Insulation Levels in the Passive Project

Nic has been bugging me to write more geeky, green posts lately. I figured that covering the insulation levels in the Passive Project that have gotten us to the Passiv Haus standard would be as good a place as any to start. We can also take a look at the differences between the 100K House Project insulation levels and the Passive Project while we’re at it.

All of the required insulation levels to obtain Passiv Haus certification have been calculated for us by PHIUS. All of the SIPs insulation levels come directly from Suretight’s information page on how SIPs work. Suretight is our SIPs manufacturer of choice for the Northeast region. The Rigid XPS insulation levels are based on the standard R-5 per inch that these panels come in from almost all manufacturers.

Below are the figures for both projects we have done to date so you can compare the differences.

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Postgreen Passive Project Insulation Levels

Under Slab = R-50 (10″ of Rigid XPS Insulation)
Foundation Walls = R-10 (2″ of Rigid XPS on exterior down to footers)
Walls = R-32 (8.25″ EPS SIPs from Suretight)
Roof = R-53 (12.25″ EPS SIPs from Suretight)

Postgreen 100K House Project Insulation Levels

Under Slab = R-7 (1″ of Rigid XPS Insulation + Insulated Radiant Air Barrier)
Foundation Walls = R-5 (1″ of Rigid XPS on interior down to footers)
Walls = R-25 (6.5″ EPS SIPs from Suretight)
Roof = R-53 (10.25″ EPS SIPs from Suretight + 2″ of Rigid XPS)

As you can see, the biggest difference is in the foundation insulation levels. We went from a paltry R7 under the slab to a whopping R50. The foundation crew double checked this figure about a dozen times as they just couldn’t believe it.

HERS Index Rating Initial Evaluation Results

Most SIPs projects use the 4.5″ EPS panels from residential construction which is vastly superior to stick and batts (more on that below) and we increased to the 6.5″ panels on 100K to gain a couple extra LEED points for our Platinum goal. I was surprised to see that we only needed to bump up one level to the 8.25″ panels on the walls. We really didn’t need to increase our roofing insulation at all, but we switched to the 12.25″ panels there for larger span loads and simplified installation.

The cost of increasing the panel thicknesses probably ran us about 10% more per square foot on the Passive Project compared to the 100K project. The cost of the extra rigid insulation ran us a bit more and came in at about an extra $4K per home compared to the 100K project if memory serves me correctly.

Actual Insulation Levels & Comparison to Standard Construction

Now let’s quickly compare the SIPs insulation levels we are using to more traditional construction techniques in the area. To get a true whole wall insulation value, we must take into account all studs and air gaps that occur in the walls as well as the sections that have the full insulation value.

Plans Submitted to Philly for Building Permits
  • The 6.5″ SIPs walls rated at R-25 will have a whole wall value of R-21.6. A 13.6% reduction from advertised.
  • A typical 2×4 wall on 16″ centers and R-13 batts will have a true R value closer to 9.7 or a 25.4% reduction.
  • A typical 2×6 wall on 24″ centers and R-19 batts will have a whole wall R value of about 11.7 for a 38.4% reduction.

This data is coming from and

If we assume the same % reduction for the 8.25″ walls we are going from R-32 to a total wall value of R-27.6. Many builders in Philly are building with 2×4 walls, and the best ones are building with 2×6 walls. That means the Postgreen Passive Project homes have total wall insulation values that are anywhere from 236% – 285% higher than standard new construction homes in Philadelphia. I’ve even seen other LEED rated homes insulated to these same lower levels and don’t even get me started on old rowhomes with plaster over brick exterior walls.

Do these higher insulation values cost more? Of course, but they reduce the heating and cooling loads by almost 90% and they are a tiny fraction of the cost of something like a geothermal or PV system that would get us to the same efficiency levels. That’s the beauty of the Passiv Haus standard.

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