Calculating R-Value and Heat Loss Using FLIR ONE PRO And Surface Temperature

Seeing all the missing insulation in my roof had me wondering if I could use my thermal camera to estimate heat loss. A quick google search found this smart guy's post Measuring your house walls R-value with an infrared thermometer with the calculations.

The maths are beyond me, but lets see if I can figure it out.

Temperature Gradient

For a given internal ambient temperature and a given outdoor temperature the temperature drop for each different material is proportional to the materials R Rating. 

e.g. 68F indoor to 8F outdoor is 60F delta T. The total R rating is 13+6=19. So the drop for the R-19 material will be 13/19*60 = 41F.

Now what if we don't know one of the R Ratings but we do know the temperature drop?

Indoor Air Film

Air is a good insulator and the approximate indoor air film for stable air is R0.14 or U7.14. This is the air between your indoor wall and yourself.

The U value is made up of radiation of around U5.1 and conduction/convection of around U1-3.

A single glazed 3mm window is about R0.18 and 80% of that is the air film.

Estimating Wall R Value

Due to the conversation of energy we know that the energy moving through the air film from inside to the wall is equal to the energy moving from the wall to the outside.

If the air film is U7.14 and the temperature difference between the wall and the inside ambient air temperature is 2C then energy transferred is 14.28W/m2.

Now let's say the temperature drop from the wall to the outside is 15C.

Uunknown * Tout = Uair_film * Tin

Uunknown * 15 = 7.14 * 2
Uunknown = 7.14 * 2 / 15 = U0.952

So the wall's providing a U value of 0.952 W/k*m2.

Runkown = 1 / Uunknown = R1.05

Calculator

R Value Calculator

Fine Tuning Air Film R Value

Compared to the source I have adjusted the radiation down based on emmissitivity being 95% not 100% reducing it from 5.7 to 5.16.

Accuracy

To test the accuracy my idea is to measure a known material. For example I could put R1.0 up against the glass window R0.0375 and measure it. Or ... other ideas?

Examples

To test the it out I have taken a few thermal photo's using my FLIR One at night. It was around 3C outside (according to BOM) and windy, and 19C inside according to my cheap kids temperature night light.

I checked the heat gradient from floor to ceiling and it appears to be minimal at around 0.2C to 0.7C. This is a good sign I'm not losing too much out the roof.

Expectation

I assume my walls are 90mm thick R0.1 Gyprock + R2.0 Wool Bats + R0.14 10mm Air Gap + R0.1 Matrix Cladding = R2.34 ish.

If the insulation is missing then a 90mm air gap with reflective on one side is R0.4, giving a total of R0.74.

Image 1 - Balcony Door

I have tried using masking tape on 3mm glass to get an idea of what the outside temperature is.

Box Bx2 - Ambient

I have used my telescope which is about 30cm from the wall. It has a lower temperature of 14.1 than the 19C I expected. I'm not sure how much of that is the FLIR One +-2C accuracy or perhaps it's not the best reference.

Box Bx2 - Glass

I put masking tape on the 3mm glass. I estimate it has an R value of R0.0375. Using my R Value estimator I get R0.027 which seems close to the expected. 

To get spot on I need to adjust the outside temperature down to 2.2C. 

I am assuming the outside air film is 0, however I have found suggestions it can be 0.02. If so I need to drop the outside temp to 1.0C.

For now let's assume outside is 2.2C.

Wall

Box Bx4 - 13.5C - R2.8

Box Bx5 - 13.1C - R1.6

Box Bx3 - 12.0C - R0.637

The interesting aspect is the relative difference in temperature on the same section of wall. It appears as though it is missing insulation in the 12C box.

Image 2 - Bedside

This image is generally 2C warmer than the previous. My guess is that is more the FLIR One than reality so I'm going to adjust outside by to be 4.2C.

Ambient

I am using the bedside table, which is a quite a bit lower than the wall boxes. But as my ambient temperature gradient is only about 0.7C the ambient at the higher wall point should be at most 17C.

Wall

Again it appears I am missing insulation.

Sensitivity

One thing I have noticed is the higher the R Value the more sensitive the calculation is to temperature differences. Taking the Box Bx2 15.4C as an example:

1. Initial R1.6

2. Change Outside 4.2C => 2.2C: R1.9

3. Change Ambient 16.4C => 16.7C: R1.2

Image 3 - Beside

This time I'll use an internal wall as the ambient reference. This has the advantage of being the same material and hence reflectivity of the actual wall, and it is at the correct height.

Compared the the earlier image using the same outside temperature of 4.2C we get similar results.

Image 4 - Window Coverings + Double Glazed Window

I am curious if my current roman blinds + roller blinds are very insulative.

The numbers are a little diverse to know what to trust. My expectation is for around R0.7. Perhaps the heater was on too recently.

Image 5 - Window Coverings + Single Glazed Window

Assuming the single glaze is R0.18 then the window coverings are adding R0.2-R0.3.