r value chart

Brad White

That is similar to plots out of a hot box lab. Keep in mind that IR guns measure the surface temperature(duh)-and this does not include the interior "still air" film which has an R of about 0.68. I would have to make sure that was factored in. But it is a fair approximation.

My $0.02

Brad

Gordy_2

ME what do you make of this chart ?

http://www.professionalequipment.com/content/how-to-determine-r-value.asp

Think it is a reliable piece of material?

Gordy

Gordy_2

Thanks Brad

So do you think using probes on the outside, and inside skins would be better? Or are you saying watch what you shoot using the black tape rule with the IR gun?


Gordy

Dave Yates (GrandPAH)_8

Brad

A big what if (in my mind's eye):

What if the exterior wall has siding? That would put an air-gap between the siding and the exterior surface of the wall, which could be any number of different materials.

Brad White

The key to accurate measurement

is equilibrium or a steady-state condition and I should have mentioned this in the first response.

In other words, say you have a wall with 70 degrees inside and 50 degrees on the outside. Suddenly the outside temperature drops 25 degrees as a cold-front moves in. Now you have a 45-degree delta-T versus the 20 degrees you started with.

The wall temperature at that point would not register a perceptible cooling nor accurate determination. The wall's mass is still holding heat even though the motive force just increased. The BTU train has yet to pick up speed.


The volatility of indoor and outdoor temperatures versus the fixed mass of the wall is always going to skew any readings. You have to accept the limitations and an approximation.

Now to your question: No need for black tape- that is used only when measuring a metallic surface such as bare copper pipe or bright metal. That is a limitation on infra-red emissivity, sort of a flash-bulb blinding if you will. It does not mean that there is a lesser temperature, just that the technology you are using (an IR gun) has a limitation in that area.

I agree, a thermistor sensor on each surface would be better and that is what the laboratories use. Minco makes nice copper foil ones, very nice and accurate.

Oh, to expand upon my point about the air films: The indoor and outdoor air films add 0.85 R value "just for showing up", based on 0.68 for still indoor air and 0.17 for a 15 MPH exterior wind speed. By measuring the surface temperatures or at any point within the wall for that matter, these factors, however minute, are eliminated. I just wanted to ask or verify that the temperature curve chart takes this into account. I suspect that they do if they are truly based on measured surface temperature.

Brad White

That is always an open question, Dave

and a good one.

Personally, I discount any materials outside of my air barrier, meaning the sheathing behind the brick and air space.

If I have a brick-veneer wall as a rain screen and a vented space behind with weeps, I do not count the brick nor the (now moving) airspace toward my wall R-value. I will take an outside air film factor (R=0.17 in a 15 MPH wind) but that is it.

That may be conservative in that there would still be that same air film on the brick exterior and another behind the rain screen, even more sheltered and beneficial. A 4-inch layer of brick adds about R-0.32 at 0.08 per inch, so we are not talking about much R value.

The mass, especially on a sun-lit side, will be the best benefit over time and operation but I do not consider it because daylight is not a constant.

Anyway, that is how I calculate a wall's R-value in that instance.

[Deleted User]

R U kidding me???

Gordo et al. I got something off the Wall a long time ago,from Bob Warner if memory serves me correctly, that uses thermocouples to assimilate the same conditions.

Just to test it under "real time" conditions, on a "known R value" wall factor, I hooked up one of my data loggers to record data on a regular basis (4 times an hour) and see if there was some consistency to it. There wasn't...

I allowed the logger to watch all conditions of heat loss and heat gain. Granted, it never got REAL cold nor real HOT, but there was enough of a temperature differential to cause energy flow.

I will have to dig deeper to find the file, but I did find the basic information used to obtain R value. As you can tell from the graphic, this is rather old information.

Another interesting thing I saw was the time lag effect of the alledged R value. It would appear that it doesn't really change with delta T.

I attempted to write a spread sheet to convert the raw data to R value, and it made so little sence that I abandoned the idea and let it be what it was, a "picture in time" of R value.

Enjoy!

ME

[Deleted User]

Parameters...

I guess those graphics are useless as you know whats on a boar hog without some splainin'

The room was isolated (door closed) and unheated. It is an outside room (12X12X8) with two windows. The sensor on the wall was placed 4' above the finished floor, dead center between two studs. The wall is 3.5" thick wood frame. The stud cavity is filled with tightly packed cellulose (ground up treated newspaper) insulation. There is 1/2" of XPS insulation placed over the top of compressed fiber board siding, and finished with 1/8" vinyl siding. (Resided exterior)

So from the outside moving in, exterior air surface, vinyl siding, 1/2" XPS, 3/8" old siding, 1/2" celotex, 3-1/2" cellulose, 1/2" sheet rock and inside air surface.

THe rise in inside temperature near the end of the graph was due to my turning the panel radiator on in the room.

QUestions?

ME

Gordy

Formula

Darned if I can make that formula work Mark to desifer a realistic r value. I asked you for that very formula you posted at one time. It seems something is wrong with the formula.

With out taking a temperature measurement of the outside skin verses the air temperature, how can this formula work?

I know r value is forever changing in the real world, but a snap shot on a design day scenerio would be helpful.

Short of ripping into walls to see what is there in an existing wall you really don't know what you have.

Adding the r values of my wall configuration together does come close to the chart method I posted.

What do you see wrong with it Mark.

[Deleted User]

Gordy...

I hear you, hence my response of a snap shot in time.

Just to clarify one methodology over the other, I brought my non surface contact IR thermometer home tonight, and shot a detached inanimate object far enough away from the house to not have its surface temperature influenced, and then shot the exterior of the wall (in the dark). They were the same. So for all intents and purposes, the OSA temps I recorded are essentially the same as the outside wall surface temperature.

With that said, I ran your calculation at numerous spots on the graph, and came up with an R 10. My wall has 3.5" of tightly packed cellulose (I saw it during a recent bathroom remodeling project), and with an R value per inch of 3.5 per inch, I have a core R value (not including inside and outside air surfaces, sheet rock or 1/2" XPS foam) of R-10.5. That makes your calculation off by a factor of 33% at a minimum, and more if I account for the other listed items. A fairly heavy/significant error factor.

Both of these methods are purely estimates of wall r value, and would definately show a wall with little to no resistance to heat flow, but as it pertains to precise R values, it's still just an estimate. Nothing beats visual observation, and for less than $100.00 you too can purchase a Rigid See Snake with a 3 foot probe and actualy go into a wall cavity to see what is really there through a relatively small hole (3/4").

But still, as I have shown with the data logger, I don't think that R value is all its cracked up to be!

Buttcha gotta start somewhere... no?

It has been my experience that infiltration is the KILLER of all heat loss factors.

ME

Doug_7

R-Value Measurement

Attached is an article that points out that R-Value of fiber insulations is greatly influenced by air infiltration and moisture.

Theoretical R-Value of a wall structure, obtained by adding up the R-Value of the components, is a valid method - but it is a theoretical design method that is true only in the laboratory. It will only be true in the real world if there is no air infiltration. Any air movement can and will reduce apparent R-Value of a wall structure.

Generally rigid insulation - such as foam board with no gaps - avoids this air infiltration problem.

Measuring temperature drop across a wall structure is the best real world method for determining actual R-value of a wall structure. Results will vary with the wind, depending on how good the vapor barrier is.

Also attached is a method for calculating R-Value of a wall or window.

Comments welcome.

Doug

Brad White

Good information, Doug

That underscores what ME was saying (and to which and with whom I readily agree), that air movement thwarts the best R-value hopes.

That is why I like the foam-in-place, rigid foam -especially continuous over framing members- or SIP construction. It is not the R-value itself (which is excellent on a per-inch and total basis) but on the integrity of that R-value.

The second attachment Measure R-Value.pdf, mirrors my points about negating the interior air film contribution. That said and as Mark pointed out, the building mass at any given time does not translate into steady-state heat flow. Nothing is static and the relationships, when measured, sure seem tentative and general in nature.

jp_2

real world vs lab test

without the controlled lab testing of R valve, nothing would exist!!! same goes for afue in my thinking.

theoretical calculation are the only "real" place you can start doing any sort of heatloss, other than measuring the heatloss directly and thats going to lead to lots of error too.

need to remember, science is a 'darn good close guess', not exact.

Doug_7

More on Measuring R-Value

Some additional information on measuring R-Value: Does not work for ceilings. Some tips on temperature measurement. Need at least 2 hours of stable conditions to reach equilibrium for valid measurement.

I assume the reason method does not work on ceilings is because the 0.68 film coefficient is not valid ?

Measurement under higher delta T (cold weather) will be more accurate.

This method is for people who want to check if the design R-Value of a structure is actually being met under real world conditions, which depends a lot on the integity of the vapor barrier.

Personally, I like Thermovision infrared camera as a method for finding thermal gaps in a structure. I just can't afford one.