From: mja@philabs.philips.com (Michael Amato)
Subject: Re: R-values
Originator: mja@hercules
Reply-To: mja@philabs.philips.com (Michael Amato)
Organization: Philips Laboratories, Briarcliff, NY 10510
Date: Fri, 13 Nov 1992 21:12:59 GMT


In article <1992Nov13.062141.2776@adobe.com>, pngai@adobe.com (Phil Ngai) writes:
|> In article <1992Nov12.155202.17035@hubcap.clemson.edu> hubcap@hubcap.clemson.edu (System Janitor) writes:
|> >I have a (sort of) related question. Everyone knows that if you cram
|> >6 inches of insulation in a 2X4 stud wall, you don't get the full
|> >insulation value of the thicker insulation. What I don't know is:
|> >how does the insulation value that you do get compare to 4 inch insulation
|> >in a 2X4 stud wall.
|> 
|> According to "The Visual Handbook of Building and Remodeling" (an
|> excellent book which I highly recommend), the R value per inch goes
|> up linearly as you compress it, from about 3.2 at 100% thickness to
|> about 3.8 at 50% thickness.
|> 
|> To answer your specific question, R-19 insulation which is 6.25 inches
|> thick gives R-18 in a 5.5 inch wall (2x6) and R-13 in a 3.5 inch wall
|> (2x4). (apparently this is a very common question, I wonder why :-)

This information is very interesting. I figured the R-19 they sell would not
be R-19 in my walls. This is why I bought and would recomend 5.5" R-21 insulation
for 2x6 framing.

|> 
|> You may have seen them selling R-13 insulation for 2x4 walls.
|> Now you know how they do it and why it costs so much.

I don't know about the cost of R-13, but for the R-21 the price increase over
R-19 (->R-18) for the same square footage was not that much. I am insulating
approx. 1000 sq. ft of addition with lots of cathedral ceilings. It requires 
16 roles the R-21 (lots of knee walls) and the price difference was only $100.
The R-19 is sold in different size roles, and the R-21, although more expensive
per role, has more square footage per role, reducing the price difference
somewhat. (I hope I'm not wrong since I'm going from memory). Anyways, the
cost difference was not that much.

|> 
|> There will undoubtedly be many postings saying you get less total R-value
|> when you compress fiberglass (even though you obviously already know this).
|> There will be some postings that go even further and claim the R-value/inch
|> goes down with compression. This is (to be charitable) presumably based on
|> a misunderstanding of the difference between total R-value and R-value/inch
|> plus the usual exaggeration that goes on when stories are passed word of
|> mouth.  Needless to say, the second claim is wrong no matter how many times
|> it is repeated.

OK. So now how much R-value do I lose when I put in rafter vents and lose
about 1" of the 9.5" for the R-30 insulation? From you numbers above

5.5" -> 6.25" causes  R-19 -> R-18

9.5*5.5/6.25=8.36, which is about what is left after the vent is installed.

Then  (R-30)*(R-18)/(R-19)= R-28.42? 

I wonder if this is why the better rafter vent is rated at R-1.5.

|> 
|> This question has come up before with responses as I described, and there
|> is no reason to believe this time will be any different.  Consider it an
|> opportunity to determine the reliability of the participants.
|> 
|> 
|> -- 
|> My opinions are my own.
-- 
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| Michael Amato  DoD#0639 | "If pro is the opposite of con,       |
| mja@philabs.philips.com | then what is Congress"? - Will Rogers |
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