Image Credit: All images: Chris Briley / Roger Normand
Image Credit: All images: Chris Briley / Roger Normand South elevation, north elevation, and the window schedule. Main level floor plan. Lower-level floor plan.
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[Editor’s note: Roger and Lynn Normand are building a [no-glossary]Passivhaus[/no-glossary] in Maine. This is the fourth article in a series that will follow their project from planning through construction.]
So far, we have been guesstimating how much this project will cost. Yes, we could use estimates based on cost per square foot, but there are are several design factors that influence that equation. For example:
- 13+” thick, R-50+ double-wall construction, additional sub-slab insulation, and an R-70+ ceiling are more expensive than single-wall construction built to current building code standards;
- Triple-pane windows are more expensive than even high-end double-pane windows;
- Nearly half of the conditioned (to be heated or cooled) square footage is below grade, which should reduce costs;
- The garage and three-season room are not conditioned, which should reduce costs.
Our architect, Chris Briley, sent a series of eight blueprints for the two contractors to price within an expected margin of error of 15%. (See my previous blog, Selecting a General Contractor, for insight on our approach.)
The north and south elevations, along with the window schedule, are posted as Image 2, below. As you can see, there is a stone “belt” around the entire perimeter of the house, along with an elliptical roof structure to highlight the front door entry.
High-solar-gain triple-glazing for the south side
All windows will be triple-paned. I suspect that not many people realize that different coatings can be applied by the window manufacturer to either reject or enhance how much solar heat is allowed to penetrate through the glass. Most major U.S. window manufacturers use a “soft” low-e coating which is designed for southern climates to reject heat penetration, thereby reducing air conditioning needs. Here in the north, a “hard” low-e coating is preferable to enhance solar penetration, thereby reducing winter heating loads. Windows with such a “hard” low-e coating are more difficult to find.
We have not yet selected the window manufacturer yet, but when we do, I’m sure it will become a blog topic.
R-50 walls and an R-70 ceiling
The main floor plan and the lower level floor plan can be seen in Image 3 and Image 4 below. Note that the areas under the three-season room and the study are designed as a slab on grade. We do plan to see how much more it would cost to excavate the area under the study so it becomes part of the lower level. Also note the four bump-out areas on the south side – that is for the window wells to daylight the lower level.
For you building geeks, the wall cross section is shown in the image at the top of this page. Key features are: Insulated concrete foundation with a flared top course to support the exterior wall 2×4 wall; double-wall framing; R-70+ ceiling/R-50+ wall insulation (blown-in cellulose); spray foam insulation where the main floor I-beams meet the exterior wall (always a difficult area to insulate); 12′ high Advantech sheathing applied vertically to eliminate horizontal seams that can allow water to penetrate into the wall cavity; 2×4 studs applied vertically, with the cavity filled with Roxul, which serves both as insulation and a drainage plane.
Does the Zip sheathing need housewrap?
I have some lingering concerns over some of these details. I am still uncomfortable about the absence of Tyvek, particularly on the south wall as there are strong, persistent winds coming off the river that can drive rain into cavities if the Adventech sheeting tape fails. I am also concerned about how the vertical 2×4 will be attached to the load bearing wall, and the ability of the vertical 2×4 to support the considerable weight of the adhered stone.
The remaining drawings include rafter plans, floor framing plans, east and west elevations, and floor heights. I see nothing of note in these drawings, so I will exclude them from this post.
We will be speaking to our two potential contractors this week, and look forward to getting a better handle on construction costs.
The first article in this series was Kicking the Tires on a Passivhaus Project. Roger Normand’s construction blog is called EdgewaterHaus.
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16 Comments
roof assembly
For a less expensive and more robust roof assembly consider eliminating the air vent, Zip sheathing and Ice and Water and rosin paper, and replace with: WRB membrane stapled (and taped) across the roof trusses (membrane is reinforced, waterproof, vapor open, airtight) - connecting it to the wall sheathing to form a continuous air seal. Place battens on the WRB/trusses and your metal roof on them - making a vented roof assembly outboard of the WRB. This gives you optimally functioning thermal insulation and vapor open construction, with an efficient use of materials, labor and cost.
See a nice article on such an approach from a fellow Mainer here: http://www.builderonline.com/blogs/postdetails.aspx?BlogId=tcushmanblog&PostId=108055
(Note: we sell this WRB, Solitex Mento Plus at http://www.foursevenfive.com)
Response to Ken Levenson
Ken,
I've raised this question before with Albert Rooks, but now it's your turn to answer. Have you ever done this? What was the roof pitch? What type of shoes were you wearing? Where did you place your feet as you attached the membrane? How did you get down from the ridge? Were you wearing OSHA fall protection?
This is a detail developed on paper. I want to hear how you do it when your feet are straddling the roof trusses.
wall assembly
We might suggest depending more on gravity and less on adhesives to keep the wall assembly dry. It should be more robust and cost effective to replace the Zip (OSB) panels with CDX plywood that is then wrapped in a WRB that is air tight, vapor open and waterproof. The WRB is stapled up in an overlapping manner, and then taped. Not coincidentally, we offer a WRB, Solitex Mento 1000 that fits the bill. http://www.foursevenfive.com. This should give you a robust construction at slightly lower cost.
Martin, building from lower
Martin, building from lower to higher one would go up standing on the battens. The Membrane and gaps would be as safe as any construction which is not as safe as non construction. Fall protection could easily be set up.
Membrane rolled out horizontally, battened and then rolled up as you add each next higher batten.
Response to Martin Holladay
You build your own ladder as you go with the 2x4 battens. While I'l readily admit I've not personally done this, here's a nice recent shot of Chris Corson in action through the WRB - using this very technique.
Thank you AJ
you beat me to it. but still have a look at the great picture of chris attached to my comment #5.
photo attachment for comment #5
let's try again here. worth a look.
OSHA
Speaking of OSHA and Not-So-Buildable ......
OSHA and buildability
Ken,
Thanks for describing the method. I can see that it is possible, although unrolling the membrane (especially the first course) is somewhat of a tricky procedure.
John,
The difference between installing rigid foam over solid roof sheathing and installing a membrane over roof trusses is that, in the first case, you have sheathing to stand on. In the second case, the first course of membrane is stretched between the trusses without any sheathing to help the installer.
I'm not saying it's impossible -- it's just trickier.
And you're right -- most residential builders ignore OSHA fall protection requirements, no matter what type of air barrier they install on their roof.
zip tape is bullet proof
Zip tape is well made. I have a very old sample I check. It is great tape. I built many ultralight aircraft that had taped coverings. Tape that is engineered to perform a function is not your dollar store wrapping paper tape.
I would never add an overkill layer of the same function over it. That said, best advice is to work with manufacture's recommendations.
Near to topic... I am thinking the backside of Zip needs moisture protection and am looking into coatings ideas. Opinions welcome!
response to aj builder
opinion 1: don't use osb on exterior sheathing, instead use CDX and WRB, as mentioned in #3
opinion 2: keep the osb but then install a vapor variable membrane like Intello Plus, which has a perm of .17 in winter but over 13 in summer. (it's an interior air barrier too, which ensures insulation optimization.) see attached image.
Ken, DB stands for?
I like
Ken, DB stands for?
I like your detail and of course it must be moisture bullet proof but I would think
1- the wall is very thick and is only adding R-1 or whatever for the empty areas.
2- critters would love the built in mega hotel areas
Where I am I have not seen a great need for rainscreens if other options are used such as vinyl siding or backpriming siding. And the inside chase for wires, I wire all my homes and see no need for the chase. I do like the idea of isolating the interior drywall though if building a home that is well insulated and low in thermal mass. The low mass idea is mine in that I work for second home customers that like to rapidly adjust temperatures. Who knows if my idea has merit, but it is one idea I toy with. Part of this idea is to use foil backed drywall, your space, foil faced foam.
Who knows the R factor for two foil faces separated by horizontal 2x2s?
Thanks Roger, Chris. Question for Ken
First, I would like to thank Roger and Chris for allowing us to get a good look at the details of the house.
Then i would like to ask Ken some questions about using the Intello Plus. I am considering using it for the house I am building early next year. Like AJ, what is the DB for? I looked it up on your website, but I do not understand why you would need that with the IntelloPlus.
Also would this assembly do well in the humid south? We can be humid year round especially with a well-insulated house where the heat doesn't run often. Thank you. Lucy
response to aj builder
AJ,
DB+ is simply the name convention for another Pro Clima membrane, one that is paper based, yet reinforced for dense pack cellulose. It has a perm variability of .8 to 5.5. Here is a link to that page: http://www.foursevenfive.com/index.php?main_page=product_info&cPath=70_76&products_id=220
response to Lucy Foxworth
Lucy,
Sorry for delayed response - catching up here.
One would use either the DB+ or the Intello Plus. (One or the other, not both together.) See note above to AJ regarding DB naming.
However, In a hot and humid assembly, things get turned around a bit. So I've attached here to this response our hot and humid assembly. With this you use a vapor retarding, robust WRB called DA by Pro Clima. (We just got this in and it is new to the website.) And then you go with vapor open Solitex Mento at interior. In this way one is limiting the vapor drive into the enclosure yet allowing some drying to exterior while maximizing drying to interior - and maintaining airtightness inboard and outboard of the insulation.
Feel free to call us to discuss , we now have an 800 number. 800-995-6329.
Stone wall area
So, if I read this right, you are attaching two 2x4's, on face, to come out 3 inches. At the top of this, you have a 2x4 on edge, coming out 3 1/2 inches. That leaves a 1/2 inch gap between the vertical, paired 2x4's and the cement backerboard. I asume that there is a vertical 1/2 inch strip of treated board or plywood there. If not traeted, would you run a house wrap product on the backside of the cement board?
Are you using 24 or 16 inch OS's?
I like the detail as I am considering something like this for a future Passive house, likely with Larsen trusses and tile as the exterior cosmetic sheathing.
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