20) Mooney walls/thermal bridging, breaking

by Gail on December 7, 2009

Today's "look"

Today's "look"

The following link demonstrates a very interesting construction technique that our architect Jonathan Ehling passed on to us. We would like to use it, if it is not too late!

Mooney Wall (this site link is filled with great advice, mostly about the use of solar energy) – retrofitable wall with high R value and low thermal bridging
This description of the Mooney wall appeared in the Breaktime discussion group of the Taunton Publishing discussion forums (Taunton publishes Fine Homebuilding magazine) — a good place to get all sorts of construction advice.

The Mooney wall provides high R value, low thermal bridging, and good resistance to infiltration.   All this at a low cost (especially if you are providing the labor).  Thermal bridging refers to the transfer of heat directly through the relatively low R value studs — this can reduce the R value of the wall substantially from the R value of the insulation placed between the studs.  For example a 2X6 stud wall with “R19″ insulation only has an R value of 13.5 when the thermal bridging of the wall structure is taken into account (see http://www.ornl.gov whole wall calculator).

The Mooney wall can be used for either new construction or as a retrofit to increase the R value of an existing wall.

Another example of using the Mooney wall here …

From the BreakTime Post:
The “Mooney wall” is the The brainchild of Mike Smith and Tim Mooney.  The primary purpose of the MW is to upgrade the insulation of a standard 2×4 (on 16″ centers) wall.   The MW is well suited to renovation work with superior insulating characteristics to standard 2×4, 2×6, and many variations.  In one thread I see Mike estimating the wall at a true R-18  (unlike the artificial R-19 of a 2×6 fiberglass wall, which doesn’t take into account the thermal bridging of the wood).

The Mooney Wall’s thermal bridge is restricted to the points where the 2×2′s (really 1.5″x1.5″) contact the 2×4 wall studs.  This is a vast improvement over the nearly 30% thermal bridge of a typical 2×4/2×6 wall.

{ 4 comments… read them below or add one }

Joy December 7, 2009 at 9:05 pm
John December 21, 2009 at 9:43 pm

I am interested in Mooney Wall technique. Do you have any comments or tips based on your experience?

Thanks

Reply

Gail December 22, 2009 at 10:10 am

Hi, John
We were more-or-less too late to use the mooney wall. Ideally, the walls will be 2×4, with the thermal breaking horizontal 2×2 adding the other 1.5″. The anchor bolts were already set in the concrete at the 2×6 width, and our 2×6 lumber was already purchased, and much of it used for formwork first.
I think our framer wanted to make me feel better about it, so he suggested (and quite rightly) that in our climate, where it seldom goes below 0 degrees, the added labour to add the horizontals plus the minimal difference in actual R value would never be justified in the amount of energy loss. We would lose valuable floor space because the walls would now be 8″, and then the windows would have to have wider sills, adding to the expense. We’re planning to use blown-in cellulose for the walls, and spray-on foam for the ceiling, so the R value will be pretty good anyway.
Depending on where you live (colder climates, for example) and assuming you’ll be doing the labour, it would be energy-efficient and cost-okay.
The other thermal break I would have liked to use is Dow’s Structural Insulated Sheathing, used on the outside. It is neither available in Canada, nor approved as a sheathing for our building code. But maybe when you get there…

Reply

Anne January 21, 2010 at 3:18 pm

I saw this used in a co-housing development in New Hampshire, but 2x2s on 2x6s, with cellulose.

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