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Another OC vs. CC Question

josephny | Posted in General Questions on

I’ve been thinking about the walls in my current project (zone 6).  They are a combination of new 2×6 (1.5×5.5) and 100 year old 2×4 (actual size).  My plan was to spray 3.5″ of CC and fur out the 2×4 to 5.5″.

Then I started learning a little more (so that now I understand a teeny tiny amount and before I know nothing).

I just did some calculations using my newfound knowledge and got some very disturbing results.

Using only 3.5” of CC, the total R value of the wall is 10.98.

Using 5.5″of OC, the total R is 13.38.

Using a combination of 3.5CC and 2OC, R is 15.94

Using 5.5″ of CC, R of 16.99

This is using the numbers on the table below and the 1/u + 1/u = 1/u method of getting a total R. 

Am I doing this correctly?

If so, 3.5″ of CC is the worst of all 4 options — and not inexpensive.

 

 

             
   

OPEN CELL

CLOSED CELL

3.5″ CC + 2″ OC

5.5″ CC

 
 

R val/in

3.7

6.5

5.482

6.5

 
 

Thickness

5.5

3.5

5.5

5.5

 
 

Airspace

0

2

0

0

 
 

R foamed

20.35

22.75

30.15

35.75

 
 

R air

0

1

0

0

 
 

Total Foam R

20.35

23.75

30.15

35.75

 
 

Total Foam U

0.049140049

0.042105263

0.033167496

0.027972028

 
 

R/in framing

1.2

1.2

1.2

1.2

 
 

Effective framing in

5.5

3.5

5.5

5.5

 
 

R framing

6.6

4.2

6.6

6.6

 
 

U framing

0.151515152

0.238095238

0.151515152

0.151515152

 
 

Whole wall U

0.074733825

0.091102757

0.06275441

0.058857809

 
 

Whole wall R

13.38

10.98

15.94

16.99

 
             

 

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Replies

  1. insaneirish | | #1

    I didn't check your math, but your observation is why this article exists: https://www.greenbuildingadvisor.com/article/installing-closed-cell-spray-foam-between-studs-is-a-waste

  2. charlie_sullivan | | #2

    You are right that "closed-cell-spray-foam-between-studs-is-a-waste" as the article that Patrick linked says.

    In fact, even open-cell is an expensive choice for little benefit over your choice of fluffy insulation. You might instead spend the money on a configuration that reduces the thermal bridging.

    Was your plan to fur it out and add insulation from the interior or exterior?

  3. user-420506 | | #3

    Hi, Joseph. You don't indicate the spacing between your studs for the different sections of wall. Is it a typical 16 inches on center? When doing this calculation, you should indeed by looking at the overall effect of the studs acting as low-insulation bridges, and they will indeed drag the overall average insulation value down. To calculate the total u-value for the wall assembly, you should be weighting the u-value contribution from the insulation versus the studs based on the percentage of the wall that is made up of studs versus insulated bays. For example, assuming 16 inches on center with 1.5 inch wide studs of your indicated depth, then 1.5/16=9.375% of the wall would be studs, leaving 90.625% as insulated bays. So, your weighted average u-value would be:

    .09375(0.238)+.90625(0.042)=.060375, or an r-value of about 16.6 for the whole wall, ignoring the complications of your older sections with true 2" widths, not to mention doors and windows. (All of which would reduce the total r-value.)

    So, obviously the studs have a large impact as thermal bridges on the total insulative value of your wall assembly. That is why continuous external insulation is so powerful, as it eliminates those bridges entirely. With that said, not every project has that as an option. (Do to a variety of considerations, my own current project is using a combination of external insulation and CC.

    So I'd vote do what you can within your project constraints. It is also worth pointing out that CC can have some great air-sealing properties when done well, and on older houses being retrofit, that can have an outsized impact on efficiency and comfort.

    Good luck!

  4. josephny | | #4

    Thank you guys for confirming my understanding.

    Studs are 16" OC, but some are 2" wide and some are 1.5".

    I need to figure out the best solution going forward. For example, leaving the plan for 3.5" of CC on the walls, would filling the gap with batting be good or would it be better to put a thin layer on the outside (preferable a single 1" or 2" sheet), and of what (ISO/XPS/EPS, faced or unfaced).

    Thank you!

  5. Expert Member
    AKOS TOTH | | #5

    Your math looks right and your numbers are in the ballpark. Interior and exterior air flims, drywall plus siding usually add another R3 to the assembly for your overall insulation value.

    Don't get lost in chasing every R value though, even in zone 6, an R10 wall gets you most of your energy savings:

    https://energyvanguard.com/blog/the-diminishing-returns-of-adding-more-insulation/

    If it cost you a lot for extra little bit of insulation, there is generally no ROI in energy savings.

    With an old house your most important item is to air seal. To do that your options are:
    -peel and stick on the outside
    -dense pack (not a true air barrier, but good enough)
    -spray foam

    Once you are air sealed, the rest is getting the cheapest insulation into your walls.

    For example, if your contractor is steering your towards cc SPF, go with only 2" of it and fill the rest with batts. I would look at another SPF installer though, the full oc SPF option should be similar to 2.5" cc SPF.

  6. josephny | | #6

    Air sealing is a huge challenge for this house -- lots and lots of air gaps (like, see the sky size gaps). That's one thing I'm very much looking forward to fixing with the spray foam.

    I'm really starting to think I'll just fill the remaining 2" of wall with fiberglass batts. They'll be between 3.5" of CC on one side and sheetrock on the other. Should they be faced or unfaced?

    Thanks!

    1. Expert Member
      AKOS TOTH | | #7

      In zone 6, you need 35% of your overall insulation to be SPF/rigid, so 3.5" of cc SPF with 2" of batt insulation is no problem. Since you have sufficient impermeable insulation, you don't need any warm side vapor retarders such as faced batts, just painted drywall is fine.

      Since the SPF is being charged by board feet, you can even go down to 2" cc SFP with 3.5" batt insulation as this still plenty for condensation control. SPF 40% less and batts are not much more with the same amount of work to install.

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