Hello all. Long-time lurker, first time poster.
I plan on building a Pretty Good House in Tucson, AZ. Zone 2, design temps 104, 32.
I've been thinking my way through a bunch of different wall stack ups, trying desperately to avoid thermal bridging when i began to wonder how much it even matters in a warm climate. Is it worth the fuss? I understand the need to limit bridging in cold climates, but what about the desert?

A simple wall assembly I've been considering (out to in):
stucco/lath/paper
rainscreen gap (not even sure this is necessary with 11" of rain per year)
wrb
taped plywood sheathing (air barrier)
2x8 studs 24" OC
Roxul cavity insulation (R30)
drywall

Aiming for not-quite-advanced framing, but will be designing to limit superfluous studs.

My goal is to approximately double the code insulation amounts (R13 for zone 2)

Caveat: I'm not a big fan of foam. Would like to avoid it when possible.

Questions:
1) Is shooting for a framing factor of 15% unreasonable?
2) Would there be MUCH benefit in using staggered 2x4s instead of 2x8s?

Thanks for your insight! Michael

Alright, after a bit more research it seems that having a higher R value wall assembly in a hot climate is not all that cost effective in the first place:
GBA's hot climate design



According to Danny Parker's model (in the comments section), doubling the wall R value from 10 to 20 only saves about $25 a year in electricity.
Not much.


Running some quick material cost numbers, going from 2x6 stud wall with Roxul (whole wall R16?) to a 2x8 with Roxul (whole wall R20?) adds about $1100. Assuming this saves about $15 a year, it'd take over 70 years to payoff.


On a different note, while trying to calculate the whole-wall R value of my hypothetical 2x8 wall, i was messing around with the ORNL Online Simple Whole Wall R-value Calculator, but found something that seems strange to me.


Keeping all other values the same, when you switch from wood 2x4 studs (24" oc) to 2x6 studs, the whole-wall R value drops 6% relative to the clear wall R.

Also, going from 16" oc to 24" oc shows a relative drop in whole-wall R.

That seems counterintuitive to me. Seems like the wider the stud, the more R value you'd have. And it definitely seems that with less framing members you'd have a higher whole wall R. I'm new to this stuff, can someone explain this?

Have you ran the ORNL mass to stick calculator to see which saves more in energy bills? If I remember right your neck of the woods has the one of the biggest bang for the buck with mass.

Thanks, TLP. i'll look into that when i can find a PC. There's no Mac version, unfortunately.

Staggered studs, seriously, you've been watching too much "Wild Alaska".

In warm, dry climates ( I lived and worked in Albuquerque) the standards change and return on investment for insulation is generally much longer. We still specify insulation beyond code, not a moral issue as it is for some, but it has more to do with acoustics than saving fuel. Your are interested in the bulk fo the energy load, cooling in your case. The rain screed is a path for super-heated air on sun exposed walls, and radiant heat is the enemy.

I could not understand, and still don't , the seemly total conversion from evaporative cooling to DX. I would have the best dang swamper available and keep thing cool and moisturized year round. In fact, I might yet if they keep raising taxes and educating idiots here in Minnesota.

An ERV for sure.

Your biggest building hurdle in Tucson is the termites. In Tucson it's not IF you will have a termite infestation, it's WHEN. You will have to stay on top of things to make sure the termites don't get out of control.

Have you looked at a concrete wall system?

Thanks Badger. Swamp coolers work great for part of the year, but for the 2-3 months of rain we get in late summer they don't do sh*t (too hot and humid) . For those months, it's nice to have AC. Some people have dual systems (evap and AC).

Lbear - I hear you about the termites. Trying to design around them. I'll look into concrete wall systems,. I'd always heard they were pretty pricey compared to wood framing. Are you in Phoenix? If so, what's typical wall assembly there? How about a better-than-typical?

The rain screed is a path for super-heated air on sun exposed walls, and radiant heat is the enemy.
I could not understand, and still don't , the seemly total conversion from evaporative cooling to DX.

I'd like to see a hot climate house design that used a double wall/roof with the gap being used as an air channel for evaporative cooling. Ie, water misters in the gap.

I've been thinking my way through a bunch of different wall stack ups, trying desperately to avoid thermal bridging when i began to wonder how much it even matters in a warm climate. Is it worth the fuss? I understand the need to limit bridging in cold climates, but what about the desert?

TLP: Absolutely worth the fuss, framing factors can bridge and degrade r-value in both cold and hot climates causing high heating and cooling HVAC loads. The daily differences between inside/outer wall temps determine the Framing Effect Coefficient, bridging, and r-value. In hot climates the temperature differences can be as high as cold climates depending on the internal set point of the house, # of annual cooling and heating days.

Questions:
1) Is shooting for a framing factor of 15% unreasonable?
2) Would there be MUCH benefit in using staggered 2x4s instead of 2x8s?

TLP:
1.15% would be much lower than 27% national average.
2. Absolutely less thermal bridging in staggeed studs. Thermal bridging (or heat transfer coefficient) is a function of the cross sectional area in contact with temperature differences at the intake and exhaust of the wood, and thermal power or flux through it that is function of how far thermal power has to travel. So probably not a lot of difference a 2 x 8 will bridge vs a 2 x 4.

Running some quick material cost numbers, going from 2x6 stud wall with Roxul (whole wall R16?) to a 2x8 with Roxul (whole wall R20?) .

TLP: There is no knock down factor to r-value according to Roxul testing (use it to design to) if installed to RESNET CL 1 fit, great choice. 2 X 6 wall, R24, 2 x 8, R30. http://www.roxul.com/files/RX-NA_EN/pdf/Comfortbatt%20Sheet_Eng.pdf .
Look at the COP between the two stud walls, since you are better than code. Not a whole lot of difference in thermal bridging and framing factor given the same geometry. Eliminating superfluous studs as you stated will help. I prefer an inter utility chase that keeps the insulation and air barrier continuous, cuts down on bridging and framing factors.
http://blog.lamidesign.com/p/swedish-platform-framing-info.html

Keeping all other values the same, when you switch from wood 2x4 studs (24" oc) to 2x6 studs, the whole-wall R value drops 6% relative to the clear wall R.

TLP: In theory a larger 2 x 6 stud will bridge a little more than a 2 x 4 dropping the WWRV to CW ratio.

Also, going from 16" oc to 24" oc shows a relative drop in whole-wall R.

TLP: Look at table 3, r-value performed best when the studs where closer together…
http://web.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal_frame/

That seems counterintuitive to me. Seems like the wider the stud, the more R value you'd have. And it definitely seems that with less framing members you'd have a higher whole wall R. I'm new to this stuff, can someone explain this?

TLP: A wider stud (2x6) vs a shorter stud (2x4) will bridge close to same since what matters are the cross-sectional area and length the thermal power or ‘flux’ has to travel. I am sure the calculator is making some crude assumptions since the math model (flux, temp differences) is quite complex. Spacing has an effect on thermal load distribution, see report.

Run the Mass Calculator, get familiar with DBMS if you are not already.. Take note of CIC(Concrete-Insulation-Concrete) the best performer per their report and industry write-ups on their test. What happens there is the thermal bridging stays within the surface at some depth (1” of 2”). The outer and inner mass “breaths” heat/coolth releasing it in a 6-12 hour lag time for loading the wall by solar design, releasing it at night. You area performed the best due to your daily temps swings per year around your inside set point (70F). You have a better chance of achieving net zero with mass and a tight envelop than others. It does not cost more if you get into it and know what you are doing, it can cost much less than stick framing as a matter of fact especially when you factor in the energy saving’s, comfort, sound deadening, safety(natural disaster), termites, etc….

Here some info,

http://web.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal/index.html

http://www.bibm.eu/Documenten/ECP%20General%20Guidelines%20for%20Using%20Thermal%20Mass%20in%20Concrete%20Buildings%20(PM%2029%2004%2009).pdf

Posted By jonr on 04 May 2014 09:27 AM

The rain screed is a path for super-heated air on sun exposed walls, and radiant heat is the enemy.
I could not understand, and still don't , the seemly total conversion from evaporative cooling to DX.

I'd like to see a hot climate house design that used a double wall/roof with the gap being used as an air channel for evaporative cooling. Ie, water misters in the gap.

I used Thermax against the corrugated steel wall and roof to the ridge vent. It's a start.