What I have been told for over 30 years is ½” or 13mm. That’s why thermopane windows are spaced ½” .
Of course that’s old info.
Janosh
From:
Sent: Sunday, February 22, 2009 12:05 PM
To:
Subject: Re: [papercreters] CMU blocks with crete mortar?
I had read one source a whiles back that stated it was a quarter inch square
was the maximum size for an airspace that won't support convection of air.
That seemed very tiny but I remember that the source (which I can't remember
now) seemed very knowledgeable.
-x-
----- Original Message -----
From: Spaceman
> I'm afraid I can't answer that one. I know in principle how it works but
don't have the figures handy. The basic idea is that air rises when it warms
because it is lighter from expansion. As it rises cold air replaces it, and
becomes warmed in turn. As the cavity air warms and circulates, heat is
transferred towards cold surfaces, the air is cooled and sinks to replace
rising warm air. A quick search shows me many websites that state the
principle, but none with hard numbers for the minimum cavity that supports
convection of air.
>
> Spaceman
>
> Michele Marcell wrote:
>
> Ok. How small is that!!
> Michele
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