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R-values

rktect 1

rktect 1

Gold Member
Joined
Oct 20, 2009
Messages
1,112
Location
Illinois
So, I just got off the phone with the person who submitted a REScheck. I asked him where he came up with the continuous R-value of 3.4 for the walls and ceiling. His answer was that for the walls he added up the R-values of the 1/2" gypsum board, the 1/2" osb sheathing, the 1" air space and the brick. The ceiling was 1/2" drywall, air space, 1/2" plywood sheathing and shingles.

I told him to provide documentation for that. Honestly, I don't think he is even allowed to use these materials for the R-value in REScheck.

Is it possible?
 
Re: R-values

I think I am being snowed here.

From the dept of Energy website regarding REScheck.

"Continuous Insulation R-Value - Enter the R-value of any continuous ceiling insulation. Continuous insulation is insulation that runs continuously over structural members and is free of significant thermal bridging; such as rigid foam insulation above the ceiling deck. For ventilated ceilings, insulating sheathing must be placed between the conditioned space and the ventilated portion of the ceiling (typically applied to the trusses or rafters immediately behind the drywall or other ceiling finish material). For structural insulated panels, enter the manufacturer-reported R-value for the entire assembly."
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Which is exactly what I thought originally. I've never heard of adding the other materials as I believe they are factored into the program when you pick wall types.
 
Re: R-values

mtlogcabin, you add R values, then divide 1 by the sum to get the U value.

I'm in project (mis)management now, so I'm not familiar with the details of REScheck, but the OP is how I used to calculate heat loads manually, except that he missed the air film R values.
 
Re: R-values

mtlogcabin said:
You do not add the "R" value of a products you add the "U" values.
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The original post was computing a combined "R" value of stacked layers of continuous material. The heat has to travel through the full thickness of every layer. In other words, the resistances are in series. Resistances in series are combined by adding the "R" values. The original post had an example of this sort of calculation.

Conductances in parallel are combined by adding the "U" values (weighted by area). For example, to combine the conductances of 2x6 studs with insulation between the studs, assuming neither air leakage nor gaps nor thin spaces in the insulation, you would add the "U" values (weighted by area). This allows for the fact that the heat will disproportionately travel through the less insulating material. So if the 2x6 studs have an "R" value of 7, and the insulation has an "R" value of 19, and the studs are 1/16 of the wall, the combined "R" value = 1/(combined U value) = 24"/(1.5"/7 + 22.5"/19) = 24"/(0.214" + 1.814") = 24"/(1.398") = 17
 
Re: R-values

I left out the units of "R" values and "U" values in my example calculation.

American "U" values are in British Thermal Units / (square foot * Fahrenheit degree * hour),

so American "R" values are in (square feet * Fahrenheit degrees * hours) / British Thermal Unit.

One British Thermal Unit is about 1055 Joules, or about 10 microTherms.
 
Re: R-values

REScheck factors in the structure when a wall or ceiling design is chosen. You do not add the structural elements to the calculations.
 
Re: R-values

ResCheck is a joke. It is pathetic that many "professionals" rely on that garbage to determine whether or not a structure meets minimum energy requirements.
 
Re: R-values

incognito said:
ResCheck is a joke. It is pathetic that many "professionals" rely on that garbage to determine whether or not a structure meets minimum energy requirements.
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I don't mind the rant, but without posting a solution all I have is negative feedback I can't do much with. Do you have an alternative besides the prescriptive path?
 
Re: R-values

Pyrguy is right on. Rescheck automatically considers the type of wall construction into it's calculations. We have been using Rescheck as a compliance tool for a long time and have found it to work very well. Is it perfect? No, but it's pretty good.
 
Re: R-values

rk - Once upon a time, SOME DPs used to add up all the elements and air barriers inside and out to come up with a 'total' R-value. More recently, via ResCheck, ComCheck and other similar software programs, that practice is out of vogue. Apparently it was not terribly reliable.

Contrary to some opinions ResCheck (or ComCheck for commercial buildings), WHEN PROPER DATA is input, is a much more reliable way to determine minimum prescriptive performance. Lacking that, the code provides other methods that are deemed acceptable, but...

From the 2006 IECCC:

402.1.2 R-value computation.

Insulation material used in layers, such as framing cavity insulation and insulating sheathing, shall be summed to compute the component R-value. The manufacturer’s settled R-value shall be used for blown insulation. Computed R-values shall not include an R-value for other building materials or air films.

Sounds like your applicant missed that part of the Energy Code... ;)
 
Re: R-values

I would agree that ResCheck is defective at best. A very shortsighted method of determing energy compliance. Our locally amended energy code is one page long and results in a superior product at completion. Yes it is prescriptive but it is VERY easy for everybody to understand and achieve and verify. As with everything else related to the code we set forth minimums and maximums as appropriate. The prescriptive method is truly a KISS approach while ResCheck is typical goverment bureaucratic smoke and mirrors(it was done on a computer so it must be better).
 
Re: R-values

JD,

I do remember that from way way back. I never did it that way myself but remember seeing some calcs for it from someone I worked with. He told me it took a long time to figure it all out. Supposedly pretty complicated. Again I don't have personal knowledge so......grain of salt time. I am not a fan of REScheck but understand why someone would want to try the tradeoff method. Of course these people don't realize that the trade off is to pay higher prices for windows to get low-e AND Argon filled as well as a high efficiency furnace instead of just using 2x6 stud walls with the R-19 batt insulation.

How much more money could a 2x6 stud cost over a 2x4? How much extra for R-19 insul instead of R-13? I know the difference between an IG window and Argon filled is a couple hundred bucks times 10-20 units per house. It adds up real quick.
 
Re: R-values

I don't disagree that higher R-value insulation and better windows are a great benefit, but the most often overlooked energy waster (or opportunity for energy savings) is air leakage/air infiltration. I know that I will get some flack for this, but I like the new blow test or air leakage assessment requirements in the new (2009) code. Seal 'em up!
 
Re: R-values

Vegas Paul, I agree that we need to seal the buildings better. I am not convinced that the blower door test is needed. Do you have any information as to how many houses fail the blower door test that have passed a visual inspection of the air sealing?
 
Re: R-values

REScheck is not prescriptive, it is an alternate method (engineering, mathematics). Prescriptive is follow the tables in the book. Neither REScheck or the prescriptive R-value tables address tightness. If you are enforcing tightness, are you also enforcing the ventilation that goes with it?

If you think there is something wrong with REScheck take it up with the DOE, or make sure your clients know you won't accept them.

I like having the choice and flexibility. If the plans don't follow prescriptive then I ask for a REScheck.
 
Re: R-values

TJ - you are correct regarding ResCheck's ignoring tightness, That's not it's purpose. However, I was referring to the soon to be adopted (in some jurisdictions!) 2009 IECC that addresses air infiltration/tightness much more restrictively. If the 2009 IECC is adopted and enforced, in addition to the insulation compliance calculation (prescriptive, ResCheck, etc.), a tightness evaluation will be required. This may be done by inspection or by blower test, or other approved method. Many jurisdictions will be using 3rd party (Energy Raters, for instance) for this. I have no idea yet how we will be doing this. I don't have any data to answer Wendell's question regarding blower tests vs. inspection results. It's a great question, and after adoption, I would like to see lots of data collected to evaluate this.
 
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