# How to determine the R-value of a wall without opening it?

| Posted in Energy Efficiency and Durability on

When I buy stuff the R-value is on the package so it’s fairly easy to calculate the total value of the wall.

Can the R-value be determined if I know absolutely nothing about the wall and the wall can’t be openend to check?

Outside wall: 32F
Inside wall: 50F
Room temp: 70F

If I would have measured those temps with a infrared thermometer, would they somehow be useful to calculate the R-value of the wall?

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### Replies

1. | | #1

Have a look at this fellow's calcs...spreadsheet is linked there as well:

https://woodgears.ca/physics/r-values.html

1. | | #2

Matthias Wandel is my hero!

2. | | #3

First: really low R value

Second, as I mentioned in response to a previous post, the problem is temperature variation and time. The inside temperature does not, can not, change instantly with a change in outside temperature. Since the outside temperature varies during the day, what you are really measuring a kind of average of the temperature differences throughout the day.

Those measurements sound like good windows, but a bad wall. I just picked up my thermometer and the worst glass in my house[R2ish] is around 60 degrees with a 40 degree outside temp. Triple pane was around 67. 69 degree inside temp. Infrared measures various interior surfaces at about 68
Having done this in the past, it must be pretty cold out to get below 50 degrees on my worst windows, so that wall sounds essentially uninsulated

3. | | #4

I have found the readings I have gotten from inferred thermometers to be very inaccurate + or- 5°
I had a bay that they did not blow insulation into when they did the rest of the house.

When it was below zero outside the wall felt like 50° all in that wall was about R4.

Walta

4. Expert Member
| | #5

Another thing to watch is air leaks. I've found with houses with balloon framing in particular, a wall could be insulated with fiberglass batts but because of the massive air leaks through the wall cavity the insulation does almost nothing. This is why lot of times insulating these older houses with batts won't be enough, dense packing is your friend.

The heat flow based R value estimate only works if the wall cavities are somewhat air sealed.

5. | | #6

All of you many thanks for the input!
The numbers I gave in my first post were just for explaining.
Now I know that the calculation is possible I'll take real measuments.
I'll try to pick a day with a stable temp and heat my room at stable temp for a few hours before measuring. I'll need a portable heater for that because the wall I want too measure has a radiator on it.

I guess a big temp difference between the inside and outside wil give the best results?

@walta110 I have a simple thermometer, so I don't expect it to be very accurate. But perhaps I can use a 'calibration' trick. I measure a boiling water as a reference, just before measuring the wall. That said it's not really about the absolute temps, but the difference. When I measure the same surface over and over again the readings are amost the same.

1. | | #13

The biggest limitation with IR thermometers is measuring surfaces with differing emissivities. As long as you're measuring the same surface, it should be pretty repeatable. I wouldn't recommend trying to verify it with the boiling water trick. It's not a good reference to begin with, since the boiling point will vary depending on where you are. But more importantly, while water does have a similar emissivity to what's expected by a typical IR thermometer, the boiling of it is going to make it difficult to actually get a surface measurement. You'll be measuring the average of a bunch of steam and a roiling surface. At best a simple waste of time.

6. | | #7

Whatever the R-value of the wall are, I plan to do at least one upgrade.
I've a radiator fixed on that wall and obviuously a lot of it's heat goes to the wall.

There's about 4"/10cm space between the wall radiator I want to fill with rockwool with aluminium foil facing the radiator.
Not the whole wall, just hidden behind the radiator. Would it best to fill up the whole space between the wall and radiator? Meaning the rockwool touches the radiator. Or is small airgap better?

7. | | #8

I have found the fairly cheap infrared thermometer I have is very repeatable, if not entirely accurate. It gives repeatable measurements on the same spot and different spots in a room.
My thermostat on the wall reads 69 degrees. If measure the floor the wall various non outside facing surfaces, they read 68 degrees or so. This means, to me, that it is a useful measurement tool, as long as I am using it to measure against itself.

8. | | #9

So, it is 35 degrees out this morning.
outside wall[2x4 sprayfoamed] reads 67 degrees
outside wall[2x4 r11] reads 66 degrees
What this tells me is that while the difference in surface temperature of poor insulators is measurable and significant, as the insulation values go up, the difference in surface temp drops and is quickly going to get into the noise.

1. | | #10

To use that heat flow method I linked in post #1, you also need to measure an accurate room temp so the more accurate you have room temp, wall surface temp and outside temp, the more accurate your results. I tried it using some old FLIR data on two identical side by side windows with and without acrylic inner panels and got R3 for the "bare" sealed double pane glass window and R7 for the window with panel added. It looks to be decently accurate. That said, measuring a wall temp, particularly with thermal bridging, brings with it a lot of variables so you need to be consistent with height and do some averaging...using a very accurate measurement device. I have a FLUKE probe that is my goto for accuracy, but you'd need to tape the probe head to a wall, wait a bit...then take a few measurements within a few feet to average them.

Having a radiator near the wall will definitely skew results.

9. | | #11

The amount of air movement at the surface will be a big factor.

Walta

1. | | #12

@walta, yes, and on the outside as well, which likely has a much larger impact with respect to heat flow if you're taking measurements on a windy day. I noticed this doing calculations on FLIR data taken with -22 C ambient, but a noted -34 C wind chill. This was a north window, and winds are typically NW in winter here with cold temps, so the data didn't line up until I plugged in -30 C (instead of -22 C ambient) for an outside temp.

The assumption I used as a reality check was that a sealed non-operating dual pane window, lowE, argon filled, should be at least R3.

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