Nominal & Effective R-values

Kevin

Anyone know of calculators for nominal and effective R-values?

I'm looking to compare two wall systems.

Conventional 2x6 construction (siding/weather resistant barrier/sheathing/2x6 stud(R20)/vapour barrier/drywall)

VS

siding/weather resistant barrier/sheathing/2" XPS foam/2x4 stud(R12)/vapour barrier/drywall

Kevin

orson

Here: http://www.ornl.gov/sci/roofs+walls/...rs/NS/Calc.htm

Here is another: http://www.design-navigator.co.nz/CRC.html

You might want to consider going to a Structural Insulated Sheathing system instead of the 2" foam.

The key is to get the wall airtight.

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Kevin

I had found those links before posting. Neither one of those links gives me effective R-values.

SIS doesn't get me the R-value that I'm looking for.

Part of my goal is to stay with a standard 6-1/2" wall as to not spend extra money on custom jambs for windows/doors. The wall system in my original post accomplishes this.

concretemasonry

Kevin -

You should make some basic accepted changes to your thinking of the two wall systems.

The insulation in between the studs is probably as you stated (R20 and R12), but the the effect of the thermal transmission of the studs reduces the effective value by 10% or more when you consider the complete wall. This depends on the spacing or the studs. If you had steel studs, the reduction in effective insulation value of the wall is even greater. - This is a feature ignored by the pink panther since they only talk about the insulating value of the mater they sell without regard to how it is used and what it is used with.

As a "seat of the pants" estimate, I am quite sure the the 2x4 wall with the 2" foam would perform much better, because the rigid foam covers the studs and is a real insulation value and not decreased the the classic "thermal short circuit" caused by the studs. You could probably wade through the ASHRAE manual and get the numbers to use in a realistic calculation.

An additional plus of the rigid foam is the reduction of the air infiltration.

If you try to calculate it, you should use the "series parallel" method instead of the simplistic parallel heat flow method.

If you are dealing with an old-fashioned prescriptive code designed for typical inspectors, you will not have to recognize the real situation or take advantages of the real benefits.

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Kevin

I'm not quite sure why you suggest I need to change the way I think. I hadn't stated my thoughts. I simply asked for a calculator to help me compare effective R-values.

I'm well aware of the differences between the two wall systems, but would like to see a comparison that shows effective R-values.

I know the standard 2x6 wall comes in close to R13.7, but what about the other system? I'm also well aware of the thermal transfer through the studs, and I'm trying to minimize this - hence the 2" rigid foam.





I'll simplify my original post - does anyone have a calculator (or a formula for that matter) to help one calculate effective R-values in a given wall system?

neolitic

I am sure that you are most welcome
for the tome others took to attempt
to answer your question, even though
their answers fell short of your high expectations.
Since you brushed off Concretemasonry's
citation, I doubt the "Building Science" site
would much please you either.

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ContractorPete

I am not sure of a calculator right off the top of my head, but there are a ton of free energy software on the US department of Energy website.

DavidC

I know of no such calculator. It may well exist, but I would use the freely given info already at hand like so;
2x6 construction - add the R values of the drywall, chosen stud bay insulation and sheathing. Decrease by 10% for your adjusted R value.

2x4 construction - same as above except add the the R value of the foam board to the adjusted assembly R value.

I'm not a rocket surgeon, but feel this would be an adequate comparison.

Now if I needed a more technical approach I would grab a cup of coffee and some visine. The building science website has a ton of trusted info on various assemblies, may even have the calculator you seek.

Is this calculation for your own consideration, for a customer, inspector?

Good Luck
Dave

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ContractorPete

I finally have 15 posts so i can post a link:
http://apps1.eere.energy.gov/buildin...velope_systems

Energy envelope calculators, most free.

DavidC

What restraint you display Pete. You've waited since Nov. when 1 intro and 14 oops, wrong threads and your in before your coffees cold.





Or so I've heard.

Good Luck
Dave

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ContractorPete

Lol...

Ya I haven't really been that active on this forum...I post mostly on greenbuildingtalk.com. I like the diversification of the trades on this forum though. Its nice.

sawyerEd

Kevin, I think the ORNL tool that Orson linked for us above will work for you even though it stops at 1" EPS or R-4. 2 in of XPS is rated at either R-10 or R-9.6. You noticed that in the results table the foam layer's effective R value is slightly less than the R rating for the material, presumably to allow for air infiltration at joints and openings. I think that if your wall has a continuous layer of foam from foundation to soffit with shiplap joints caulked and openings caulked, you could add either 6 or 5.6 to the stated values for 1" foam on the ORNL calculator. I think you need to pay attention to vapor transmission issues in a wall that contains absorbant fiber insulation depending on the humidity where you live. Getting moisture out of a wall takes energy. I'm sure you know this already. sawyerEd