Distribution Efficiency in upper Midwest basement
I am an energy auditor in the upper Midwest, and I am trying to figure out how to accurately account for the benefits of duct sealing in what is very common in our climate zone: semi-conditioned storage basements with no insulation on either the walls or ceiling.
Most energy auditor programs have a black/white definition of conditioned vs unconditioned basement, and the on-the-ground reality of it just seems not that clear cut. I’ve seen plenty of cold basements that are barely 50° but technically have a register on the furnace to keep pipes from freezing.
Actual technical research tests on homes on the matter has shown that sealing ductwork in these kind of basements does save energy (albeit less than attics or vented crawlspaces, of course). Examples:
https://weatherization.ornl.gov/wp-content/uploads/pdf/2001_2005/ORNL_CON-480.pdf
https://www.aivc.org/sites/default/files/airbase_7474.pdf
And it sort of seems that the world is starting to recognize the importance of sealed ductwork. It’s now a requirement in 2021 IECC, and Technical Reference Manuals for Minnesota & Illinois, for example, are identifying the savings from sealing in “semi-conditioned” basements. The pressure imbalances leading to increased infiltration alone is worthwhile, it seems. Yet programs like Snuggpro show $0 benefit from duct sealing in a basement that isn’t fully “unconditioned”.
Herein, then, lies my question:
I have access to an energy modeling program that let’s me control the estimated distribution efficiency. I’d like to be as accurate as possible, living in a world of imperfect data, but I don’t want to run afoul of BPI-2400/ASHRAE 152 definitions. What should I use to track effective ductwork distribution efficiency?
… would suggest a basement with leaky ductwork in climate zone 6-7 would have distribution efficiency of 92%, rising to 95% if sealed (3% efficiency improvement)
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Replies
Sealing the ducts is just the wrong place to be putting emphasis.
They need to be working on air sealing and insulating the basement. When that's done the duct leakage becomes a non-factor.
If the basement is being heated more than is needed to keep the pipes from freezing, just close the damper. But that will make the first floor use more energy and be less comfortable, so it probably won't save much.
By focusing on duct sealing you're encouraging people to put their money into something that does them no good, which ends up being counter-productive.
Interesting. Your premise of doing them "no good" isn't backed up by the actually studies of homes linked to above. But don't get me wrong, I am a big proponent of basement wall insulation, even if the relatively low price of natural gas per therm in our area may not validate it looking strictly at ROI/SIR. The benefits to the building and occupants will never be ignored by me. Our firm insulates basements all the time.
I just have never been one to accept the argument that "if you can't do it perfect, don't do anything". I deal with a lot of low income residents who don't have the means to afford to have their basement insulated. Do I tell them "tough", and effectively allow a home to use more therms than it necessarily needs to? (Even if the effects of duct sealing is relatively small compared to wall, the material cost is very low). That's even more carbon emissions, albeit small, contributing to climate change. From that perspective it is irresponsible of us to ignore. Especially since the side benefits of sealing in even conditioned spaces is enough for the 2021 IECC to require testing and sealing in new construction.
My original questions still stand.
I guess my question still stands of how sealing the ducts is any different from just closing the damper on the register.
There's nothing magical about ductwork. Basically what you're talking about is heating the basement less and maintaining a lower temperature. You could model that. The heat flow through the walls of the basement would decrease. The flow down from the living space would increase. Infiltration would increase as the stack effect would be greater.
To implement it you might look into a thermostatic damper on the basement vent. That way you'd be able to avoid closing it too much and risking pipes freezing.