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

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

> 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.

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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Duane C. Johnson wrote:

*> 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*

*> > 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.*

*> *

*> 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.*

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

*> *

*> 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*

*> *

> 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> > 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.>> 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.