ASHRAE Journal - April 2015 - 56

COLUMN BUILDING SCIENCES
Joseph W. Lstiburek

Continuous Insulation and Punched Openings

Windows Can Be A Pain

BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

Sometimes we make easy things hard. And sometimes we make hard things easy. With
continuous insulation and punched openings both things are true.
The physics is easy. A wall has to control water, air, vapor and heat. A window has to
control water, air, vapor and heat. Both have a water control layer, an air control layer,
a vapor control layer and a thermal control layer. All you have to do is connect the water
control layers to each other, the air control layers to each other, the vapor control layers
to each other and the thermal control layers to each other. Oh, yeah, one other point.
You don't want the windows to be sucked out of the wall when it is really blowing.
Now it gets interesting. Where is the wall water control
layer? With continuous insulation it can be the continuous insulation layer itself-or it can be behind it. It is
pretty dumb to put a separate water control layer-specifically a film or thin membrane-over the exterior of
the continuous insulation layer because it is impossible
to install the additional layer in a practical manner-one
that prevents it from getting sucked off and one that has
constructible details.†
Now, I don't have a problem with water control layer
films when they are used correctly. Historically, they
have an awesome track record. Tar paper, impregnated
felt, coated paper, and polyolefin films go back a long
way. The best performance from a wind load perspective
* A "Straube-ism"... after Professor John Straube. He is a master punster. I stole this line from him
to make this column work.
† The only folks that recommend the practice are the folks that sell water control layer films (aka
housewraps, water-resistant barrier films and coated papers). The only reason I can come up with
as to why is that they don't want the rest of us to figure out that you don't need their products if you
turn the continuous insulation layer itself into the water control layer.
‡ Not good if you are in the water control layer film business-but pretty good if you are in the
liquid-applied over structural sheathing water control and air control layer business or if you make
structural sheathing that is itself the water control and air control layer. Of course, both of these
groups hate the continuous insulation people who argue that the continuous insulation can do both
on its own. Ah, the marketplace is getting interesting and the squabbles are getting ugly. Each group
is trying to screw over the other groups to either hold on to market share or capture market share.
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and a durability perspective comes from installing such
films behind the continuous insulation layer and over
structural sheathing. That way the film is supported on
both sides-it is sandwiched typically between OSB/plywood/gypsum structural sheathing on one side and the
continuous insulation layer on the other. Neither sucking nor blowing cause it to flex. Of course, you can turn
the structural sheathing itself into a water control layer
and air control layer and not need the film layer at all.‡
I think the winning technologies are to make the structural sheathing itself the water and air control layer-
and to install continuous insulation over the structural
sheathing (Figure 1). Back in the day we called this the
"perfect wall" (see ASHRAE Journal, May 2007). Or make
the continuous insulation itself the water and air control
layer and include or exclude the structural sheathing
based on-wait for it-structural considerations (Figure 2).
Note that this is an "opinion" so everyone relax. You get
to have your own opinions, too.
So, according to me, there are two locations for the
water and air control layer-behind the continuous
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford,
Mass. Visit www.buildingscience.com.

ASHRAE Journal - April 2015

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