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Passive House: What Do You Think?

Opportunity for Interactive Decision-Making with GreenBuildingAdvisor Community

Image Credit: Rob Moody, Organic Think Inc

Passive House Project in Asheville, N.C., Poses Many Questions and Lessons

Check out floor plans, site plan, a model and details plans of this project.

Over the next few months, I will be following an interesting infill project here in Asheville. It’s an 842-square-foot house, following the Passive House Institute standards.

I checked out the project in West Asheville for the first time last week. The homeowner is a single fella who is contracting the home himself, with a fairly flexible time line. That will give us an ongoing opportunity to dissect different aspects and stages of the project very closely, and hopefully engage the readership to weigh in with philosophy, experience, and opinions. I’ll be shooting some video of the project as well, interviewing the architects and owner, so stay tuned. I live close by, so I will follow the project through to completion and habitation, chronicle a comparison of modeled to actual performance, and share the results of the HERS testing as well.

The home belongs to Chris Otahal, a structural engineer at Kloesel Engineering here in Asheville. He is working with Aaron & Calder Wilson at Wilson Architects, a husband-and-wife architecture team. I have had the pleasure of working with Aaron & Calder on a number of projects, including my own home. They are a very talented couple and bring a deep understanding of green building to this project, though this is their first Passive House.

Before beginning, Chris sought information from the US Passive House Institute in Urbana, Ill., which provided the team with energy modeling, insulation strategies, and HVAC sizing. (I’ll share some specifics on the services in later blogs.) With the Institute’s info and some further modeling, the team concluded that this particular design would be cooling driven—there will be more traditional energy used for cooling the home than for heating it. To keep the heating load as low as possible, the team implemented such strategies as airtight construction, superinsulation, passive solar design, and the use of custom glazing on the south side of the home (I’ll get into these in more detail in the future as well). Here’s a list of green features that the team has incorporated or will incorporate on the remainder of the project:

  • The home has a small footprint, which minimizes land disruption and material use (the lot is .10 acres!).
  • The structure holds a trim 842 square feet of living space.
  • The marginal infill lot was unattractive to most, but Chris saw a diamond in the rough. The site has a stream on the south end and a sewer easement on the north end, limiting the potential building area. But Chris’s needs and the creativity of the team took the opportunity to put an interesting home on an interesting lot. The home makes use of the entire buildable area of the lot.
  • Chris opted for a combination of triple-glazed windows, some with a low solar heat gain coefficient (SGHC), some with a relatively high SGHC.
  • Exterior walls are 2×8 construction, 24 in. o.c. The cavities will be filled with blown cellulose, with 3 inches of rigid insulation on the exterior installed perpendicular to the studs.
  • A rainscreen siding system was utilized, and the exterior finish will be corten steel panels attached to 1×4 furring strips.
  • They used a high-performance weather barrier and drainage plane, VaproShield.
  • A mini-split will be used to condition the home.
  • They will be using a high-efficiency tanked water heater.
  • Passive solar design utilizes large south-facing windows and limited north, east, and west glazing. South-side overhangs were properly sized.
  • The 16-in. roof and floor TJI joists will be filled with insulation.
  • An energy recovery ventilator will provide fresh air, pressure balancing, and preconditioning.

The project is at rough stage, so it’s a great time to investigate while the walls are still open. Interior insulation will start up soon, which brings us to our first opportunity to make this an interactive blog and construction process. Chris is still deciding on the best possible insulation product or products to use for the roof. It’s a 16-in. cavity that you can think of as a clean canvas. Free your inner green building Michelangelo to share your opinions and help the team find answers to the following conundrums they’re facing:

  1. Should they install a foam skin on the underside of the sheathing?
  2. 1. If so, should it be open cell or closed cell? (We are in a mixed humid climate, in the Blue Ridge Mountains. It’s a fairly unique climate: humid, some snow, some heat.)
  3. 2. Should another type of insulation be used, either by itself or in conjunction with foam?
  4. 3. What would the most effective depth be, taking energy efficiency and budget into consideration?
  5. 4. Should the roof be vented or unvented?

Here are my responses. Let me know if you agree or disagree and why:

  1. 1. For air-sealing purposes, I would opt for 3 to 5 inches of foam.
  2. 2. I would choose open-cell foam for its breathability.
  3. 3. I would fill the remainder of the rafter cavity with a blown-in product, possibly dense-pack, no-VOC fiberglass.
  4. 4. I would fill up the cavity. The mix of the two insulation types would be less expensive than all foam.
  5. 5. Unvented. It’s not necessary to vent with the use of foam insulation, which ensures that the conditioned space is pushed all the way to the exterior sheathing. The heat stays out of the envelope, so there is no need to vent hot air from the assembly.

What would you like to know about this project? I plan on investigating the choice and installation of windows, exterior cladding, and mechanical systems. Do you readers have burning questions that you would love to have addressed here more than anything else in the whole, wide world? If so, take a moment to post your question. I’ll kick it around to the team and respond in subsequent blogs. In the meantime, check out the Passive House Discussion at GBA, and another at JLC.

Check out pictures of the project at my Flickr page. Also, I will make a few architectural details available periodically. Here are a couple:

36 Comments

  1. User avater
    Dan Sunderland | | #1

    Passive Choices
    Rob, Good article and it looks like it will be a great project. I had a couple of questions as it also relates to how others make the decisions. #1. How did they go about trying to refine and decide to use the equipment or products you've mentioned? #2. With the exception to the guidelines provided why have the resources they have used stood out or proven themselves above others?
    I agree wholeheartedly about the use of spray foam insulation on conventionally framed projects but I would go ahead and spray enough on the underside of the roof skin to do the whole job. This makes the attic area if any, conditioned space and m better location for ductwork. I would also go ahead and use it on the walls as well but that may fall into the above questions. Often we are engaged in many of the same type projects here at http://www.earthenconcepts.com.
    Dan

  2. Dean Dowd | | #2

    Sounds like a great project
    Sounds like a great project Rob, and Asheville is a wonderful place. I don't know to much about energy recovery ventilator's, maybe add some information about those?

    Dean

  3. Bruce Chyka | | #3

    Triple Glaze windows
    Thanks for sharing this forey into Passive Haus design. I met Katrin at the ACI conferenc a few weeks ago. If the expense of foam is a concern, I'd rather forego the triple pane windows for more foam. I'd be interested in how the decision on the windows came to be.

  4. User avater
    John Semmelhack | | #4

    Heating load versus cooling load
    I'm very surprised to hear that there is going to be more cooling energy used for the house than heating energy. Heating degree days to cooling degree days in Asheville are about 5 to 1. According to ASHRAE data, Asheville has relatively mild summers. In a typical year, there are only 20 total hours where the outdoor temperature is 90 or higher. It seems that proper shading and night-time window ventilation would be adequate to maintain comfort.

    I have a recently completed house in Charlottesville, VA (very similar climate to Asheville, but warmer summers) built to the Passive House standard. Our summertime strategy is shading, night ventilation, and a little bit of "free" cooling courtesy of our heat pump water heater.

  5. Boone Guyton | | #5

    West ASheville passive house capital of US
    I saw a presentation about another passive house in West Asheville http://thenauhaus.thewiredmouse.net/index.php this also plans to meet passive house certification. They will be using hempcrete http://www.hemtecusa.com in thier wall system which promotes itself as carbon negative. Interesting stuff and I look forward to see how both houses do and if an optimal envelope efficiency to cost can be figured. At some cost it would seem there is a reduced return on investing in envelope efficiency that might be better spent on mechanical or generation efficiency. A lot to be learned from these two houses. Go West ASheville.

  6. John Brooks | | #6

    Cooling vs. Heating
    Rob...great blog concept(interactive)and great subject.

    John S.,
    I am not familiar with the Asheville climate...but I can see how the cooling load can be greater in many US climates.
    An Extremely High performing home (like passivhaus) should require very little energy to heat.
    The house itself can effectively reduce the number of heating degree days.
    My climate is different (North Texas) and I only consider my house only HALF-passive... my heating load is almost negligible...
    The heat from occupants, pets and appliances is practically heating the house.
    In the summer I have reduced my cooling load .. but the occupants and appliances are still heating the house(and adding moisture) during all of the hours that the outside air is not in the comfort zone.

  7. User avater
    Rob Moody | | #7

    Degree days.
    I'm definitely not saying that Asheville has more cooling degree days, just that cooling the home will require more load than heating. That's due to the measures that the team has taken towards passive heating: appropriate window choice and placement, superinsulation, etc. Asheville's climate has more HDD than CDD.

  8. John Brooks | | #8

    More details
    Rob,
    Give us more to go by...how about more photos?
    How about posting a Site plan.... floor plans,wall sections, HERS report,
    Nudge your Architects to Post comments.
    How about the Energy Rater .. Let's hear his/her thoughts.

  9. John Brooks | | #9

    oops ... on more photos
    Rob, I missed the flickr link the first time I read the blog
    http://www.flickr.com/photos/organicthink/sets/72157618500940728/
    By HERS report .... I meant preliminary.

  10. Pat Morgan | | #10

    foam and more
    I am a builder in Atlanta and have been trying to figure out the insulation question for highly efficient home building. I just read today an article in the July '09 Fine Home Building on foam that has info germane to this discussion. Most importantly, that the efficiency of foam (open or closed cell) diminishes drastically beyond 3-4" thickness, so filling a 16" cavity is probably a waste of money. Also important is that installation of closed cell can be tricky, and if not applied in thin lifts causes problems. I agree with you about the issue of breathability for closed cell foam, although the manufacturers say its perm rating is as high as a piece of OSB, so maybe it's OK. Mixing other insulation in the ceiling with a good dose of foam instead of 100% foam will definitely save some serious money.

    I noticed in your architectural details that the wall cavities are filled with cellulose. I would recommend foam in the walls too for the air sealing qualities it provides. Maybe do it like you are thinking for the roof with foam against the sheathing and then another type inboard of that. Notwithstanding all the elements of that wall, it will still leak some air -- unless there's foam in there to seal it all up. Owens Corning is also coming out with a new product they call Energy Complete which is spray applied to the inside of the the sheathing at all studs, seams, penetrations, etc with insulation inside of that. I'm not affiliated in any way with Owens Corning, but I thought it looked pretty slick and you can see more info at http://www.ocenergycomplete.com/system/.

    Thanks for following this project for all of us. I'll be watching for more.

  11. User avater
    Rob Moody | | #11

    Thanks!
    Great comments from everyone. I'll consolidate and address them next week. I'll also see if the building team can put their two sense in as well. This is going to be fun.

  12. John Brooks | | #12

    Air Barrier at the wall
    I agree with Pat Morgan about reconsidering the air barrier at the wall... I don't see how the vaproshield as installed will provide "airtightness"
    http://www.flickr.com/photos/organicthink/3547615630/in/set-72157618500940728/

  13. User avater
    Rob Moody | | #13

    Agreed
    Vaproshield calls their product an air barrier, but I would think of it as a weather barrier. The team is air sealing the envelope to ensure a tight envelope and not relying on the membrane to do that job.

  14. John Brooks | | #14

    Weather Barrier or Air Barrier
    Rob, Vaproshield and your blog represent it as an air barrier.

  15. User avater
    Rob Moody | | #15

    I mentioned that in my last comment.
    I'll make the change in the blog since using the wrap for a weather barrier and drainage plane is the project team's intent.

  16. Pat Morgan | | #16

    readership
    Somebody's reading your blog. I got a call from a guy at Owens Corning to say thanks for my earlier comment on their product on this blog. Again, I want to make clear there is no affiliation, but I think the product is worth a look. The good news is people are reading.

  17. User avater
    Brian Knight | | #17

    Manual J versus Passive House
    What a great discusion. Thank You Rob! I also am working on a West Asheville project that is aiming for passive house criteria and agree with Mr. Semmelhack that a cooling dominated load is very surprising. My HERS rater recently did a Manual J of the home and it is calling for twice as much heating as cooling (6.1 to 3.1 MMBtu/yr). This home has a very strong passive solar design.
    I wonder if Manaul J is underestimating the passive solar input or is whatever modeling that passive house using overestimating? From what I understand of Passive House in Germany, is they rely less on passive solar heating because of fewer days of winter sun. Is the US passive house energy modeling software kicking HERs and Manual J's butt?

  18. Chris Otahal | | #18

    Keep the Comments Coming
    Thanks for all the comments! I'll try to address a few of them.

    What appeals to me about cellulose is the extremely low embodied energy in the material. My insulation guy claims that dense packed cellulose, properly installed, is unrivaled:) we're thinking of installing the drywall and blowing the cellulose into the cavity to achieve the proper density. I do like the idea of a thin layer of foam on the inside face of the plywood sheathing, and cellulose for the remainder of the cavity.

    As for the vaproshield, that became an element of the wall system when I decided to use Corten siding. There are two different products, wallshield and wrapshield. I'm using Wrapshield, lapping the horizontal and vertical joints, and taping all of the joints with tape provided by Vaproshield. The data on their website suggests that this will be an effective air barrier. However, we are not relying solely on it.

    The heating and cooling loads were determined by energy modeling performed by the passive house institute US using the passive house planning package. I don't have the data in front of me, but at some point, I can share the loads. These depend on meeting the air tightness criteria of 0.6 ACH at 50 pascal.

  19. User avater
    John Semmelhack | | #19

    Annual loads vs. Design/Peak loads
    Brian K. - Rob M. clarified in an earlier response that it was the Cooling Design Load, not the Annual Cooling load that was larger than the heating side.

    Regarding the Passive House Planning Package software versus HERS rating software (specifically, Rem/Rate): the

  20. User avater
    John Semmelhack | | #20

    Annual loads vs. Design/Peak loads
    ...biggest difference I've come across so far is in how the PHPP treats windows and thermal bridges. For HERS ratings, window performance values are taken from the NFRC rating for a standard size window which may or may not represent the actual window installed. In the PHPP, each window has a very detailed entry, including specific size, frame performance, glazing performance, thermal bridge of the glazing spacers, as well as thermal bridge of the installation.

    There are quite a few other differences...too much for a post...but it seems that PHPP is much more accurate when dealing with very low energy buildings.

  21. John Brooks | | #21

    Furring Strip attachment
    Looking at the "Jamb and corner detail" drawing...
    I am surprised to see that the furring strips are attached only to plywood and not anchored to the framing....
    cor-ten siding sounds heavy and it seems like the fastener would tend to rotate and sag in the foam.
    Maybe not right away...but over time.

  22. John Brooks | | #22

    Even more worry-some
    Looking at the "Jamb and Corner Detail" drawing again
    the furring strips in the corners apear to be fastened only to the foam?

  23. User avater
    Rob Moody | | #23

    Yep.
    Good catch. The detail is drawn with furring strips offset from the studs, but I saw on site that he in fact anchored them into the studs. We actually caught just a couple of spots where the furring strip fasteners missed the studs. Chris was going to double back and check those spots. As a builder, there were many instances when I witnessed clapboard and fiber cement siding anchored into sheathing only. You hit the nail on the head (funny punny) that best practice is fastening directly into the framing members. The architect mentioned on site that he would make the change in his drawings the next time he encountered that detail.

  24. User avater
    Rob Moody | | #24

    Righto.
    Chris attached furring strips to the corner framing assembly. I'll try to get a picture of this next time I'm there.

  25. Chris Otahal | | #25

    Furring Strips
    Actually, the 4'x8' foam panels are applied horizontally. In between each row of 3" thick foam is a double 2x4 'band', screwed into the studs. So basically every 4', there's a horizontal band constisting of (2) 2x4. We sacrificed some R-value, and there is some additional thermal bridging (although the 2x4 bands are perpendicular to the studs, so that helps), but the trade off is a well supported steel siding! So, the furring strips have really solid attachment at these bands, and we'll put a few intermediate screws into the sheathing as well. I'll send a photo to rob, and maybe he can post it.

  26. User avater
    Rob Moody | | #26

    I was oversimplifying.
    I was calling the 'band' furring strips for lack of a better term on my part. 'Band' is a great terminology for it. Thanks for the clarification Chris. I posted the pic of the 'band' and foam on my Flickr site under Westville Passivhaus and named "Chris' Detail Pic." It shows the "band" that is attached to the framing members, the builder's felt, then the proper, vertical furring strips on the surface. The thermal breaks occur at the perpendicular intersections of the vertical studs, horizontal "band" and then again at the vertical furring strips.

  27. Donald Lintner | | #27

    Roof insulation
    I'd like to hear more comments on insulating the roof as I am about to start building and want to hit about R-60. I was thinking of 16" parallel chord trusses to get an open ceiling with 1/4" ply to get a vented roof, 2" XPS foam with the seams taped below the trusses and the space filled with 15" of blown in fiberglas insulation. Would the foam on the underside of the sheathing as you detail it be more effective at air sealing the assembly?

  28. User avater
  29. alan abrams | | #30

    passive house roof insulation
    just a note of caution regarding the use of open cell foam in roofs--

    we've had two instances of water leaks, and open cell will absorb alarming amount of water--and retain it with annoying persistence.

    it suggests that we should apply the same care we take with vertical drainage-to roofs--redundant planes, provision for ventilation, etc.

  30. clarke olsen | | #31

    wall framing
    Using 2x8's for studs is a very wasteful way to gain to gain wall thickness. Two 2x4 walls make more sense thermally and ecologically. The benefits of more foam - thermal break, sheet barrier - diminish after..... 1"? 2"?.....

  31. Chris Otahal | | #32

    2x8s
    clarke:

    2x8's aren't ideal, and for various reasons I wouldn't use them again. However; for this project I had several 'tall' walls due to a stair located at an exterior wall, and a large floor opening in the second floor. Thus, deep framing members were required for the long spans (18' plus). It made more sense to have my framer use one type of wall construction.

  32. Alexander Davis | | #33

    Critical and Passionate
    This discussion is so good. I wish every building design fostered this much discussion and passion about the details. I wish more buildings had even a fraction of the care and thought that everyone here is expressing about Chris' Passive House in Asheville. I hope to build one in the same manner. Thank you Chris for going before most and walking the walk.

  33. solarcraft | | #34

    roof assembly
    I'd have a look at the vented roof assembly on the Smith House, The 1st Passive house in the US. They filled roof TJI's with high density fiberglass , but used structural fiberboard as Sheathing creating a breathable roof, over that ,they applied wood strapping for air space, then OSB as roof deck, vapor retarder membrane, and metal roofing.

  34. Bryan Seigneur | | #35

    Link to next in series?
    This is so interesting, I'm very psyched...in the body, LINK TO THE NEXT POST IN THIS SERIES, PLEASE. Meanwhile, I guess I'm using site search and crossing my fingers!

  35. Rob Moody | | #36

    Link to next in series?
    Bryan,

    Glad you are interested. Here's a link to my Passive House 2 blog entry, and another to Martin Holladay's blog, Passivhaus For Beginners - The History of a Superinsulation Standard.

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