2x4 or 2x6 studs

[Redoverfarm]

So Scott are you saying that there is just a 1% gain between R-19 that R-13.  If that is so then why are we paying more for R-19 .   3-1/2 verses 6-1/2 Huh

[MountainDon]

Well, that forced me to delve a little deeper. I fired up my REScheck program and loaded the information I'd saved for my cabin. It's set up with R38 ceilings, R30 raised insulated floor, 3 high quality U-factor 0.35 windows and one door in a 14 x 26 foot structure.

When I run walls of R13 vs. walls of R19 the program gives me a 13.3% difference in the energy efficiency. The 2x4 R13 walls fail the test. In NM that means the building would not meet code and would not have the plans approved.

When I run the R19 walls vs R13 walls plus R5 foam on the exterior there is a 12.2% difference in energy efficiency rating. This also passes code like the 2x6 R19 walls do. The program does not take the effect of the foam being a thermal break at the studs. It would seem that the foam over 2x4 would beat the real world performance of plain 2x6 though.

:-/ :-/ :-/

FYI, the R19 2x6 walls beat the code by 3.3% while the R13 + R5 walls beat code by 2.2%. There are a number of codes ranging from 1992 through 2006. NM is one of the states that follows one of the strictest codes, 2003 IECC.

These figures are for my mountain location. When I run the same numbers through my year round residential location those same values (R13+R5) come out to better code by 27.9%

I have mentioned the REScheck program before. For the curious here's where you can find a link
http://www.countryplans.com/cgi-bin/yabb2/YaBB.pl?num=1177221623

[ScottA]

Well I should have been more specific I guess. 1% relative to no insulation at all is what I meant. Don's program comes up with 13.3% which I assume is better relative to R-13. So relative to no inulation it would be 1.33% so I was off 1/2% sorry. As I understand R value R-1 = no insulation but also no air movment. R-10 = 10% of the heat flow of R-1. R-100 would be 1% of R-1 and so on. Unless I'm wrong on this which I admit I could be R-19 is not that much better than R-13 relative to no insulation at all.

Scott

[Preston]

My goal in using the 2x6 vs a 2x4 isn't really about money, it's about stayin warm without forced air    It was nice to hear what you had to say about immediate cost vs future satisfaction Don! <<That thought has been playing through my head I just couldn't get it out in words.

ScottA, You mentioned 2x8x12's, you'll be cutting a lot to get an 8 foot wall.

[MountainDon]

As I understand R value R-1 = no insulation but also no air movment. R-10 = 10% of the heat flow of R-1. R-100 would be 1% of R-1 and so on. .....R-19 is not that much better than R-13 relative to no insulation at all.

I'm trying to understand what is being said here and having trouble.

I understand, R-value to be: A measure of a material's resistance to heat flow (BTU's per hour, through one square foot of wall area for one degree Fahrenheit temperature difference). R value is derived from the U-factor of a material; that's how well a material conducts heat. R value is the reciprocal of the U-factor.

R-value figures do not apply only to things we recognize as insulation, they apply to all materials. A 1/2" of plywood is rated at R=0.63, 1/2" of drywall at R=0.45, a 4 inch common brick receives an R value of 0.80. A single pane window glass has an R of 1.0  :-/  Complicating the calculation of the R value of a total wall is the fact that doubling the thickness of a material does not necessarily double its R-value.

I don't see how it can be said that R1 = no insulation, or that R100 would be equal to 1% of R1. I don't understand where that comes from?  Help!

Anyone??

[MountainDon]

From Finehomebuilding.com

R-value

Created as a way to sell insulation, R-value is a measure of resistance to heat flow. The higher the number, the greater the insulating value.

Because doubling the R-value cuts heat loss in half, adding R-1 to R-1 will make a big difference; but adding R-1 to R-30 will reduce heat loss by only about 3%......

http://www.taunton.com/finehomebuilding/how-to/articles/thermal-resistance-ratings.aspx?nterms=65826

[John Raabe]

If you have an R-2 wall, basically an uninsulated 2x4 frame with just the air films and materials, then adding R-11 batts will only let about 1/5 the heat through the wall. A five fold increase in efficiency. Very cost effective! If you were paying $10 per 100 sf of wall for heating you have now reduced that to $2.00 - an $8.00 saving.

With that R-11 wall you can cut the heat flow through the wall in half by putting two inches of isocyanurate insulation (blueboard or pinkboard) on the inside or outside. This will add an R-10 and cut the heat loss through the framing. This will be cost effective in cold climates and where heat is expensive. It may also be cost effective in very hot climates where electric costs are high. You have reduced your cost of heating from $2 to $1 saving $1.00 per 100 sf of wall by doing this.

If you want to cut the heat loss of the above wall in half again - then you have to add another R-20 to R-25 to the mix. Very difficult to do and not cost effective in most climates (perhaps in the Antarctic ). For all this additional work and expense the savings are only $0.50 per 100 sf of wall. You can quickly see that the law of diminishing returns is at play!

[ScottA]

I guess what is confusing is the fact that the air film around the building envelope both inside and out has an R value of about 1 so even with no inulation you have R-1. Add a single pane of glass R-1 + R-1 for the air  to this and you have R-2 cutting the heat flow in half so even with just a single pane of glass you have cut your heat loss by 50%. Each additional doubling of R-value saves you half of the previous doubling as John said so the second doubling is only a 25% improvment over the first and so on.  For walls figure into all this that the studs won't have the same R value as the insulation the losses for windows and the fact that the walls don't comprise 100% of the building envelope and you can see that by simply adding 2" (R-6) of fibergass by going from 2x4 to 2x6 walls doesn't really gain you much when the whole picture is considered.  The money example John gave says it best I think. When it comes to insulation the more you spend the less you get after a point.

Scott

[ro]

Back home I grew up in a neighborhood full of 1 1/2 ti 2 1/2 story houses, stick built with 2x4's Never saw one of them fall down either. They're now 100 years old and for the most part still there, but mostly updated, renovated, ext. foam added, etc. But underneath the same old, same old.

I live in such a stick built house which is still standing after 60 years, mind you in my case, one difference is that the 2x4's used in it were old school lumber. They are full dimension rough cut beauties. At a full 2x4, they are 8 full sqare inches, whereas modern 2x4's are planed down to 1.5x3.5's which @ 5.25 square inches is actually 34% smaller. The cross-sectional area of a modern 2x6 @ a planed down size of 1.5x5.5 is only 8.25 square inches, and therefore comes very close to a full old school 2x4.