ASHRAE Journal - April 2020 - 8

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LETTERS

Interior Spray Foam
February's Building Sciences column is about using
closed-cell and open-cell spray urethane insulation
inside a building. Closed-cell is air impermeable while
open-cell is not.
In the past, Dr. Lstiburek warns against using vapor
retarders on both sides of a wall or roof, but yet the closedcell insulation is recommended inside in conjunction with
an exterior vapor retarder. Please have Dr. Lstiburek clarify.
R. M. Joki, P.E., Member ASHRAE,
Montgomery, Ala.

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I am quite sure that I have not in the past warned against
double vapor barriers in all assemblies. I have, however,
quite emphatically warned against them in air leaky or
air porous assemblies particularly walls. Walls are notoriously more air leaky than roof assemblies because of architectural complexity and the use of air porous materials.
However, they do not have to be. Double vapor barriers are
quite common in roof assemblies. I wrote about them in
January 2016 ("How Come Double Vapor Barriers Work?").
It is also common to have exterior vapor barriers in wall
assemblies-specifically the use of foil faced rigid exterior
continuous insulation. The key is to control the temperature of the condensing surface. I wrote about the topic in
October 2017 ("Hybrid Attics and Hybrid Walls").
In terms of the specific recommendation of highdensity closed-cell spray foam on the interior of exterior
sheathings that are vapor impermeable there are no
issues as the high-density closed-cell spray foam is air
impermeable and has a very low vapor transmission.
Interior vapor does not pass to the exterior in sufficient
quantities via air flow or vapor diffusion to be an issue.
Note that this is for residential occupancy. For special
use facilities such as indoor swimming pools, pressurized and humidified hospitals and art galleries in cold
climates, I would not install high-density closed-cell
spray foam on the inside of a vapor barrier sheathing.
There are no issues in attic assemblies where the roof
sheathing is covered with an impermeable fully adhered
membrane underlayment (a vapor barrier) and highdensity closed-cell spray foam is applied directly to the
underside of the roof sheathing for similar reasons as
discussed for wall assemblies. Note, again that this does
not apply to special use facilities in cold climates that are
humidified and pressurized.
Joe Lstiburek, P.Eng., Fellow ASHRAE, Westford, Mass.

ASHRAE JOURNAL

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ASHRAE Journal - April 2020

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