Superinsulation Using Double Walls

Most people are aware that homes built in the last 15 to 20 years have made some attempt to be energy efficient. However, there has been significant increase of builders and consumers raising the bar to achieve Superinsulation. Which one is passive house

No, ‘superinsulation’ does not have a cape, or a big S on it. It is also not a specific product of insulation. However, it is more of an overall approach to building design, construction, and retrofitting, that dramatically reduces heat loss (and gain) by using much higher levels of insulation than typically used. Various types of superinsulation concepts are available in modern construction. In this article I would like to bring a few to light, so you are aware of some significant changes to construction of green and modern buildings.

Superinsulation concepts can come in many forms and are often what some call passive homes standard. A passive house will use an estimated 90 to 95 percent less energy for heating and cooling and 60 to 70 percent less overall energy than a typical code-built home. So now you are probably wondering what a passive home is. The term passive house (Passivhaus in German) refers to a rigorous, voluntary standard for energy efficiency in a building, reducing its ecological footprint.While there are many techniques and technologies that can be used to achieve the passive house standard I want to only focus on how wood frame structures achieve these higher standards since the majority of homes built in North America are built of wood.

Because wood frame and passive houses are such large topics I am also only going to spend the rest of this article only dealing with walls. Super Insulated walls primary purpose is to keep the cold air outside and the conditioned air inside and not allow air infiltration. In most houses, air leaks occur in numerous places, including cracks in the basement, walls, floors and ceilings; through gaps around windows and doors; and through leaks in the ductwork. Air leaks in and out of the house, and within the house air moves between insulated and uninsulated spaces, so that heated (or cooled) air is constantly being lost.

This air lose is commonly called thermal bridging and it is important that you understand thermal bridging before we get into how super insulated wood frame walls are constructed so here is a couple videos that will help with understanding thermal bridging.

Passive house thermal bridging video:

Now that you have a basic understanding of thermal bridging let’s take a look at wood and it R- Value.

The R-Value of wood is dependent on it thickness and it moisture content. Here is a R-Value chart (Link)

This video does a great job of explaining whole wall R-Value. While the video was made for the product called Rastra however the same concepts of whole wall R-Value also applies to wood frame.

As you can tell wood doesn’t stack up very well against most common insulation products and in light of thermal bridging and wood ability to transfer heat through it. Wood needs to sealed very well or have an additional cavity for more insulation to achieve the passive house standards.

The two most common methods used are Double Walls or Larsen Trusses

Although I want the rest of this article to be focused on how superinsulation is accomplished by the use of different types of double walls I have to at least briefly explain advanced framing since I mentioned it.

Double Walls/ High Performance Wall Systems

Essentially, the technique involves building two stud-framed 2 by 4s walls (the interior one is load-bearing), with the air/vapor barrier placed behind the inner wall. Insulation is sandwiched between the studs of both walls. (This kind of construction allows most of the electrical, plumbing, and ducting work to be located within the interior wall, thus maintaining the integrity of the vapor barrier.  The reason the space between the walls is filled with insulation is so there’s no wooden thermal bridge from inside to outside. Today, this technique is still used, but other less-expensive approaches are often used.

The amount of framing lumber for a double wall can be cut by staggering the 2 by 4 studs to the inside and outside.  All of the members are structural, but the plates — the only conductive paths from inside to outside — must be as wide as the walls.

Double walls can come in many variations and there is no way I could cover every type in this article however I would like to discuss the Larsen truss.

Larsen Trusses

Is a double wall system that usually has a traditional 2×4 framing load bearing wall for interior wall and an exterior chord of trusses called the Larsen truss.

A Larsen truss is usually site-built. Because the truss is not required to bear any roof load, its components are light. The original Larsen truss consisted of two parallel 2x2s connected by small rectangular gussets of 3/8-inch-thick plywood. The gussets measured 6½” x 8¼” each and were spaced 24 inches apart.

The exterior Larsen truss extends from sill to rafter tail and attached to studs. This creates a balloon-framed outer wall (which is erected after the roof is on).

Note: The large window well in this video. We will discuss it later in article.

Fire Service Concerns:

The whole purpose of these super insulated passive houses is having a structure that is extremely air tight. This extreme tight structure is great 364 days a year but the day it catches fire it could be a whole different story to use firefighters.

The same design principles that keep inside environment in and the outside environment outside will also play a role in fires in these structures with very little smoke leakage to the exterior until a window fails.

Firefighters should approach any fire in these structures as ventilation controlled fire and anticipate that conditions could also be under-ventilated resulting in backdraft of smoke explosion situations.

There is also a good chance that fires in these structures could present as Nothing Showing so make sure and not let your guard down, because opening the front door will allow air the missing component of the under-ventilated situation.

Since there will not be much leakage of smoke except the neutral plane to drop to floor rapidly and zero visibility.

Another concern with these double walls systems is overhauling and locating smoldering fires.

Larsen Trusses also pose a balloon frame exterior wall so in the event of a fire from exterior look for an open channel for the fire to travel.

Sizing up of these structures can be complicated for a couple reasons. Size Up Clues for wide walls

The first is that from the rig they will appear as any other structure and will not have a sign on they caution super air tight structure.  One must develop a trained eye to look for a few possible clues on their size up.

The only real clue is the size and depth of windows and walls. Since you can usually tell the thickness of a wall from just looking at it? Look for deep window well or wide door jambs. Super Insulated Passive House in TIC view Green Maltese

The second thing that can be complicated on your size up of these structures, is because they are so air tight and double walls you will most likely not have very much thermal signature from the exterior on your thermal imaging camera. The photo above is a thermal view of a passive house in NYC on a cold 12 degree night. Now image if there was a fire inside.

Deep Window Well with smoke stainsBe sure and look around cracks in door frame for any signs of soot that has been deposited on it. This might be the only sign of walking into a serious situation.


In this article we have only dealt with one aspect of a super insulated house and that is the walls. There are many more factors such as roofs, windows and fresh air ventilation systems that pose potential problems to the firefighters. So as with all types of construction it is paramount that you the firefighter do your own research and walk through every building that is being built in your jurisdiction so you will know your own hazards.

Stay Safe

Captain John Shafer


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