R-Value Approach Vs. U-Value

[Phil B]

I may need to insulate an exterior concrete wall on the interior side and have always been a little confused on the difference between R-Value approach versus the U-Value approach. For example, the R-Value requirement for an exterior Mass wall in Climate Zone 2 is R-5.7ci. The R-values in the wall below add up to approximately 7.3. This wouldn't comply because the insulation is not continuous. However, if you look at the wall based on the U-value method, the U-value would be 0.137 which would comply because it is below the maximum 0.151 allowed.

This seems to be contradictory. Am I looking at things wrong, or simply misunderstanding the code?

 

[RLGA]

Technically, it is U-factor.

The difference between the two methods is to what the two units apply. With the R-value method, only the R-values of the insulating materials are considered. With the U-factor method, the thermal conductance of all materials in the assembly, including surface air films, are included. The thermal performance of the insulation within the metal studs must be reduced per Table C402.1.4.2, because of the thermal bridging of the metal studs.

 

[Phil B]

Technically, it is U-factor.

The difference between the two methods is to what the two units apply. With the R-value method, only the R-values of the insulating materials are considered. With the U-factor method, the thermal conductance of all materials in the assembly, including surface air films, are included. The thermal performance of the insulation within the metal studs must be reduced per Table C402.1.4.2, because of the thermal bridging of the metal studs.

Yes, I understand all that. What I don't understand is that how can the code allow a detail that is clearly non-compliant under one method be compliant under another?

 

[wwhitney]

Why is the rigid insulation labeled as R-4.5 but U-0.33? Those don't correspond, R-3.0 would be U-0.33. And is the 0.6 factor in 7.5*0.6 = R-4.5 the steel stud correction factor?

What I don't understand is that how can the code allow a detail that is clearly non-compliant under one method be compliant under another?

If the two methods always gave the same answer, why have two different methods in the code? The code would just include the simpler one.

So we have two method precisely because the simpler one (R-value of the insulation only) isn't that accurate, so you also get the option to use a more complicated method (U-value of the entire assembly) when that is to your advantage.

Cheers, Wayne

 

[RLGA]

That’s why they offer two methods (actually, three methods, if you consider the component performance alternative method). If you cannot comply with one, you use the other.

For the R-value method, the assembly shown would not comply because it is not continuous insulation and the fact that the insulation only has an R-value of 4.5, which is less than what is required to comply with the R-value method.

In other words, if you cannot get enough insulation in the assembly to comply with the R-value method (or cannot provide the continuous insulation as required) then you might be able to provide an overall assembly in which its individual components contribute in some way towards reducing thermal conductivity.

 

[RLGA]

Why is the rigid insulation labeled as R-4.5 but U-0.33? Those don't correspond, R-3.0 would be U-0.33. And is the 0.6 factor in 7.5*0.6 = R-4.5 the steel stud correction factor?

The only reason I can think of is they were reducing the effectiveness of the insulation because of the metal furring. The IECC table does not have a correction factor for that depth of furring, so maybe it was calculated using ASHRAE formulas.

 

[Phil B]

That’s why they offer two methods (actually, three methods, if you consider the component performance alternative method). If you cannot comply with one, you use the other.

For the R-value method, the assembly shown would not comply because it is not continuous insulation and the fact that the insulation only has an R-value of 4.5, which is less than what is required to comply with the R-value method.

In other words, if you cannot get enough insulation in the assembly to comply with the R-value method (or cannot provide the continuous insulation as required) then you might be able to provide an overall assembly in which its individual components contribute in some way towards reducing thermal conductivity.

Thank you both for your answers. Very helpful!

On a related note, I've heard differing opinions, i.e., yes and no, to attaching metal furring through continuous insulation so that you get the 'ci' required by the R-value method. Is the approach below OK? Polyiso is rigid but it does compress a little.

 

[RLGA]

Fasteners provide a negligible loss in thermal performance. If the insulation is not too thick, the installation shown in the illustration should be okay. If you get too deep, the moment force on the fastener may be too much.

Is this a single story building? If not, how is the insulation continuous through the floor above?

 

[Phil B]

Thanks again! This is single story.

 

[jar546]

However, if you look at the wall based on the U-value method, the U-value would be 0.137 which would comply because it is below the maximum 0.151 allowed.

What code cycle is that for zone 2? Unless I am reading it wrong, Zone 2 in the 8th Edition of the FBC Existing 2023, the mass wall requirement is R4 or less than U 0.165. Providing more than half the insulation is on the inside.

 

[Phil B]

What code cycle is that for zone 2? Unless I am reading it wrong, Zone 2 in the 8th Edition of the FBC Existing 2023, the mass wall requirement is R4 or less than U 0.165. Providing more than half the insulation is on the inside.

Yes, 2023 FBC Energy Conservation 8th Edition, but for new commercial construction. Zone 2 would be U-0.151 max. for mass walls. I don't see the requirement for more than half of the insulation on the inside.

 

[jar546]

Yes, 2023 FBC Energy Conservation 8th Edition, but for new commercial construction. Zone 2 would be U-0.151 max. for mass walls. I don't see the requirement for more than half of the insulation on the inside.

You are absolutely correct. I scrolled right down to the Residential section of the Energy Code. My mistake. Sorry about the confusion.

 

[Phil B]

No apologies necessary. It keeps me reading thru the codes for stuff I always miss.

 

[steveray]

Don't we all wish all the interactions were this nice? Happy Monday!

 

[tmurray]

This interaction is an example of open minded search for the truth, not the attempts to force others to agree with you we often see today.

 

[Paul Sweet]

HVAC engineers use U values (or factor) to calculate heat gain & loss. Having both methods lets you use the U values they calculated for energy calculations.

Glazing is rated by U value instead of R value because the edges transmit more heat than the center of the light.

 

[Penguin87]

Thank you both for your answers. Very helpful!

On a related note, I've heard differing opinions, i.e., yes and no, to attaching metal furring through continuous insulation so that you get the 'ci' required by the R-value method. Is the approach below OK? Polyiso is rigid but it does compress a little.

View attachment 14986

Yes, that works except you will need to surface mount your electrical or plumbing as 7/8" hat channel depth isn't enough to fit in services.

In fact, as long as your insulation is continuous and uninterrupted, any configuration works. Make sure the insulation runs floor to ceiling, is taped and sealed.