paybacks, the energy consultant’s guess of 15 years is a pretty reasonable number for enclosure improvements, but one of the obvious questions is, “How long will you stay in this house?”
Overall, Good luck Slate, and I hope you’re finding the greater GBA community helpful instead of more confusing!
Originally published on GreenBuildingAdvisor.com. Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine. Kohta Ueno is a senior associate at Building Science Corp. Comments may have been edited for clarity. Use the link at the top of the post to see the original thread. Photo courtesy Scott Gibson.
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