U values and R values ?

[Tom Bainbridge]

the title has most of the question

are they both a way of measuring heat loss / insulation values

or are they meausring different "things"

[DickRussell]

They are both related to heat transfer, and each is the reciprocal of the other. For calculating heat transfer through something under steady-state conditions, U is the heat transfer coefficient; in British units it is a measure of BTU per square foot per hour per degree Fahrenheit. For the rest of the world it is in watt per square meter per degree Centigrade (or Kelvin). The bigger the number, the more readily heat will pass through that something (wall, window, insulation, etc). In the US anyway, stickers on windows give the whole window U value; double-pane units might get 0.32 or thereabouts, and a good triple-pane one can be below 0.2.

R (think "Resistance to heat flow) is 1 divided by U. It is used to characterize insulation, or actually any material through which heat flows. The nice thing about R is that when you have layers of materials (sheetrock, insulation, sheathing, siding, air film coefficients), you can add the R values of the materials to come up with the total R of the assembly. You can't do that with U.

If you are doing a heat load calculation for a house, without software doing calculations for you (and sometimes making incorrect assumptions about things), you are basically adding up the heat transfer rates through the various parts of the shell as U*A, to get total BTU/hour/degree F.To get the U for walls of a particular design, you would add up the R values of the layers and take the reciprocal of that.Then you'd multiply by whatever temperature difference (in to out) represents your design condition. And of course you'd then add in any heat losses that are constant, not really depending on outside air temperature, such as loss through basement slab. Finally, you would add in the heat loss for a particular rate of air leakage, usually expressed as air changes per hour (ACH), using air density 0.075 lb/cu.ft, heat capacity 0.24 BTU/lb/degree F (those are close enough numbers for most purposes). For a superinsulated house, losses due to air leakage can be huge relative to conductive losses through the shell.

[Tom Bainbridge]

dick that's the answer I needed thank you

[Dick Seibert]

This is the subject of debate in the industry now. In my opinion the window industry went to U Factor to confuse the public so they wouldn't know how inadequate most windows were compared to walls, why go to the extent of installing a R-19 wall then put a R-2.86 window in it? By calling that window a U 0.35 nobody knows how to calculate the actual R-Value, and those who do know don't want the public to know, like window manufacturers and salesmen.

[Martin Holladay]

Dick,
While you may be right that some window manufacturers would rather use U-factors than R-values to avoid comparisons with insulation, don't carry your conspiracy theories too far.

Many of us who have completed the sixth grade are able to calculate 1/U. I'm one of them, and I resent the implication that "those who do know how to calculate the actual R-value don't want the public to know." I tell anyone who asks me.

Since the NFRC labels have been providing U-factors for years, many builders and homeowners have become comfortable with U-factors. The lower the U factor, the better. Frankly, I get a little miffed when a window manufacturer trumpets R-value -- I say, "Come on -- what's the U-factor?"

[Dick Seibert]

Martin:

Instead of "...and those who do know" I should have said "...and many of those who do know", I didn't mean to make a blanket statement indicting everyone " who knows"..

[AndyGump]

Thank you Dick for that very enlightening and cogent answer to the OP.

Andy.

[Ted S.]

Many of us who have completed the sixth grade are able to calculate 1/U.

I actually still screw this up from time to time.

[Bluewoodrock]

I actually still screw this up from time to time.

I gots a button on my fancy calculator that does it for me...

Otherwise I'd probably screw it up too.

[Dick Seibert]

Ted:

Yeah, Martin and his group of 6th graders must be really smart if they can calculate a reciprocal in their heads, first how do they know the formula? Nobody tells them that it is the reciprocal, how many people even know how to calculate a reciprocal if they do know? I can do a simple one in my head, if I see a window with a U factor of 0.50 I can calculate it as an R-2 window, and if I see a window with a U factor of 0.25 I can calculate it as an R-4 window, beyond that I have to get a piece of paper and pencil, or calculator.

There are other factors like air-leakage,air-leakage is a bigger factor than conduction when determining energy efficiency, is air leakage included in U factor?¹ At one time they said it wasn't, now we get equivocal answers. When the NFRC was first formed there was a fight with AAMA, AAMA kept air-leakage, does anybody here even know where to find the AAMA label? If they do find the AAMA label where do they get the book to look up the number to get the air-leakage? Do the stores selling windows even have the books? I was told that building inspectors would carry AAMA books around to check the AAMA labels, I have yet to hear of any building department that has ever heard of AAMA much less bought their books for their inspectors. I see AAMA now has a "Certified Products Directory" online, good the same name the NFRC uses, so I go to plug in a common window, Andersen, it's not there because AAMA only handles aluminum, vinyl, fiberglass, and wood composite windows, I thought AAMA and The National Wood Window and Door Association's (NWWDA) stopped fighting and got married, apparently they got divorced and the NWWDA changed it's name to The WDMA, but nowhere on their site do I find a directory giving me any information like air-leakage.

BTW, get a window like this with 0.20 air-leakage and you can forget all about your sealing foams, sealants, plugging all the infinitesimal pinholes in your walls, and your blower door machines, air will rush through a 0.20 window, I get windows from Canada and Germany 100 times more air-tight.

Oh well, the NFRC announced a few years ago that ti was taking over air-leakage, the AAMA be damned, so I plug Andersen into the NFRC online Product Directory, darn, air-leakage isn't shown, but it's supposed to be there, they promised us they were taking it over. Most labels are now like this, others are including air-leakage like this, maybe some day they'll all stop their territorial fights.

How's that Andy for an answer to thoroughly confuse you, we apparently need Martin and some of his 6th graders to explain all of this to us. And Tom the Limey who asked the original question, is the American system clear to you now?


¹ From a physics definition it doesn't, I've got a 1997 NFRC directory that says it does, in the 1998 Directory it no-longer says that.

[John B]

Oh ,
Just Nevermind

[Dick Seibert]

Dick,
Do you think this principal applies only to windows?

John:

No it applies to walls too, if you've got cold or hot air blowing through forget about any insulation. I've argues with the scientists at the Daylighting Institute that windows shouldn't be tested and rated separately form the walls they go into, the NFRC was a step forward making window ratings apply to the entire window and not just the glass, it should be carried to the next level and the approval should be in an installed wall assembly. They agreed with me in principal but stated that it wasn't practical, that window manufacturers would test gold-plated assemblies, not assemblies put together in the real world.

To further complicate this U factors and R ratings fail to take into account beneficial solar heat gain in cold climates, the Daylighting Institute pioneered the superwindow concept by proving that in Reno on cold days R-5 windows outperformed a R-15 wall, the windows I use are only rated at R-5.6 (U-0.18) but they estimate they perform at the level of an R-18 wall, that's why I never build walls higher than R-19, generally speaking it's a waste of money to build walls more energy efficient than your fenestrations in them, but that applies to air-leakage more-so than conduction.

[George Roberts]

In general, the use of U or R is historical not conspiratorial.

But I do agree with some of the posts that neither R nor U provide the true answer.

[Dick Seibert]

For those interested in knowing how to specify good windows, Cardinal has come out with a new product, their i80 hardcoat, it is designed for colder climates to trap heat in a building, as a hardoat it can be installed on the #6 surface of triple pane installations, attached is a performance chart from Cardinal that I just received from my Cardinal rep today.

Going to a hard coat is a real departure for Cardinal, they originated soft coats and I never thought I'd see them go to a hard coat, but this is sorely needed in cold climates. I will be using it in my moderate climate on my northern and easten elevations to help heat my homes with the free heat from the morning sun.

[John B]

I will be using it in my moderate climate on my northern and easten elevations to help heat my homes with the free heat from the morning sun.

Dick,
I don't understand how you expect to get much solar gain from the North during the heating season...during the cooling season there actually is a fair amount of solar gain from the North.