Wow, studs really knock down the net R-value!

17 years ago

I was doing some calculations about the net UA for my new home design, and so I needed to calculate the net R value for a wall or roof. Presuming a 1.5" stud every 16", that would mean a proportion of about .1 for the area of the stud. A 2x4 stud (i.e., 3.5" deep) has an R-value of 4.4.

So, if I had a nominal R-value of the wall (i.e., the non-stud part) of 19, then the net R-value would be:

net R = 1 / ( .9 / 19 + .1 / 4.4 ) = 14.3

Intuitively, it would seem that the net R-value should be about .1 of the way from 19 to 4.4, or about 18!

I wonder if when the energy efficiency organizations say that homeowners should have a certain R-value, that they mean that for the nominal application, or for the net as described. I'm barely going to be able to get a nominal R-value of 19 in a 2x4 wall!

Featured Answer

energy_rater_la

17 years ago

swamp whiz,
there are a lot of ways to build better walls.
there is an insulation company that does a lot of work
with my clients. his company installs a glue based rockwool
for a R-15 in a 2x4 wall. this combined with a 1" polysi
foam board to the exterior of the wall is how we get
R-22.
if foil faced foam is used it will act as a radaint barrier (not as well as roof install which gets sun all day, walls only get direct sunlight at various times of day)..radiant barriers need a minimum of a 3/4" air space
so this wall application works when brick exterior is used.
it can also work if furring strips are attached to walls and hardi of vinyl attached to strips.

the interior of wall is drywall with air tight drywall approach.

keeping air from passing though the insulation is the key.
we start from the exterior..foam boards with all seams taped all holes sealed before exterior cladding is installed. then insulation (you can use R-15 batts)
the final air sealing is the ADA to interior.

most folks mis spend their insulation dollars by
using spray foam in walls. the above is a cost effective
proven equal. then dollars saved can be used where foam
performs most effectively. roofline applicaions (unvented attic no soffit or ridge vents).
this is an unvented attic.
the benefits are these
insulation on roof line keeps ducts in a semi conditioned space, so duct leakage is not an issue. equipement is also
in attic seminconditioned which extends the life of the unit.

sounds like you are doing a lot of research.
there are people who can help you to make the correct decisions for your specific climate.
visit www.natresnet.org
unbiased third party independent information for hire.

2x's are the weak part of the walls wood has an insulating value of R-1 per inch. a 2x4 stud in the wall is R-3.5
which reduces the overall value of your insulated walls.
using foam sheating to exterior reduces thermal bridging.

the only way to eliminate thermal bridging is a wall without studs. this is called SIPs Stuructured Insulated Panels.

when you build a home with these upgrades it is very important to size the hvac system for the upgraded house.
the person who sizes the house has to accurately input these upgrades. the end result for all this is a smaller hvac system ..bigger is not better.

efficiency is all about trade offs. better insulation
better windows (solar heat coefficients of .35 and u-values of .35 or lower)..smaller hvac system..higher efficiency hvac system. low utility costs, reasonable payback, and the most important to the homeowner...
comfort.

best of luck.

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Comments (13)

jakethewonderdog
17 years ago
Without checking your calculations carefully, thermal bridging from the studs is a known problem. You are correct, it's tough to get the R19 in a 2x4 wall. Are you including an exterior insulating sheeting? Since that installs over the studs, it breaks the thermal bridging. I think a 1/2" of closed cell foam has an R 3.5 without looking it up.

Other alt are going with 2x6 walls or using closed cell foam (or foam and bat). It doesn't solve the thermal bridging problem, but foam has about twice the R of fiberglass, provides a good air seal and (believe it or not) provides wracking and penetration resistance.
swampwiz
Original Author
17 years ago
Yes, I'm going to go closed cell foam, with the exterior insulating sheeting.
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srercrcr
17 years ago
There's some kind of sheathing, perhaps urethane, that gets R4. I actually double layered it on my western exposure, giving me R8 plus R13 for the pink stuff.It doesn't compress the fibreglass much. If you touch the inside of the wall at 7pm, it's not even warm.
swampwiz
Original Author
17 years ago
Actually, I may go open cell on the walls, and use a 1/2" polyboard on the outside, that is R-3.5. That seems to be the best for the outside.
jakethewonderdog
17 years ago
I'm not sure how you are designing the walls (vapor barrier, etc.) that said, open cell has about half the R value of closed cell while being more permeable.

Are you sure about the R value of polyboard? I'm not familiar with it in practice, but what I was finding on the web didn't have that R value. That's the same as closed cell foil backed boards with low-R blowing agents (Tuff-R) which is about as high as an R value for 1/2" material as I can find for insulating sheeting.
creek_side
17 years ago
I believe he is talking about polyisocyanurate board. It apparently runs from R6 to R8 per inch. My builder is planning to use doubled 1/2 inch polyisocyanurate insulating board between the sheathing and the siding. I forget which brand he is planning to use, but he said we should get R7 out of it.

The house plans spec its use.

Here is a link that might be useful: One Brand Of Polyisocyanurate Insulating Board
PRO
N/A
6 years ago
Your computation is wrong. 90% R19 and 10% R4.4 is R17.54 (.9*19 + .1*4.4)
Matt Peterson
4 years ago
last modified: 4 years ago
His computations are correct. You use the harmonic mean, rather than the arithmetic mean, when calculating parallel resistance. Alternatively, convert R-values to u-values, calculate the average u-value, and then convert that average u-value back to an R-value.

For example, if you had a wall where 20% was an R-value of 2, and the remaining 80% had an R-value of 20, then the R-value for the wall would be 7.14.
ADV Planners, LLC
4 years ago
Styrofoam Green exterior use green board if building a new home .. R 10 for 2" thick $46 32 ft2. roughly 3x cost of R13 could cut 6" x 8' for base and then 2" strips for the studs if install time no issue. or just run in in full sheets also comes R5 at half cost ; 1" that'll make your stud R9.4 and wall R23. (net 1/ (.1/9.4 + .9/24) = R20.7 .
ADV Planners, LLC
4 years ago
Then again what are the size and R value of your windows skylights etc.
My House
3 years ago
Props to Matt Peterson for coming up with the most ridiculous reason the authors obvious calculation error is correct.
Matt Peterson
3 years ago
Please educate yourself: https://youtu.be/NAE6TOWz8nw?t=443

The first author was correct: you transform the R values to U values (1/R), then take the weighted arithmetic mean. This is equivalent to the weighted harmonic mean of the R values.

Theory: https://youtu.be/NAE6TOWz8nw?t=258