ASHRAE Journal - April 2021 - 54

COLUMN BUILDING SCIENCES

the airtightness. We showed this with fiberglass and
mineral wool insulation ( " Venting Vapor, " ASHRAE
Journal, August 2015 and " Conditioned Unvented Attics
& Unconditioned Unvented Attics, " ASHRAE Journal, June
2020).
Foam sales people and marketing people forget one
of the code arguments against unvented conditioned
attics-when you move the thermal layer from the ceiling to the underside of the roof, the surface area of
heat gain/heat loss goes up 30% to 40%.** I had to argue
aggressively to not have spray foam roofs penalized
because of the increase in surface area. Remember the
big issue was leaky ductwork in vented attics. By moving
the insulation layer to the underside of the roof deck, I
argued that the ducts were now " inside, " and if they now
leaked it wouldn't matter.
The counter code argument was, " Who cares? We can
make ductwork tight, so it does not leak. " Yes. True.
Ouch. Ducts today are tight wherever they are. (We were
here before, " Cool Hand Luke Meets Attics, " ASHRAE
Journal, April 2014.) But I won the battle by saying even

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tight R-6 ducts were pretty bad in a vented attic when
they are above all that R-30 ceiling insulation (now
it is R-38 and going up) exposed to all that radiation
exchange from the underside of the roof deck. If I could
just keep to R-30 on the underside of the roof deck and
not be made to increase the thermal resistance of that
layer, the increase in surface area of heat gain/heat loss
would be compensated for by the way more effective
thermal performance of the ductwork that was no longer
exposed to the radiation exchange.
The next battle was from attic vent people who argued
that vented attics and vented roofs were more energy
efficient because all that airflow from the outside would
cool the underside of the roof deck. Turned out that was
not the case, because for the airflow to be effective, it
had to be well coupled to the underside of the roof deck.
The airflow was not sufficient to compensate for the
radiation exchange between the underside of the roof
deck and the top of the insulation layer on the ceiling.
I need to repeat once again that it was lucky to just break
even thermally with unvented conditioned attics compared to vented unconditioned attics in the code arena.
So now I chuckle in amazement when I hear the current
argument: " If I use spray foam on the underside of the
roof deck, the increase in airtightness is so significant I
can, therefore, reduce the minimum Model Code thermal
resistance roof insulation R-value by 20% or 30%. "
Let me go back to the beginning. Again, it is a very,
very bad idea to try to trade off thermal resistance for
increased airtightness because the Model Code minimum
airtightness is already so low. Let me repeat. Again, it does
not matter because we are already so " tight. "
And guess what? Let's say the argument works. Guess
what comes next? We already have manufacturers of
solar panels, battery storage and high efficiency geothermal systems saying, " Who cares about thermal resistance and airtightness if you use our stuff? We can trade
off building enclosure efficiency with solar collectors
and conditioning system efficiency improvements. We
don't need all that airtightness and thermal insulation. "
Sales people and marketing people who sell and market thermal insulation. Be careful what you wish for.

References
1. BSC. 2013. " The Thermal Metric Project. " Building Science
Corporation. https://www.buildingscience.com/project/thermalmetric-project
**It's

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this Pythagoras and hypotenuse thing.

http://www.ashrae.org/technologyportal https://www.buildingscience.com/project/thermal-metric-project https://www.buildingscience.com/project/thermal-metric-project http://www.ashrae.org

ASHRAE Journal - April 2021

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