Wall thickness and diminishing returns
gonyo
| Posted in Construction Techniques on
So I live in Fairbanks Alaska and I’ve begun the process of building a home up here decided on the plan got the land pending. The standard construction up here is 2×6 24″ on center I read this book http://www.uaa.alaska.edu/civilengineering/arctic/upload/Building_in_the_North.pdf called building in the north it says that the best wall thickness based on money to the amount of R-Value you get is 2×8 but that part of the book hasn’t been updated since the 80’s. Just kind of curious if anything has changed, is there a wall thickness that just gives more back fot the buck.
In Fairbanks Alaska we had 13940 Heating Degree Days in 2014
Thanks
Replies
Keep in mind that there's a diminishing return on R value. An R of 20 is not twice as good as an R of 10, since at some point other heat losses (eg, windows, ventillation) swamp the losses through the walls.
Thanks thats kinda what im wondering when does adding blue board or making the walls thicker start to get pointless because of any limiting factor like windows or money
It's fairly easy to figure out how many square feet of window you have, pick an R value for them (eg, 5), then do the arithmetic.
The heat loss through an insulator is, for a given temperature differential, proportional to the square footage divided by the R value. If you have a house with 5,000 sq ft of exterior area and 10% of that is windows, and your insulation (other than the windows) is R 30, your non-window heat loss is proportional to 4500 / 30 = 150, while the loss through windows (at R 5) would be 500 / 5 = 100. Going to R 50 would give you a heat loss of 90, while R 100 would be a heat loss of 45.
Given those numbers (which are highly artificial) it might make sense to go to R 50, but not R 100, since you'd be losing twice as much through the windows as through the insulation.
Of course, this isn't using "real" numbers, nor does it account for differences in, eg, the attic vs the walls. YMMV.
(One would think that someone would have an app online that would help you run the numbers.)
Also ...
Even without window openings etc. an R 20 is not twice as good as an R 10. Not sure I can explain it, but the relationship is not linear, it is exponetial (spelling). It is something like doubling an R gives you 50% gain in reducing heat lose so to go from 20 to 40 will cost you twice as much and only give you 50 % reduction in heat loss, and going from R 1 to R2 to R 4 to R 8 to R 16 has already gotten you well down the curve from 100% heat loss to 10%, so is doubling the cost to go from 10% to 5% "cost effective"?
That is roughly how "THEY" have come up with the R 20 for walls and R 38 for roofs for most heating zones.
Ok, now we await the hestria.
You'll gain more from keeping the same thickness studs and adding the appropriate amount of foam board to the outside of the sheeting than you will from going to thicker studs. This is because the foam, properly installed and taped, provides a continuous thermal break between the inside and outside. The "appropriate" amount depends on what zone you live in; and ends up being more complex than that. Other things such as vapor retarders/barriers and where they need to be placed in the building envelope are all location and situation-specific; and getting this wrong can lead to mold and structural deterioration. Event the type of foam (EPS vs polyiso vs XPS) have different vapor permeances and performance at different temperatures. As long as you do your research and go about it the right way, the payoff in comfort and heating/cooling costs will be substantially more than you'll ever get out of thicker studs. For specific advice regarding your application, I'd go here:
http://greenbuildingtalk.com/Forums.aspx
And it needs to be noted that tightly air-sealing the "envelope" is incredibly important. It's easy for air leaks to cut your insulation effectiveness in half.
exterior foam
Agree with strawmeyers. I'd look at two layers of 2" thick XPS on the exterior of the shell.
There have been a few studies that show polyiso's R-value can decrease in lower temps. XPS will save you money and will still give you very good thermal performance.
Two layers of 2" is pretty easy. Three layers it can get a little more complicated. But it might be worth it depending on your heating load and fuel costs.
For an exceptional envelope, run the layers on the walls and roof, tying them together. You can then overframe your gable eaves, etc, so you keed a nice tight and continuous envelope.
You'll find decent references on buildingscience and greenbuildingadvisor.