Maximizing Passive Heating Potential
Buying a regular code built new home in climate Zone 4 with 4000-5000 degree heating days. I would like to maximize passive heating of the house. Front of the house faces south and am maximizing number of windows there. Garage also faces south. Builder standard is to provide insulated garage door with glass windows. Garage also has 2 west facing windows. I think all this will mean that the garage could be a good heat source. Is that correct? If so, how can I transfer that heat usefully to the rest of the house? Appreciate any guidance. The house building will start in the next month, hence looking for suggestions that the builder wont find difficult to implement.
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A garage for a code built home is not going to be a source of passive heat in the winter, and that's putting it mildly. Garage doors leak like crazy and aren't well insulated. The windows are probably going to be mediocre at best, possibly poor.
Passive heating is a tough nut to crack, even in a house specifically designed and built for it. Adding the restriction of code built is going to make it pretty much impossible. Code minimum windows are going to be energy losers, guaranteed. South ones will lose less than north ones.
I agree with Trevor. The first and best thing you can do at this stage is to make certain that your house is airtight. As soon as the house is framed and windows installed, and before the insulation goes in, have an energy auditor come in and do a blower door test. Spend time while he's there and while the fan is going, to go around and find all the leaks, then fix them. A leaky house is normal; it's also usually uncomfortable and expensive to heat because you're heating your yard. Passive heat only works if the heat stays inside the house. It does not work if the heat promptly goes out through cracks, through the cracks in the sill to foundation, through the framing next to windows and a lot of other places. Find them and fix them. No amount of insulation nor south or west facing windows will make up for a leaky house.
Bob, Trevor; Thanks for the prompt response! I think I over used the word "passive" since the house will be built with a decent sized HVAC unit. I was just trying to see how to make best use of the heat from the garage, that is all. Understand your comments about air leaks. I will try to include the blower test as suggested by Bob to find and fix air leaks. How long does it take to prepare for and do the test and what would it cost if the builder is going to ask me to get it done? Once again, impressed with this website and the speed of matter-of-fact response. I know I am listening to experts!!
Check with your Town to see what codes they go by; the national building code calls for 3ACH 50. I don't know about your area, but here, most builders and inspectors ignore that and the house is never tested. If the code applies there, the builder is required - required - to build the house to that level, whether he knows it or not. So if it's over that it should cost you nothing. That said, houses can get much better than 3ACH50, and this makes for a more comfortable house. I'd suggest that below 2ACH50 you should install a ventilation system which can be simple through the wall "Lunos' type units.
Typically tightening a house requires caulk, cans of spray foam (Cheaper to buy a foam gun and canisters) and good acrylic peel and stick tape. None of this will break the bank.
The article in this issue of GBA - "How to Meet Air Leakage Requirements" is exactly what you need at this point.
What size heating system you have is irrelevant. Most heating systems are vastly oversized and therefore don't work as well as they should anyway. Building a tight house means it will use less heat, but it primarily means that the temperatures will be more consistent and even throughout the house and everyone will be more comfortable throughout the year.
My home has lots of windows on one side for the view. I had already built the computer model of the home to select insulation and heating systems. I ran the model with the house in every direction. I found for my house in my climate orientation made almost no difference in energy usage.
Also be aware most fire codes will not allow you to transfer air from a garage to your living space because it would allow a fire to spread.
Generally the air quality in garages is poor because of the exhaust fumes and the car off gassing when off.
Walta
Folks, as I read your responses, I see that I am trying to figure out the size of the umbrella to use to stay dry in a rain storm in Houston! My current house in Houston built to standard code in 2014 did go through a blower test (5ACH?) and hence I was taking it for granted that the new house In Virginia being built 8 years later would be much tighter. Alas, on your suggestion, Bob, I looked it up and the way I understand it, while VA has updated the code recently to include blower test (3ACH?) effective June 2021, the builders have a 1 year grace period to implement. May be I am mistaken. If I am not, then it is really up to the builder for my house that is starting to be built now. Any suggestions on how best I can bring this into the project?
Start by telling the builder you want a tight house; minimum 3ACH50. He needs to learn how to do that anyway, so he might was well start on your house. Basic stuff: use a peel and stick, moisture permeable membrane on the exterior. Henry Blueskin is one brand. (Note the moisture permeable - you do NOT want an Ice and Water Shield type moisture BARRIER - that's a fast way to a huge mold problem.). Use window & door spray foam around your windows. The most common leakage issues are between the sill and the foundation - so that needs to be sealed well. Also the ceiling - attic is a huge area for common leaks, but I can't make recommendations without knowing more about your construction.
Also - Blaise Pascal figured out how to measure pressure, so the measurements are called "pascals". So the code for a building is so much air (here, 3) at 50 "pascals", or 50p. 25p, for instance, is a different quantity. So the term is 3 Air Changes per Hour at 50 Pascals, or 3 ACH50, or 3ACH50p.
+1 on improving air sealing. Ideally you would have an agreed target with the builder (<3ACH50) and you'd do a blower door test prior to insulation so that you can find and fix the biggest leaks easily. A final blower door test should then be uneventful.
I don't think you want a house to be closely coupled with the garage. They tend to leak and have contaminated air (think gas, fertilizer, paint, insecticide, and all the other crap that gets stored in the garage because it'd be crazy to store in the house.). Also, a garage typically becomes superheated during the summer, and you don't want that heat in your house.
I understood what you meant, so my answer remains the same. Trying to get heat from the garage will result in the opposite, a net loss of heat. Andrew made an even more important point, which is that you absolutely do not want to share air with the garage for health reasons. Even having an attached garage at all is frowned upon, so you want to air seal between garage and living area as best you can, probably even having a positive pressure in the house relative to the garage, so any air transfer (not just leaks, but when you open the joining door) goes in the direction of the garage.
Thanks folks for well thought through, candid, comprehensive responses, all within a day! Really appreciate it. I will post the outcome of my discussion with the builder, hopefully within the week.
You might find this article interesting: https://www.greenbuildingadvisor.com/article/gba-prime-sneak-peek-reassessing-passive-solar-design-principles
Money quote, from Dr. Joe Lstiburek:
“Don’t bother with the passive solar,” Lstiburek wrote. “Your house will overheat in the winter. Yes, you heard that right. Even in Chicago. … You should go with very, very low SHGCs, around 0.2, in your glazing. If this sounds familiar to those of you who are as old as me, it should. We were here in the late 1970s when ‘mass and glass’ took on ‘superinsulated.’ Superinsulated won. And superinsulated won with lousy windows compared to what we have today. What are you folks thinking? Today’s ‘ultra-efficient’ crushes the old ‘superinsulated,’ and you want to collect solar energy? Leave that to the PV.”
I've been in my passive house, with pretty big, 0.5-0.6 SHGC windows for over three years. It has not overheated in the winter; not once, ever. I guess if you tried to, you could probably do it. But when all you have to do is open a window to regulate the temperature of needed, his statement is just silly. The real challenge with passive solar is getting the right design and assemblies such that you're not losing more at night than you gain in the day.
Dr Joe is dead wrong on this. I can second what Trevor said. I have a sun tempered house, get about 50% of my heat passively (modeled), and overheating is not a problem.
Location is important. I'm in Colorado, I don't know about Virginia. But with proper overhangs over the south windows, overheating shouldn't be a problem anywhere.
I'll join the others in noting that blind opposition to passive solar is anti-science. Joe is a brilliant (and humorous) building scientist, but he's being one-sided in this one rant.
To make passive solar work, you need to:
1. Be building in a suitable climate--cold, reasonably sunny winters. It won't work in a climate with overcast winters, and isn't sensible in a climate with mild winters. "Zone 4" is helpful, but if you provide your location or a nearby metro area, we can look up the solar potential. How sunny or overcast are your winters?? Look up your location in https://weatherspark.com then look up Denver, CO (very sunny cold winters) and Buffalo NY (overcast cold winters) for comparison.
2. Passive solar space heating only works if the building is airtight and superinsulated, so the heating load is fairly low. Arguing that "superinsulation beats passive solar" is silly. This isn't a competition of what is more important: Airtightness? Insulation? Capturing daylighting and passive solar heating gains? Its doing all three in a balanced approach (i.e., do the math, figure out the losses and gains). Trying to claim one is important and others are not is anti-science but can make for some interesting articles or quotes. You can optimize the three together, along with cost-effectiveness, avoiding bump-outs in the building envelope, and other criteria, if you want an optimum design for energy efficiency.
3. During winter in Zone 4, the sun only shines about a third of the day, and most of the solar heat gains occur in only one quarter of the 24-hour day. The solar heat gain must absorbed during the time when the interior COULD overheat (Joe makes this point), then re-radiate that heat into the interior overnight, when there is no solar gain. That's commonly referred to as "thermal mass" by passive solar designers, although DCC would prefer it be called something else, like passive heat storage. A high thermal mass material like concrete slab floors, brick or stone walls, water tanks, etc. will readily absorb heat from the surrounding air or sunlight striking it. It will readily radiate that heat back into the surrounding interior air when the air temp gets cooler than the passive heat storage mass.
Here's a home in Zone 5 (colder than your Zone 4), that needed no wintertime space heating on the main floor, and some backup space heating on coldest nights on the upper floor, resulting in extremely low winter heating bills (electric backup heating):
https://www.greenbuildingadvisor.com/green-homes/a-passive-solar-home-from-the-1980s
I spent considerable time air-sealing during construction, and doubled the amount of insulation compared to typical construction at that time forty years ago. R-60 ceiling, R-25 walls, attached garage all reduced heating load by about half and minimal extra expense.
(Note that this passive solar design would NOT work in the Pacific Northwest, northern NY and VT, due to their overcast winter climates. Unobstructed south-facing windows would still be better than other windows at daylighting and wintertime solar heat gain, but just not enough to provide substantial wintertime space heating.)
Here's an article that goes into detail about solar gains, climate, and solar potential near major northern US metro areas:
https://www.greenbuildingadvisor.com/article/a-quantitative-look-at-solar-heat-gain
Take a look at the map or lists to guess at your wintertime solar heating potential.
Note that if your house DOESN'T have much solar thermal mass for heat storage, the interior will be warmer during the day from solar gains through south-facing windows, and you won't have to pay for any heating system during those times (probably a third of your day). That's called a "solar tempered" home. It is superior in energy usage than a home that ignores and fails to capitalize on wintertime solar heating gains (and daylighting).
Note also that south-facing windows are only useful for wintertime solar heat gains IF their is an unobstructed view of the sun during December-January, when the sun is surprisingly low on the southern horizon (not overhead like in summer). Rule of thumb: Objects directly south of your south-facing window need to be about twice as far from your window as their height (assuming flat ground). Otherwise they shade your south-facing window and your solar gains are much less.
One way your attached garage helps keep your home warm is it provides a warm buffer space between your home and the cold outdoors. So that shared wall space between the home and garage loses far less heat during the cold winter. As others noted, leaky garage doors etc. make a garage a poor place to capture solar heat gains, but at least in the garage itself your car will be warmer than outdoors, or you might be able to do some work in the garage that you wouldn't want to do outdoors in winter.
Folks, just settled that the county will mandate a blower door test to demonstrate 5ACH50. While this a good first step, I am trying to figure out how to get to 3ACH50 with the builder who otherwise is interested in just meeting code. I am prepared to pay for going from 5 to 3, of course. Is there a list of questions I can ask the builder to see what their standard practice covers and then to figure out what extra I could pay for to improve to 3ACH50?
Perhaps reach out to AeroBarrier and see if they have a service provider nearby? If I understand correctly they are marketing their approach as a step in the builders usual process. I reckon it's worth a chat
https://aeroseal.com/aerobarrier/
Thanks James. This was a very practicable suggestion that worked both for me and the builder. I worked with the only(?) AeroBarrier agent in VA who easily convinced the builder to include this step into the construction project after dry wall and before trim. While the builder is looking at this to ensure he doesn't fail the final blower door test that is now mandated by the county and is new to him, I am aiming for as low below 3ACH50 as possible. My next step is to work with the builder (perhaps with the help of the AeroBarrier agent, who is obviously energy savvy) to agree on a few steps the builder could take (caulk and tape party perhaps?) before the dry wall. I will update you with the results as this progresses over the next 6 months or so. Once again, thanks.
Wanted to update with my experience with Aero barrier application. Got it done after the dry wall. The initial reading before they started the sealant was 8 ACH. After the application (took about 2-3 hours of applying the sealant) air leak came down to 2.5 ACH. Thereafter the builder did the remaining trim work and the final blower test done for the county inspector came in at 2.05 ACH 50 PA. Thanks for the suggestion. I think it worked out well.
To my ear starting at 8 would indicate the builder made zero effort to make you home air tight.
Anything over 1.5 in new contraction seems like a fail to me.
Walta
I tend to agree, 8ACH50 is a leaky starting point. But, thank you @Bhasker for posting a followup note. I find AeroBarrier intriguing, because a) it's really difficult to find builders that actually build to 3ACH50 or better on a regular basis, and AeroBarrier is one avenue for overcoming that issue. And b) I think that AeroBarrier may be a good option for retrofits where a lot of leaks are no longer accessible to seal. As the company (AeroBarrier) grows and operators get more experience, I think this will become more common.
I recently finished a project that was mostly new construction, but for environmental reasons we had to keep a century-old masonry foundation and build an addition on helical piers. The architecture of the house was such that the footprint of the second floor wasn't the same as the first floor, and the footprint of the third floor wasn't the same as the second floor. Imagine a wedding cake on acid. And the first floor was split level. So this house had a lot of surface area, and a lot of transitions. I thought we did a good job on construction but the blower door came in slightly over 4. We had AeroBarrier come in, they sprayed nine gallons of caulk into the building and got us to 1.4.
I talked a bit to the AeroBarrier guy, he said that DC is now requiring blower door tests on all new construction and major rehabs. He said a typical job for him is a panicked GC or builder who has been denied a certificate of occupancy and can't get paid until they pass the test. He said he frequently sees initial blower doors that are over ten, he doesn't get paid if he can't get it under 3.0.
His second most common job is condominiums, DC has legalized recreational cannabis and some people just can't stand the smell.
Yup, as soon as he heard the aero barrier was going in, he took that as the sign to no longer give a shit. A blind man could construct a new house with 2ACH50 with minimal effort.
While 3ACH50 may be a goal, what you're really trying to do is make the house as tight as possible. That means identifying leaks and stopping them. The DIY way of doing that is once the blower door test has been done you get your own fan, pressurize the house. Then walk around the house with an incense stick and the smoke will find the leaks. Use calking and spray foam to fill the leaks.
The time to do this is when the drywall has been finished but no other interior finishing is done.
It should be trivial to get to 3ACH50. In my house there was no test & fix, just test & it was at 1.6. An air barrier, good workmanship, and sealing around the ceiling J boxes (me) was about all they did on my house.