Our Passive House Design Process

When designing a passive house it is VERY important to have your Architect and Passive House Consultant working together (if they are not from the same firm). With Grater Architects and myself it was pretty easy to coordinate changes, and how those changes effect the passive house performance. When window sizes, overhangs and window locations constantly changing - it could have serious consequences in the performance of the home. It is very beneficial to have the designer understand how changes can effect the energy model of the home.

We entered the project with an idea of a floor plan and it was modeled in PHPP (Passive House Planning Package) - an excel spreadsheet developed by the Passive House Institute. 

The passive House Standards are:

1a) Heating Demand under 4.75 kBTU/square foot/year

1b) Cooling Demand under 5.39 kBTU/square foot/year

2) Airtightness under 0.6 Air Changes per House at 50 Pascals (average of pressurized and depressurized)

3) Total Primary Energy Usage under 38.1 kBTU/square foot/year (Degraded for source of energy)

The calculation of square feet is done using the European standard for calculating area

- You measure the inside floor area

- You don't count any area taken up by stair cases or interior wall area's

- You only count 60% of the open basement floor area

Along with a preliminary floor plan; here are some of the goals of our project that we discussed with the architect in our initial meeting:

1. Open floor plan, to include dining room, kitchen and family room. 
2. Larger bedrooms
3. Unconditioned breezeway to connect garage to house (Passive House doors don't meet fire code, so the fire door needed to be outside the house)
4. 2nd Floor Laundry
5. 1st floor "future" master for aging in place
6. Large Kitchen Island
7. Lots of South Glass
8. Future space above the garage

After each iteration I would update the PHPP (Passive House Planning Package) model- an excel spreadsheet developed by the Passive House Institute - to optimize things like wall thickness and window area, and overhangs. A sample of the graph I would produce looked like:

As the overhang (shading) of the window increases, the demand for heating would would increase due to less solar gain - the opposite would occur for cooling. 

Finding the point where the cooling demand and heating demand meet would optimize the length of overhang. 

After multiple iterations we came up with the final floor plan:

Once the area of the house and volume of the house had been determined I was able to optimize the wall thickness and come up with an R-54 wall as what we needed to meet the passive house standard. (16" double wall construction - 2 x 2x4 walls spaced 7 inches apart). Since the house is 2 stories, we decided the interior wall should be the load bearing wall - so we could maintain continuous insulation between the floors (at the band joist). With the help of 475 High Performance Building Supply we were able to get many really helpful building details that just had to be modified for the wall thickness. Here is a cross section detail of our wall assembly:

The red line is our defined air barrier. For our house we will be using the ZIP wall system. The concern for condensation on the ZIP board is almost zero since it will never be cold enough to reach dew point because there is so much outboard insulation. This was verified by 475 using a computer model called WUFI Passive. The line can be traced around all of the different sections of the house without lifting your pen, and the build sequence is such that multiple blower door tests will be done prior to drywall. In areas where you see a jog in the red line - chances are there are strategically placed "Floppy Bits" that can be sealed to during later stages in construction - Photo's of this will be taken during construction.

The blue line is our weather resistant barrier. This is a smart membrane called Solitex Mento Plus, it is essentially a check valve for water vapor - not letting it into the wall cavity, but letting it out if necessary.