Net-zero help!
Hi GBAers! Just joined and this is my first post, and it’s a noob question from someone who’s never built a home before: how “real” are heat studies, really?
To help me understand how to build my net-zero home, I’ve created some spreadsheet-based heat studies. From my simplistic and idealistic studies, I’ve learned that with little infiltration, good ventilation with recovery, and great insulation, it appears possible to build a net-zero home here in at 4200 feet in Salt Lake City proper. [1]
Specifically, in a 20 feet tall, 25 feet wide, and 40 feet long house with east-west axis with 289 square feet of windows oriented primarily due south with infiltration less that 0.05 ACH50, ventilation at 0.3 ACH50 with 80% recovery, R-90 ceiling, R-60 walls, R-40 floor, and R-6 windows, the model is very close to net-zero (not including ventilation power, water heating, electrical, etc.). To heat beyond direct insolation gain, I modeled electric ventilation heating and ground-tube ventilation cooling requiring less than 3 ACH. [2]
From my reading, these values are extreme and on the cusp of build-able, but the PH literature aligns generally. From my reading of Building Science Corporation articles, infiltration less than 0.05 ACH50 is difficult to build; R-60 wall enclosure has significant long-term risk moisture compromise, etc.; and, yet, net-zero homes are built successfully…presumably. But how real is this really? Are these extreme values just an artifact of simplistic modeling?
Also, one specific question: to achieve the necessary cooling in the home in the month of October, the model requires about 2.7 ACH through a 53F ground-tube…which seem like a lot to me. Is 3 ACH achievable in ventilation-only designs? (I have not been able to find much on this topic, so article pointers would be greatly appreciated…)
Many thanks for your patience, and all the great information you folks dispense.
Cheers,
Jan
[1] http://1010-design.blogspot.com/2011/12/better-study.html
[2] https://docs.google.com/spreadsheet/ccc?key=0Ale1SfajN4iydG9rdkZzanRES3NCVC05VklFUlR5NWc#gid=6
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Replies
jan,
are you talking net zero heating?
a couple of thoughts on SLC. we're working on a passivhaus project that is close in size (25' wide, 37' long, 21' tall). when i move to SLC, my insulation levels are roughly 60% of yours to meet PH utilizing serious or cascadia windows w/ cardinal 180 - once i account for required shading. this puts my specific space heat demand under 4.0kBTU/ft2a. going with something like zola (rep is in CO) could bring those assemblies down even more.
you'll definitely need to pay attention to shading (especially in shoulder seasons)
the earth tube might be better utilized for solar DHW to keep your primary demand down
i've never seen anything close to 0.05 ACH50 - the best i've heard of was 0.17ACH50. was this a typo?
there are some PH folks in your neck of the woods - it may be more cost effective than attempting to DIY (especially if they're able to optimize assemblies/windows/etc).
good luck!
Jan,
A couple of points:
1. As Mike said, it doesn't sound like you are talking about a net-zero house, since most Americans use electricity for television, computers, lighting, etc. It sounds like you are trying to provide all of your space heat and space cooling from passive solar gain and earth tubes.
2. The trouble with getting all of your space heat from passive solar gains is that even if your computer model says that your gains are high enough to meet your needs, the solar heat gains may not occur when you need them. A sunny week in February will introduce a lot of heat into your house, but if you have two cloudy weeks in December, that February sunshine doesn't mean much.
3. Be wary of earth-tube cooling. Soils vary widely, and many people have been disappointed in the performance of these systems. While you may get cooling in June, when the soil is cold, I doubt whether you'll get much cooling in September and October, when the soil has been heated by all the ventilation air you pulled through the tube all summer long.
Hi Mike @1:
Yes; you're right, I did not state my question very clearly. I'm trying to understand if I can heat my home with passive solar gain and cool it with ground-tube ventilation. My question was really whether my model numbers "smell" like they could work from the eyes of experienced folks who have done something like it or similar to it.
I'll try my modeling with substantially less insulation and review the specs for Serious and Zola - thanks for the pointers!
While I have modeled solar-position shading, I have not added object shading which will be an issue with my 52' wide city lot with a 2-story house to the south - I'll take a look at that. The 0.05 ACH50 was not a typo - I'll chase down the PH reference...
Agreed on the cost-effective point - I just feel like I need to understand this modeling before I can have a conversation about trade-offs with the final design.
Thanks for the info!
Hi Martin @2:
I did not express my question very clearly but you're exactly right - I'm trying to meet my heating needs with direct passive solar gain and my cooling needs with earth tubes, as a first step toward a net-zero design which will include appliances, lights, etc. [1]
The design goal is to work well all year, including that two week period in February with the use of the back-up heater and in the hot September with the use of additional cooling. I have read your point on thermal build-up in earth tubes systems in number of places, mostly in the context of the improper design or installation, e.g., not accounting for the specific heat of the soil at the site, tubes installed too close together or not deep enough, etc. I will read some more on this...
Thanks for the feedback!
[1] http://1010-design.blogspot.com/2011/11/reducing-energy-consumption_27.html
.6 ach50 is the Passive house maximum for infiltration. Salt Lake City has 5,607 hdd, I think R-40 to R-50 walls will get you to PH performance.
Hi Mike @1:
I found the reference I was recalling and in that reference the low end of the range is 0.17, as you mentioned. [1]
When I model this with 5/25/40/60 at 0.17ACH50 infiltration and 0.15ACH50 ventilation with 80% recovery, the annual energy loss is: 32% in infiltration, 11% in ventilation, 6% in R-60 ceiling, 19% in R-40 walls, 13% in R-25 slab, and 19% in R-5 windows, resulting in 10.38kBTU/ft2/year which is over the 4.75kBTU/ft2/year PH requirement, FWIW. And beyond PH, the 32% loss to infiltration just seems excessive; but, I don't know what is actually required improve substantially beyond 0.17ACH50...
[1] p. 58, http://www.passiv.de/07_eng/PEP/PEP-Info1_Kronsberg_150dpi.pdf
Hi Doug @5:
Yes; that my understanding, too. When I model [1] this with 5/25/40/50 at 0.60ACH50 infiltration, the annual energy loss is: 67% in infiltration, 4% in R-50 ceiling, 11% in R-40 walls, 7% in R-25 slab, and 11% in R-5 windows, resulting in 31kBTU/ft2/year which is over the 4.75kBTU/ft2/year PH requirement, FWIW.
[1] Modeled like these studies: https://docs.google.com/spreadsheet/ccc?key=0Ale1SfajN4iydG9rdkZzanRES3NCVC05VklFUlR5NWc#gid=6
Jan,
Roughly speaking the annual maximum heat loss for PH is .6 Btu/sf/hdd, or a 2,000 sq. ft. house in Salt Lake would have an annual heating load of 6,728,400 Btu's.
Jan,
I'm a builder in Salt Lake City with experience in passive design, super-insulated building enclosures (exterior foam/double wall assemblies), and tight construction.
Though I don't know your exact location and access to solar, I am fairly confident that .6 ACH50, R45-ish walls, and a R65-ish roof will get you to Passive House.
At any rate, it sounds like you've got a very exciting project on your hands- I would love to hear more about it! Feel free to contact me at.... [email protected]
Jan, back to one of your original questions:
"From my reading, these values are extreme and on the cusp of build-able, but the PH literature aligns generally. From my reading of Building Science Corporation articles, infiltration less than 0.05 ACH50 is difficult to build; R-60 wall enclosure has significant long-term risk moisture compromise, etc.; and, yet, net-zero homes are built successfully...presumably. But how real is this really? Are these extreme values just an artifact of simplistic modeling?"
The PH infiltration rates are quite reasonable to build. At this point in late 2011, so many people have meet this requirement that it's no longer an "if" it's possible for the average builder, it just takes the decision.
The PH infiltration rate or 0.6 ACH50 will help your modeling certainly, but will also reduce moisture into your assemblies and thus create a higher quality and more durable envelope.
Most of the work is in the design stage. Choose your air barrier and method and follow through on the details while in design. For a good PH design, I like taped interior OSB or plywood behind a service cavity. That method will protect your cavity based assemblies rather than relying on the exterior layer, which will not.
As a point of illustration, many builders that I talk to are hitting in the high .20's and low .30's after their first few projects without substantial effort.
Building to the PH 0.6 ACH 50 infiltration rate is a perfectly reachable goal these days. Just seek out those who've done it while your still in design and you can do it too.
Best of luck!
Hi Doug @8:
From PH, the annual heat requirements must be less than or equal to 4.75 kBTUs/ft2/year [1] which is 9.5 MBTUs for a 2000 ft2 home for the entire year. But PH, for me, is really just the first step toward net-zero, so I think I will need to get well below the maximum PH heating requirement.
[1] http://www.passivehouse.us/passiveHouse/PassiveHouseInfo.html
Hi Brett @9:
When I model [1] this with 5/25/45/65 at 0.60ACH50 infiltration, the annual energy loss is: 67% in infiltration, 3% in R-65 ceiling, 10% in R-40 walls, 8% in R-25 slab, and 12% in R-5 windows, resulting in 36kBTU/ft2/year which is over the 4.75kBTU/ft2/year PH requirement, FWIW.
Like @7, infiltration heat loss is so large at 0.6ACH60 that improving good insulation to great insulation really does not seem to help. The problem is infiltration. If we can limit infiltration to 0.2ACH50, the heating requirements drop to 15kBTU/ft2/year with 41% being lost through infiltration, but more than allowed by PH. The R-5 windows are second at 21%, and the R-45 wall is 19%.
Perhaps something is wrong in my modeling, but I have no idea what that might be. Maybe I just need to buy PHPP to try this out. Perhaps the 289 square feet of R-5 windows are just undersized and not good enough: more insolation, more gain, less loss; therefore, less annual heat requirements.
[1] Modeled like these studies: https://docs.google.com/spreadsheet/ccc?key=0Ale1SfajN4iydG9rdkZzanRES3N...
Hi Albert @10:
In Kronsberg, PH showed builders were able to achieve 0.17ACH50 to 0.40ACH50 [1]. But even at a low infiltration rate of 0.2ACH50 with a super-insulated envelope, we lose 41% [2] to infiltration which to net-zero is difficult to recoup.
After spending all this effort to get a super tight envelope, the last I want is a little electrical or plumbing work to compromise the envelope and cause moisture damage over time, so I really like the idea of "OSB or plywood behind a service cavity" - you don't have a diagram somewhere that you would share, do you?
Thanks for helpful tips!
[1] p. 58, http://www.passiv.de/07_eng/PEP/PEP-Info1_Kronsberg_150dpi.pdf
[2] @12
Jan,
I am currently building a house that uses an interior taped plywood air barrier system with service core.
Here is a link to my blog:
http://ourhouseuponmoosehill.blogspot.com/
I have details posted there that you may find useful.
Good luck with your project.
Hi Lucas @14:
Your very nice drawings answer my question perfectly:
http://ourhouseuponmoosehill.blogspot.com/p/details.html
Behind the 2"x3" wall service core is 1/2" t&g plywood and similarly behind the 2"x2" ceiling service core. Will you apply the 4" air-stop tape to warm-side of the wall and ceiling t&g seams? Floor t&g seams, too? Are you planning on doing an infiltration test? Unrelated, have you chosen a solution for the water resistant barrier?
I really appreciate the pointer and the clear information in your blog - thanks so much! I'll keep an eye on your blog.
Jan-
It appears that you are confusing ACH50 with ACH natural. 0.05 ACH natural is about equal to 1.0 ACH50. The PH standard of 0.6 ACH50 is about equal to 0.03 ACH natural.
Hi Michael @16:
You are absolutely right - this is a huge help! I have been using ACH50 instead of ACHnat for my infiltration losses. In my case, my normal shielded, two story building, zone 2 house in Salt Lake City has an LBL factor of 14.8 [1], so a (very tight) 0.20ACH50 building results an ACHnat of 0.01, accounting for 3% heat loss via natural infiltration. Now re-running the model with the suggestions from Mike, Doug, Brett from above (windows/floor/walls/ceiling):
5/25/40/50: 7.8kBTU/ft2/year
5/25/40/60: 7.6kBTU/ft2/year
5/25/45/65: 7.0kBTU/ft2/year
7/25/60/70: 4.4kBTU/ft2/year
I have renewed hope that my net-zero goal is possible with known building techniques.
Thanks for spotting my error, Michael!
[1] page 5, http://www.energystar.gov/ia/home_improvement/home_sealing/ES_HS_Spec_v1_0b.pdf
Hi Jan,
At corners and framing transitions I've been using 4" tape - the extra width is nice in these areas.
All plywood seams will be taped with 2" tape - I take a length 4" tape and "rip" it lengthwise with my utility knife to make 2" strips.
No, as the plywood sub floor falls within the pressure boundary.
You bet. I'm planning two...
The first one is to satisfy my curiosity about the integrity of just the air barrier system by itself.
I'll do this after plumbing and electrical penetrations are in but before dryer/range vent and stovepipe installation.
The second I'll have done once everything else is in but before drywall.
Yes, Typar.
Jan,
As I look at your example I see Passive House guidelines and performance varies depending on the site. In Minneapolis it is almost exactly .6 Btu's/sf/hdd for Salt Lake it is .847 Btu/sf/hdd. Keep up the good work and keep us posted on the progress.