NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/misc/consumer/house/14696

Internet Message Format | 1992-11-17 | 2.9 KB

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From: [e-mail] (Mark C. Lawrence)
Newsgroups: misc.consumers.house
Subject: Re: R-values
Date: 16 Nov 1992 17:23:43 -0800
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In article <[e-mail]>,
[e-mail] (Phil Ngai) writes:
>In article <[e-mail]> [e-mail] (Harry Funk) writes:
>>Well, at the risk of calling into question my reliability :^), I'll take a
>>stab at why someone who's *really* pedantic (me, for example :^)) might
>>maintain that you get less total R-value *and* less R-value per inch when
>>you compress fiberglass.  First, let me say that the loss is negligible for
>>most applications.  That said, I'd base my conclusion on the following data
>
>You are talking about a type of compression that is way outside the
>question asked. You can talk about solid glass if you want, but that's
>not really relevant to the subject of home insulation.

Most (all?) insulation materials have much higher thermal conductivity than
air.  They work by preventing convection.  That's why foam-type insulation
works so well.

In the case of glass fibers, one can presume that at very low density they
will be ineffective in preventing convection currents, but their conductivity
would be negligible.  As the density increases, loss from convection will
decrease, and the loss from conduction will increase.  Presumably there is an
optimum that gives best insulation (maximum R-value) in a given wall
thickness.  I would even go so far as to assume that the insulation you buy is
in fact at this optimum density when installed in a wall of the specified
thickness.

My reasoning:  If a lower density would achieve a better R-value, that would
be how they'd sell it, as it would be cheaper to make and thus more profitable
for the manufacturer.  If a higher density would achieve a better R-value, you
would be able to buy insulation with higher R-values for a given thickness,
e.g., 3-1/2 inch R-19 or some such.

>not really relevant to the subject of home insulation. Why don't you
>talk about performance at temperatures around 10 to 100 degrees K,
>where the "air" might be solid or liquid? Or go even further with
>compression to neutron star densities? Going the other way, why bother
>with fiberglass, just leave the walls empty, since your K factor
>for air seems to indicate an R-value per inch of nearly 6?

Or better yet, leave your computer tuned to USENET and let the flames
warm your house.  With Flaming Phil online, you don't need a fireplace...

Mark C. Lawrence
Systems Programmer     Internet: [e-mail]
Stanford Data Center     Bitnet: M.Lawrence@STANFORD
Stanford, CA 94305-4136     Tel: [phone]