Wall Assemblies: Have your Cake and Eat it Too!

When it comes to wall assemblies, you can have your cake and eat it too!

I grew up in a home built in 1911. Houses were different back then. This home had sawdust for insulation and single pane windows. As it got colder, we put more wood in the fireplace. There were days in the winter when the house was so hot that we kept the front door open to cool off the home. Neighbours would drive by and think, “There are those Crazy Clays, it is -20° and they have their doors wide open!” That was the past.

Conserving energy was not a concern, since we just put another log on the fire. Firewood was free. Insulating the wall and achieving air tightness was just not a priority.

Homes eventually made a move to oil/gas fired furnaces. This new fuel cost more than the wood, which was free, so energy efficiency became more important. Thus, people began to place a greater emphasis on putting insulation between the studs.

In the 1970’s, the energy crisis hit us hard. We increased the insulation levels even further, and improved air tightness. The vapour barrier poly sheathing became standard construction. While increasing insulation values and airtightness can be a good thing, it can also have some negative consequences. Remember the condo crisis with wall assemblies rotting from the inside out?

Allow me to share a building science principle.

Take one cold coke out of your fridge and put it on your coffee table. Take another one out of the pantry and put it beside the first coke can. Wait 5 minutes. What do you notice? Yes, the can out of the fridge has accumulated condensation on the outside of the can, while the other is dry. Moisture in the air condenses on cold surfaces.

One important objective of a well-built wall assembly is to warm up surfaces where condensation may occur.

There are three problems with traditional batt insulation:


  1. We can stuff only so much insulation between the studs.  There is a limit.
  2. Eliminate thermal bridging. While insulation has an R-value of 22, the wood studs have an R-value of around 6. The framing is about 23% of the wall. So 23% of the wall is only R-6. The wood promotes thermal bridging.
  3. Poly sheeting does not allow the wall assembly to breathe.
Is there a better way?

Dr. Joe Lstiburek is a world-renowned building science expert. He has been a proponent of the perfect wall assembly. This wall assembly reduces thermal transfer and keeps the exterior sheathing warmer, thus reducing the propensity for moisture buildup.

For greater detail on this wall assembly, see my article, The Perfect Wall.

By choosing the correct wall assembly, you will achieve the following benefits:


  1. Thermal transfer is eliminated.
  2. Inside of the exterior sheathing is now warmer reducing the propensity for condensation.
  3. Effective R-value can easily double.
  4. No poly sheeting allows the wall assembly to breathe.


A well-built wall assembly is breathable and free of moisture.

I speak to builders who remember the condo crisis. They make comments like, “I just slash the poly, so that the wall can breathe.” This is prehistoric thinking; there is a better way. Why sacrifice air tightness to improve the breathability when building science proves you can have both?

Homes of the future will be more energy efficient and air tight. This wall assembly will breathe and be free of moisture and mildew, making homes healthier. Yes, you can have your cake and eat it too!

The Perfect Wall

Your choice of wall assembly will not only affect the energy efficiency and the longevity of your home but will also affect the air quality through the presence of moisture and mould in your wall. A small investment into this wall assembly will provide benefits to not only the operating costs but more importantly the air quality and longevity of your home.

The wall assembly is comprised of the cladding, framing structure, insulation and air/vapour barrier. When considering the best wall assembly for your home the following issues need to be addressed: how do you prevent water from getting in, how do you keep air from getting out, how do you establish insulation levels as high as possible and how do you construct the wall assembly to increase its ability to dry out in the event water gets in?

One challenge of our traditional wall assembly is that almost ¼ of the wall is comprised of wood that has an R-value of 1.5/inch thus degrading the effectiveness of the R-value of the wall. This leaves only ¾ of the wall to shove our insulation into. The framing lumber acts as a thermal bridge. Since we are limited by how much insulation we can shove between the wood studs, the next generation of wall assemblies will involve moving insulation to the outside of the wall. This will greatly reduce the thermal bridging and moisture in the wall assembly whilst increasing the energy efficiency, longevity and health of the home.

Most of us are familiar with the poly plastic sheeting placed between the insulation and the drywall. This traditional approach has the poly performing two functions: the air barrier and the vapour barrier. Unfortunately, this poly can trap moisture inside the wall assembly. A better approach would be to move the air barrier to the outside of the home sandwiched between the plywood sheathing and exterior rigid insulation. Removing the poly will allow any moisture that penetrates the wall assembly to dry to either the interior or exterior. Since moisture promotes mould, a wall assembly that remains dry will be healthier for its occupants. Cutting edge builders embrace positive change.

Joe Lstiburek a world-renowned building science expert has coined this wall, the “Perfect Wall.” Any builder familiar with building science will be able to advise you on the best wall assembly for your current and future needs.


Effective vs Nominal R-values

The City of Vancouver has recently implemented their new Vancouver building code. One small change affects the insulation of exterior walls. Previously, exterior walls needed to be at a nominal R22 insulation level. The new code requires effective R22 insulation levels. What difference does this make?

Nominal insulation is the R-value of the insulation batt we stuff between the wood studs. This is the R-value we see printed on the bag. Don’t be fooled, this is not the R-value of the wall. Wood has an R-value of R 1.5 per inch and makes up approximately 23% of a traditional framed wall. A 2×6 stud will have an R-value of 6 while windows range from R2-R5. Relative to the insulation property of fiberglass, wood and windows are poor performers.

What will it take to achieve Vancouver’s R22 effective insulation values in exterior walls? Here are three options.

1. The traditional 2×6 wall insulated with R22 batt will achieve R17.89 effective. Fail!

2. A 2×8 wall insulated with R28 batt will achieve R22 effective. Pass!

3. A 2×6 wall with R22 batt and 1” exterior rigid insulation achieves R22 effective. Pass!

There are many variables at play to achieve R22 effective. Make sure your builder considers the many options and doesn’t default to what is easiest. There is one wall assembly considered by the world experts to be the perfect wall assembly. Check back for my next blog post on the “Perfect Wall” and its many advantages.