[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 2Ã—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); 12â€² high Advantech sheathing applied vertically to eliminate horizontal seams that can allow water to penetrate into the wall cavity; 2Ã—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 2Ã—4 will be attached to the load bearing wall, and the ability of the vertical 2Ã—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.