Hyper Insulated Walls: A Guide

To achieve the highest home energy performance standards globally, like Passive House, exterior walls must be heavily insulated (R-40 and higher) and airtight. Previously standard 2x6 building techniques don’t achieve this because:

• They aren’t thick enough to reach that R-value using reasonably affordable and sustainable insulation types, like cellulose or mineral wool.
• Stud walls have thermal bridging, which increases heat loss
• The air barrier is penetrated in many places by electrical boxes, making it hard to reach high airtightness standards.

Here we explore how builders are achieving high-performance walls today. To help explain, we will break a wall into the layers shown in this sketch:

Thick insulation cavity: deep wall stud + blown cellulose

This layer is the main body of the wall and is thick enough to hold a lot of blown-in insulation. There are a few framing options for achieving a thick wall:

• 2x8, 2x10, or 2x12 lumber (this takes a lot of wood)
• I-joists (like the ones used to build floors – takes less wood)
• Larsen trusses (similar to I-joists)
• Double-stud construction – two rows of studs with space in between.

A continuous Layer of Rigid Insulation

One or more layers of rigid insulation are added to the exterior of the building. Several insulation types will work, including EPS, XPS, GPS, fiberglass, polyiso, or rigid mineral wool boards. This provides a continuous layer of insulation with no thermal bridging, cutting heat loss dramatically.

Interior “Service Wall”: Studs + Batt Insulation

The service wall is a simple 2x4 or 2x6 stud wall built inside the house's main structure, creating a space for wiring, plumbing, and electrical outlets. This method has lots of advantages:

• Your wiring and plumbing do not penetrate the air barrier.
• You can easily get at the services in the wall in case of future repairs.
• Bonus – you can insulate the service wall with batt insulation for extra warmth.

Air and Vapor Barriers: Where To Put Them?

The beauty of having multiple layers in the wall is that the air and vapor barriers can be placed out of the way of services like plumbing and wiring in between the structural and insulating walls and the interior service wall.

The vapor barrier is for slowing down the flow of both humidity and air leakage into your wall. It is most often installed on the warmer side of the wall, especially in a cold winter climate. In a multiple-layer wall, a good place for the vapor barrier is on the interior side of the main insulation cavity, directly behind the service wall.

The air barrier is slightly different from the vapor barrier. The air barrier stops air, and bulk water (rainwater), from leaking into the walls, but it is “breathable,” allowing humidity to escape. The air barrier is most often installed on the exterior side of the wall, between the main insulation cavity and the siding.

A wall must only have one vapor barrier, whether that barrier is on the inside or the outside of the main insulation cavity. Any other air barriers must be permeable to allow humidity out, so the wall can breathe. If not, humidity can build up in a wall layer and condense, causing the wall to rot.

Whatever the vapor and air barriers are made of (polyethylene, house wrap, plywood, oriented strand board (OSB), or advanced membranes), they are typically sealed along their seams with high-performance sealing tape. Another method is to use a continuous layer of expanded polystyrene (EPS) rigid boards, sealed with small seams of expanding spray foam. This method works because EPS is permeable enough to let the humidity out.

Challenges: Intersections

The biggest challenges in high-performance wall construction are corners, floors, roofs, and openings for windows and doors at the intersections. Every opening and intersection is a place that must be sealed tight. The photo below shows an example of a window opening being sealed in a Passive House under construction in Flatrock, Newfoundland, Canada. The sealing materials include expanding spray foam and high-performance construction tape.

Conclusion

For a high-performance building in a cold climate, you need thick walls that are well-insulated and airtight. There are a few different ways to achieve that, so you can choose the way that suits you best.

Article by:

Dane George

Dane George holds a Bachelor of Civil Engineering and a Masters of Applied Science in Mechanical Engineering from Dalhousie University. He has three years of experience working with residential contractors with a focus on energy efficient renovations, and has worked with the Clean Foundation as a Certified Energy Advisor conducting energy audits of homes. Most recently, his graduate research involved analyzing electricity consumption patterns. Dane has also prepared and delivered workshops on home energy sustainability, and is currently teaching Renewable Energy and Energy Efficiency at Saint Mary’s University in Halifax.