[Bldg-sim] Vented cavity behind brick or precast panels and effective R-value

[Leonard Sciarra]

Christopher –
The ASHRAE SSPC 90.1 discussed this in preparation of addendum ac 4th public review to 90.1-2010.  The handbook values for “air” are based on the somewhat subjective description of an air space where “air movement is minimized”.  Minimized of course means different things to different people, and the description of the test procedure was a ‘sealed’ box with sheet metal and felt (the report we think was from like 1950).  In general, the final discussions that resolved all the parties was (there were also modification to the allowances for assumptions of reflective barriers).  There is a request for research proposal on this topic to better understand the effect of movement and effectiveness of radiant barriers.

However, in response to your request of the effectiveness of the air space in a cavity exposed to exterior conditions (wind, temperature, pressure differentials) it was pretty clear the handbook values would not apply as there is just too much air movement to create a stable mass of air. (note you could apply the air films listed for a semi-exterior space in A9.4.1)

The final text of ac 4th PRD:  (the table did not change and you will note the current table in 90.1 does not exactly match the handbook as the committee simplified the table with only up, down, and side to side air spaces and made then all Delta T neutral)

A9.4.2 Airspaces. The R-value for airspaces shall be taken from Table A9.4A based on the effective emittance of the surfaces facing the  airspace from Table A9.4B provided the following criteria are satisfied:

a) The airspace shall be an enclosed and unventilated cavity designed to minimize airflow into and out of the enclosed air space.  Airflow shall be deemed minimized when the enclosed airspace is located on the interior of the continuous air barrier and bounded on all sides by building components.
b) Reflective insulation as defined in ASTM C1224, where used, shall be fitted closely around all non-heat producing components and taped or otherwise sealed to eliminate gaps or voids through which air, dust, or water vapor has the potential to pass.
c) Non-parallel spaces shall use the average distance to determine the thickness of the airspace.
d) Airspaces less than 0.5 in. (13 mm) depth thickness shall have no R-value.
e) The R-value for 3.5 in. (89 mm) airspaces shall be used for airspaces of that thickness or greater provided that that airspace does not exceed 12 in. (300 mm) between the surfaces at any point.


Leonard Sciarra, AIA, ASHRAE, LEED ap+
Senior Associate
+1 312.577.6580 Direct
+1 312.456.0123 Main
+1 847.345.5619 Mobile

Gensler
11 East Madison Street
Suite 300
Chicago, Illinois 60602
USA

From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Keith Swartz
Sent: Friday, March 04, 2016 3:16 PM
To: Jones, Christopher <Christopher.r.Jones at wspgroup.com>; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Vented cavity behind brick or precast panels and effective R-value

Christopher,

The ASHRAE Handbook has a section about thermal resistance of air spaces. See the 2013 Fundamentals Handbook starting on p. 26.12. Is that what you are looking for?

Keith Swartz, PE | Senior Energy Engineer
Seventhwave
608.210.7123 seventhwave.org

From: Jones, Christopher [mailto:Christopher.r.Jones at wspgroup.com]
Sent: Friday, March 04, 2016 12:52 PM
To: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: [Bldg-sim] Vented cavity behind brick or precast panels and effective R-value

Is anyone aware of any analyses of the effective insulation properties of brick or precast veneer with a vented air space? I assume the air in that cavity is moving slowly but it isn’t completely still therefore you can’t include all of the insulative properties of those two layers on the outside of a wall?


[cid:image003.png at 01D09C46.E75BA0D0]
Christopher Jones, P.Eng.
Senior Engineer

WSP Canada Inc.
2300 Yonge Street, Suite 2300
Toronto, ON M4P 1E4
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