Question

What is the most efficient wall assembly for new construction? I'm having trouble talking my contractor into 2x6, OSB, and house wrap.

Asked by Laura Sainz
Sacramento, CA

The contractor has particular issues with the rain screen concept and how to deal with the window installation given the increased depth.

Answer

Elizabeth DiSalvo

Answered by Elizabeth DiSalvo

Norwalk, CT

Trillium Architects

September 19, 2011

Hi! I have been designing high efficiency wall assemblies for 12 years, in the North East primarily, though I just finished a house in San Diego.

Your climate

The most efficient wall assembly depends on your climate really.

  • We are in Climate Zone 5 - which is a heating zone.
  • Your exact zone may depend on your exact location in Sacremento (I just looked it up and it seems that your zone does vary) but for the most part you are likely in a fairly moderate zone.

That said, what you are proposing: 2x6 walls with OSB and then a rainscreen is a good wall for a moderate climate.

  • It will keep you warmer when you need it and cooler when you need it. It is not overkill.
  • If you were in LA I would say use a 2x4 wall.

But in general if you are in a climate in which you are somewhat concerned about either heating or cooling, building a 2x6 wall and filling it with insulation will always help you use less fuel to do either. In our climate I use that wall and then I wrap the whole thing in 2 inches of rigid polyiso insulation board. In our climate that is a good answer. In yours it may be overkill.

A rain screen will make your siding last much longer

The rain screen is ALWAYS a good idea. Yes, it is a bit more money and labor but it makes whatever siding you put on your house last much much longer.

  • And if you are using wood siding - or even a composite siding your paint job will go many years longer than it would without a rain screen.
  • Also make sure your OSB is coated or covered with a moisture barrier. Tape the seams of your OSB if you can.
  • If you can create a good air and moisture protection layer at the back of the rain screen you will make the wall itself last a lot longer and function in an even more energy efficient way.

Get a new contractor?

The above is a pretty simplistic answer to what can be a really complicated question. But I think the big answer is 'get a new contractor!'

  • The biggest obstacle to a high efficiency house is often the guy who is building it.
  • Window in a deep wall? Every single house we do has that issue. The easiest way is to justify the windows to the outside of the wall and build a box around the window inside. It creates a great plant shelf or place to put your coffee cup!
  • We also put the windows in the middle or interior of the wall. Also fine but you have to be meticulous about moisture control.

Many contractors are close friends of mine. I truly value my smart, curious, creative and open minded contractor friends. If you have a contractor who is none of the above then maybe look for one who is happy to help you achieve what you are looking for.

It is said that mid-wall is most efficient but more dangerous to build because of the water leak potential. But really it is not that hard if you simply justify the windows to the outer wall. Oh! And wait you are only building a 2x6 wall? They have jamb extensions for that. It is a normal thing in the window world. How does the saying go? 'If you aren't helping then you are part of the problem'...

I know for a fact that there are plenty of green builders in Sacramento who would not blink an eye at a rain screen nor a well insulated wall. They will refine what I said and suit it exactly to your climactic condition.

Ok- Best of luck!

(And please remember: this is just a primer ~ there are books written about the most efficient wall assemblies....)

Elizabeth

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