So far the Passive House construction shows very little obviously out of the ordinary. We’re into the lock-up stage before anything unusual becomes visible. But there have been some serious details to consider. Let’s take a minute to find common sense in the complex science.
Sense
Close the window! Don’t let all the heat out.
In the middle of winter, it doesn’t matter how hot you crank the radiator in your car if you’ve got the windows open.
Science
Convective Heat Loss
Convection is heat transfer by mass motion of a fluid such as air or water. When a mass of hot air is adjacent to a mass of cool air, the temperatures will rapidly flow between each other until equal.
Sense
To stay warm in winter, wear a coat not a suit of armour.
Sitting around a campfire you see a wooden stick and a metal rod each half in the fire. Which one is likely safe to grab, and which one is likely to burn you?
Science
Thermal Conduction
Thermal conduction is the transfer of heat by particle collision and movement of electrons within a mass. The rate at which energy is conducted as heat is relative to the temperature difference between the two bodies and the properties of the conductive mass.
We need the energy we put in the home to stay in the home. Nobody would choose to drive their car around with a hole in the gas tank or dragging a pallet of cinder blocks behind it if they didn’t have to. It would be an obvious waste of gas and money. So why ask your house to do the same? Plug that hole and ditch the extra weight!
Convective Heat Loss
Why it matters
In conventional buildings, convection is the dominant form of heat transfer. Controlling air leakage is one of the critical elements of creating an energy efficient home.
What we did about it
To keep our air barrier continuous, we ran overhanging strips of OSB on top of the upper wall plates and then taped all joints and the outer edge to the exterior wall sheathing. Then we dropped the trusses, sheeted them, and applied the roofing membrane.
This allowed us to install two layers of R30 fiberglass batts in the trusses – well ahead of when you would see insulation typically. This had to be done early since we then sheeted the underside of the trusses in full sheets of OSB and taped those seams. Now we have a solid airtight barrier to frame our interior walls up to.
We left our interior wall heights 1 ½” lower than the OSB and framed a 2×4 dropped ceiling chase below the trusses to add lights, ducting, and wiring without penetrating our air-barrier.
On the exterior walls, we used a series of compatible Siga products for the weather and air-barrier. Siga Majvest is the barrier product and a variety of amazing Siga tapes create the seal against surfaces such as wood or concrete.
Thermal Conduction
Why it matters
Highly conductive materials protruding through our assembly from a warm side to a cold side will ruin our chances of achieving suitable energy efficiency. The more conductive a material, the faster our precious heat will walk across that bridge. The more of that material we have, the wider the bridge and the more heat will be constantly escaping.
What we did about it
The 6” of rigid insulation we see on the outside of the foundation is a pretty simple concept and one of the first visible steps taken to reduce heat loss through thermal conduction. Even our bottom run of flashing does not go completely to the wall sheathing. There is a block of rigid insulation behind it and the weather barrier is lapped over and taped to the flashing. This creates a thermal break between the conductive bottom wall plate and the highly conductive metal flashing.
On the framed walls, we used ACS clips fastened to each stud through a R-4 rated PROLOFT pad to reduce thermal bridging. We installed every clip prior to our first blower door test but before installing any Roxul batts. This was important since we could test our air-barrier for leaks before it was covered over and inaccessible.
We cut each block of Roxul CavityRock easily with a regular utility knife to fit tightly around windows and corners. Most pieces need no trimming and simply fit snuggly between each of the previously installed ACS clips.