# OT: approximating R values and fun with IR thermometers

Posted by Jeff on December 26, 2006, 5:34 pm

So, how do you calculate what the R values of our walls and floors
actually is? (Googling reveals little)

Here's what I'm thinking, let me know where I've gone astray. Or if
there is another way.

We know that Q (heat flow) = delta T / R

Now, if we want to find the R value of our wall, we can measure the
outside temperature and the temperature of the inside wall:

Tw = T outside - T inside_wall

So the heat flow is: Q = Tw / R

But we don't know what Q is.

We do have another R value we can look at, this is the R value of the
boundary between the inside wall and the interior of the room. The heat
flow through that is the same as the heatflow through the wall:

Ti = T inside_room - T inside_wall

Q = Ti / R boundary

and

R = R boundary * (Tw/Ti)

The Passive Solar Energy Book has a table with boundary R values, for a
vertical wall with (still air) and horizontal heat flow we have R = .68

So, vertical wall:
R = .68 * (Tw/Ti)

Floor:
R = .76 * (Tw/Ti)

Ceiling:
R = .61 * (Tw/Ti)

Caveats:
1) Since hot air rises and stratifies, you'd want to take your inside
temp near the boundary level

2) for higher temp differences you may need to consider radiation losses.

3) Air changes are bundled into the R value

Another way of doing this would be to have a known R value and add
that at the wall boundary, measuring the temp at each side.

What do you think?  My IR thermometer is on loan, I'll run
calculations here later.

Jeff

Posted by Duane C. Johnson on December 27, 2006, 1:59 am

Hi Jeff;

> So, how do you calculate what the R values
> of our walls and floors actually is?
> (Googling reveals little)

> Here's what I'm thinking, let me know where
> I've gone astray. Or if  there is another way.

> We know that Q (heat flow) = delta T / R

> Now, if we want to find the R value of our wall,
> we can measure the outside temperature and the
> temperature of the inside wall:

> Tw = T outside - T inside_wall

> So the heat flow is: Q = Tw / R

> But we don't know what Q is.

> We do have another R value we can look at, this is
> the R value of the boundary between the inside wall
> and the interior of the room. The heat flow through
> that is the same as the heatflow through the wall:

> Ti = T inside_room - T inside_wall

> Q = Ti / R boundary

> and

> R = R boundary * (Tw/Ti)

> The Passive Solar Energy Book has a table with
> boundary R values, for a vertical wall with (still
> air) and horizontal heat flow we have R = .68

> So, vertical wall:
> R = .68 * (Tw/Ti)

> Floor:
> R = .76 * (Tw/Ti)

> Ceiling:
> R = .61 * (Tw/Ti)

> Caveats:
> 1) Since hot air rises and stratifies, you'd
>    want to take your inside temp near the
>    boundary level

> 2) for higher temp differences you may need
> 3) Air changes are bundled into the R value

> Another way of doing this would be to have
> a known R value and add that at the wall
> boundary, measuring the temp at each side.

Yes, make a tool like this:
http://www.redrok.com/images/r_factor.gif
Make the tool fairly large, maybe 2 foot square.
The larger the size the less fringing effects
will affect the accuracy.

The aluminum plates tend to spread the heat and
average variations in the wall. I would use
thermocouple temperature sensors although I have
used diode sensors which can be cheaper. Make the
plates quite a bit smaller than the insulation
sheet.

The reference insulation should be high quality.
I use Polyisocyanurate foam. The R value for mine
is 5.30 (ft^2 F hr)/(BTU) for 1" foam.
http://en.wikipedia.org/wiki/Polyisocyanurate
http://en.wikipedia.org/wiki/R-value_%28insulation%29

Make 3 temperature measurements:

10 Tin   = 65   :REM inside plate temperature in degrees F
20 Tmid  = 60   :REM middle plate temperature in degrees F
30 Tout  = 37   :REM Outside wall temperature in degrees F
40 Rref  = 5.30 :REM R-value of the reference in (ft^2 F hr)/(BTU)
50 Rwall = (Tmid - Tout) / (Tin - Tmid) * Rref
60 PRINT "R-factor for the wall = " ; Rwall ; "(ft^2 F hr)/(BTU)"
99 END

> What do you think? My IR thermometer is on loan,
> I'll run  calculations here later.

>   Jeff

Duane

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Posted by Jeff on December 27, 2006, 3:34 am
Duane C. Johnson wrote:

Thanks Duane, It looks like a good design. I just happen to have all
the parts on hand (except it'll be 1/2")!

Roughly how long does it take for the temperatures to settle and get
a good reading? The polyiso is fairly low density, I'm thinking 5 or 10
minutes?

Cheers,
Jeff

Posted by Steve Barker LT on December 27, 2006, 3:45 am
Speaking of polyiso, who sells that stuff?  I need 2 4x8 sheets 1 inch
thick.  KC area.

--
Steve Barker

Posted by jaall on December 27, 2006, 7:46 am
My Home Depot store sells it.

ja

Steve Barker LT wrote: