Deep energy retrofit questions
Finally, I am buying a home after 30 years. It’s a wreck of a foreclosure, which means I’m doing a deep energy retrofit, and want to do it right!
1250 sf. 1930s 2-story with unconditioned basement. Halfway between NYC and Albany. Existing ductwork has supplies upstairs, but no returns. We’re taking it to studs.
1) I’m planning 4″ dense pack cellulose plus an R8+ foam sheathing on the exterior. ** Is sheathing rot a problem under foam board if there’s not enough R value on the foam? Any rule of thumb ‘not worth it’ for foamboard over well-insulated dense pack walls?
2) Triple pane windows on upstairs for sound attenuation and design considerations. ** Opinions if triples are worth it, particularly for skylights? ($100 or so more per 8 sf skylight)
3) We’re doing air source heat pump and I’m assuming the house will be well within the air tightness range of needing an HRV/ERV. There are several returns on the first floor, and I’m thinking of putting some on the second, and then using the ASHP fan to circulate the ERV supply. ** Are any particular variable speed ASHP fans up to that task – to run low, often, and quietly? Aside from a switch from the ERV for the handler to turn on, are there any other common considerations? Not sure we can run separate ducting to the rooms unless we go with a fan assisted small diameter tubes.
4) Other pitfalls for converting from an old oil furnace to an ASHP using existing ducting? Any leads for info for me to understand air flow needs better?
Tons of questions. Thank you so much for your time!
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Q. "Is sheathing rot a problem under foam board if there’s not enough R-value on the foam?"
A. Yes -- but your plan is safe. For more information, see this article: "Calculating the Minimum Thickness of Rigid Foam Sheathing."
Q. "Opinions about if triple-pane windows are worth it?"
A. In your case, you're doing it (in part) for sound attenuation, so you're the only person who can answer that question. I doubt if you'll have a fast return on your investment for the upcharge in terms of energy savings, but there are other reasons to choose triple-pane windows (including improved comfort and lower sound transmission).
Q. "I’m thinking of ... using the air-source heat pump fan to circulate the ERV supply."
A. Don't do it. Instead, you should install dedicated ventilation ductwork. This article explains why: "Ducting HRVs and ERVs."
Q. "Are there other pitfalls for converting from an old oil furnace to an ASHP using existing ducting?"
A. Yes, there are other pitfalls. Ducted minisplits from Asia don't have as powerful an air handler as American air handlers -- so you have to design your ductwork for a stricter static pressure limitation. Use Manual D to design your ducts -- or, if you don't feel up to the task, hire an experienced mechanical engineer to do your duct design.
Thank you, Martin!
Re skylights - exercise restraint here. Skylights are notoriously hard to insulate well and will, at some point, leak water. Increase your window area, add sidelights around the entrance door, hire a lighting designer (or study some lighting design guides), and if you absolutely have to put a large hole in your roof, consider a solar tube instead of a skylight.
Aside from Martin's cautions and references for ducting, think about it: what's the part of HVAC that's always wrong? Duct work. And duct work that was appropriate for a very old system is not likely to be appropriate for a new system. Start over. (Easy to say with someone else's money.) And if you do a good job of air sealing and insulating, you may not need much/any duct work other than for your ventilation system and bathroom exhausts.
P.S. One of the big benefits of mineral wool or cellulose insulation is the sound damping that it provides. I've had cellulose added to the attic in several houses, and the difference can be dramatic. I haven't lived in a house with exterior insulation, but I imagine that that could make a big difference as well. I suspect you'll be amazed at how quiet your house is with cellulose + exterior insulation, assuming you pay attention to air sealing.
Thank you, Andrew. Good points.
Look for reclaimed foam insulation. And I agree with Andrew. Take out the skylights.
As for triple pane windows, a big plus is comfort. In your pretty cold climate, if you're replacing the windows anyway, I'd go with the best performing windows you can afford. Use the savings you'll get from deleting the skylights.
Triple pane is better but not by a huge margin. Sound is mostly the function of the distance between the two panes and the thickness of the glass. Double pane with laminted glass (sometimes sold as a security option) with wide spacing is what you want.
Before you pull out the existing furnace, get a decent micromanometer and measure the flow/pressure through both supply and return ducts.
Since you are significantly increasing the insulation, the heat load will be a lot less, which means you will need much lower CFM to heat the house.
Generally the ducts that were sized for the old unit will flow enough for a medium static pressure ducted mini split at this reduced flow. Without checking is just a guess, so check first. Adding in a properly sized return on the 2nd floor is definitely important.
With a gut job, go with dedicated ductwork for the ERV. It is easy to put in and just works better than sharing the HVAC, plus it uses less electricity in the long run.