Thursday, June 10, 2010

Plywood Baffles

I have lately been putting energy into my web pages, instead of this
blog. Since more people come here, I should transfer web site
 information sometimes, to be in both places. I apologize for the
 redundancy, and will do this with limits. Here is a first installment.

Plywood Baffles
I no longer accept that expanded polystyrene or cardboard baffles
 have acceptable durability. Cardboard baffles see driven moisture,
 and decompose. At elevated  temperature, EPS evaporates.

This baffle for 24" oc 2x4 roof joists, has dimensions 22" x 24".
Plywood is 1/2" thickness CDX. 3/8" thickness is ample for future

I place top-layer insulation batts, here Johns Manville AU333, R19,
24" x 48", all the way out to the exterior wall. The adjacent baffle here
 will be withdrawn and reset, to place the next batt.

Here is a look down into a finished vent pocket. Insulation under the
 baffle is protected against soaking or other insult, by driven weather.
I will learn to shorten nearest baffle support legs, to give a little more
room for insulation. This baffle clearance is much larger  than area in
 the soffit screen.


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Efficient Farmer said...

I'm wondering if I need to do baffles like these for my house, when I retrofit fiberglass batt to R49 or better when vermiculite is all out.

The house has externally fastened metal roofing and no soffit vents (all is gable/ ridgecap). Part of the attic has spaced sheathing (1x6s) with metal directly over it. Part has plywood with metal over that.

There is surely some airflow underneath the metal roofing, particularly along the raised "D" profiles. Does it make sense to baffle these?

I see plusses of keeping insulation out of soffit(ice dams) and ensuring any moist/dirty airflow that occurs isn't THRU fiberglass.

I see minuses of time & materials spent to construct, more time to install, and leaving even less space on a mild pitch roof for insulation over wall top plates.

Curious what your thoughts are.