Value of significant insulation for a garage
Hi all,
I’m in Zone 5 (Colorado Springs, Co) and in the process of designing a carriage house (2 car garage below an apartment). General plan is to do 2×6 on 24″ centers, blown in cellulose, air barrier external to sheathing, then exterior rigid insulation sufficiently thick for my climate zone.
Where things get more complicated is when looking at the garage level. It does not appear that there are very good garage doors both in terms of thermal bridging/insulation and very good sealing around the doors. That being the case, does it make sense to go to pretty good house levels of insulation on the garage level? The garage ceiling will need a good air barrier to keep garage air from going into the apartment anyway (tentatively thinking of a couple of inches of spray foam on the ceiling coupled with careful air sealing/construction). Prior to realizing the achilles heal that is the garage door, I had been planning on rigid insulation for the entire exterior of the structure, insulated slabs and footers. I’m considering doing some modelling to put some numbers to this but guessing some folks have already looked at this, does it make sense to do more than standard insulation (cellulose studbays, air sealed exterior) on the garage level?
Cheers
Tim
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Replies
Tim, it depends to what level you intend to condition your garage. I've been involved with projects that have up to Passive House levels of insulation at the garage, but as you note, the door is the weak point. In terms of environmental impact, it very likely would not make sense to use rigid foam anywhere at the garage level, but to use code-minimum levels of cellulose and keep the garage as cool as you can stand.
Thanks for the response Michael. It is a garage so I don't intend to be overly aggressive in maintaining a set temp. I will likely want to keep it 50F or above in winter and 85 or less in the summer, with occasional times I'm projecting it where I'll set it more comfortably. Likely will only put a small electric heater to maintain a minimum temperature (planning all electric in the house and designing for addition of PV).
Tim, with a 97.5% design temp of 2°F and about 6400 heating degree days in Colorado Springs, keeping your garage at 50° all winter is a significant energy load, so I would probably go above the building code for insulation. And/or I would consider keeping it cooler except when I'm working out there.
Real numbers, nice! I'm an engineer but my world hasn't been in green building except on the side. Can you elaborate on the 97.5% design temp of 2F? I do understand the heating degree days. Yeah, perhaps 50F is too high, 40-45 could be more realistic. Don't want pipes freezing in there (and will have a heat pump water heater in there, not helping with the temp).
Tim,
Saying that "the 97.5% design temperature is 2°F" means that Michael Maines looked up the outdoor design temperature for your location (Colorado Springs) and discovered that your 97.5% design temperature -- in other words, the temperature you design your heating system for -- is 2°F. That means that for 97.5% of the hours in a typical Colorado Springs winter, the outdoor temperature will be at 2°F or above. If your heating equipment can keep the space warm at 2°F, everything will be fine.
My own approach would be to insulate the garage ceiling very well, and let the garage get cool. But I think it's nuts to put a heat-pump water heater in a garage in a cold climate, and then heat the garage with electric-resistance heat just to keep the heat-pump water heater warm -- especially since the heat-pump water heater acts like an air conditioner.
So choose a different type of water heater, and use electric heat tape on your water pipe.
Thanks for the feedback Martin. I'm tempted to ask how often the heating degree days stats are updated. I've lived here 18 years and our climate has shifted significantly to the warmer.
In terms of the heat pump water heater, i was under the impression that most were designed to at worst behave like standard electric water heater (actually engaging an electric heat element below a certain ambient temp) and at best get significantly better efficiencies than a standard electric water heater. Sounds like you're saying that's not the case....
Tim,
With most heat-pump water heaters, it's up to you how you want to operate it. In winter, you can set the controls so that it acts as a heat-pump water heater, in which case it will cool the room in which it is located. Or you can set the controls so that it acts as an electric-resistance water heater.
But if you are using electric-resistance space heat to keep the garage warm enough for the water heater to be happy, the energy picture gets complicated. Briefly, though, heating the garage for this purpose is an expensive way to heat water.
Martin, I looked for some data on hybrid heat pump usage in a garage setting. I thought this report was interesting in measuring the impact of different locations. In a perfect world it would involve some real world testing, but the trend in their data is pretty strong. Your point is still very well taken, don't heat the garage except when absolutely necessary if at all. https://www.nrdc.org/experts/pierre-delforge/very-cool-heat-pump-water-heaters-save-energy-and-money
Tim,
The report you linked to notes, "In cold climates, heat-pump water heaters perform best when installed in the basement rather than in a garage or vented closet." I agree with that assessment.
For more information, see these two articles:
"Heat-Pump Water Heaters Come of Age"
"Domestic Hot Water: No Perfect Solution"