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dwoloz 12-05-2010 12:22 AM

Advanced framing or conventional?
 
I'm building a 12x10' cottage (slab on grade, vegetated shed roof, 1 story with loft) and weighing the option of whether to stick frame conventionally (2x4 16"OC) or advanced (2x6 24"OC).
I like the idea of added R-value with the thicker walls but without yet doing the stick by stick math, I think conventional framing may actually be less expensive for such a small project. The winter lows in this climate only dip down into the 30s so Im actually not even sure the added insulation is necessary

Advice would be appreciated

jklingel 12-05-2010 01:07 AM

low 30s? you likely don't need huge insulation, like you said. what do codes require? if structurally ok there, 2x4, 24" OC and GOOD insulation, like cotton or cellulose batts (treated w/ borates) or rock wool is another option. leave the fiberglass at the store. then use the ada (airtight drywall approach). this eliminates the vapor barrier, unless your area mandates a perm rating near zero. if you decide you want more insulation, i can buy two 2x4's cheaper than one 2x6; double stud the walls as thick as you want to. check local prices.

mrgins 12-05-2010 10:21 AM

Even in that climate, you're going to save on heat and AC bills. Double 2x4s is good, but maybe consider single 2x4s, fiberglass with a vapor barrier, and then 1" polyicosyanurate sheathing. Just make sure you get the kind that ants won't eat. I made that mistake on my own house and could hear ants chewing on it at night!

jklingel 12-05-2010 12:37 PM

I'd check w/ local code folks before I installed a vapor barrier, esp in CA. If you only need a perm rating of 1 or less, then the ADA is what I would do. VBs are being abandoned in all but zone 7 and 8, which is way over CA (unless you are way up in some nasty mountains that are freezing-arse cold). Research fiberglass batts on buildingscience.com, greenbuildingadvisor.com, etc, before you use it. It is the worst choice of batts going. Mrgins: I hope you have sprayed for the ants? Synthetic pyrethroids should be fed to them. They will carry it back to the nest, and nuke the whole bunch. Boric acid kills them quickly, but too quickly. Let them take the bomb home w/ them. Good luck.

dwoloz 12-05-2010 12:42 PM

Thanks for the tips
Somehow I totally forgot about doing double stud or staggered stud, thats another good option. I'm pretty sure for my region as well, two 2x4s is still cheaper than a single 2x6, have to check though. ADA sounds like an excellent choice also
I'm wary to even consider 2x4 24"OC because of the unusually heavy roof load (vegetated roof). If I did a single top plate and point load configuration, the rafters would also be on 24" centers so I'd need pricier 2x10s and thicker decking.
Because of the seismic activity here Id like to do 3/4 ply sheathing for sheer. Possibly I could do foam on top of that; 1" maybe. Would that be worth it? Being in this mild climate I've never actually seen a house built with foam board. Is tyvek then put over the foam or are foam boards made to handle drainage?

jklingel 12-05-2010 01:21 PM

Quote:

Originally Posted by dwoloz (Post 545886)
I'm wary to even consider 2x4 24"OC because of the unusually heavy roof load (vegetated roof). •• That is out of my league. Engineer time. My gut would say 2x4, 12" OC, but see an engineer.

If I did a single top plate and point load configuration, the rafters would also be on 24" centers so I'd need pricier 2x10s and thicker decking. •• The idea of two top plates has always appealed to me viscerally. With a "normal" roof, 50 psf snow load to boot, a 2x4 wall 16" OC and a double top plate does not require trusses be on top of studs; 8" stand-off is fine. That is from an engineer; no idea what a weed roof weighs.

Because of the seismic activity here Id like to do 3/4 ply sheathing for sheer.•• May be a good idea. Again, out of my league.

Possibly I could do foam on top of that; 1" maybe. •• 1" won't help w/ thermal bridging via the studs, but it will insulate. IMO, no need if you double stud.

Being in this mild climate I've never actually seen a house built with foam board. •• I'd never seen a 13" double stud wall up here when I built in '80, either. People laughed, but they don't know. You'll have to run the numbers and look at payback time for whatever you do.

Is tyvek then put over the foam or are foam boards made to handle drainage? •• Apparently Typar is a better product, and the debate of over/under is minor.

see after bullets.

mrgins 12-05-2010 05:11 PM

Quote:

Originally Posted by jklingel (Post 545881)
I'd check w/ local code folks before I installed a vapor barrier, esp in CA. If you only need a perm rating of 1 or less, then the ADA is what I would do. VBs are being abandoned in all but zone 7 and 8, which is way over CA (unless you are way up in some nasty mountains that are freezing-arse cold). Research fiberglass batts on buildingscience.com, greenbuildingadvisor.com, etc, before you use it. It is the worst choice of batts going. Mrgins: I hope you have sprayed for the ants? Synthetic pyrethroids should be fed to them. They will carry it back to the nest, and nuke the whole bunch. Boric acid kills them quickly, but too quickly. Let them take the bomb home w/ them. Good luck.

I suggested the vapor barrier because it comes to mind automatically, I've only lived in colder states! But his elevation says he's at 3000'+. Good idea to check with authorities first tho:thumbsup:

jklingel 12-05-2010 08:00 PM

The first three houses I was involved w/ up here in Zone 8, from the late 70's to now (and only a couple others, btw; I am a DIY cat) all had VBs, because "that is what we do here". I am trying to avoid them in my 24x24, 2-story addition, and have just learned about NOT using them in the last couple of years. I will soon check w/ the city to see what code calls for. I have heard that it is "1 perm or less"), and if so, I will use the ADA w/ my dense packed cellulose. VBs are a PITA, but, if necessary, I'll do 'er again. Too, if I read anywhere about moisture, rule #1 is AIR SEAL. Now that I know how little air diffuses through a wall, compared to running through holes, I understand why air sealing is the name of the game. Read, talk, learn, then do.

dwoloz 12-05-2010 08:18 PM

I'm actually at sea level


I think that I've decided to do a staggered 2x4 stud wall 12"OC and skip the foam. Typar looks interesting but Im leaning towards Tyvek StuccoWrap since it has drainage grooves (Typar does claim it works just fine with stucco though).

Hohn 12-05-2010 08:48 PM

Quote:

Originally Posted by dwoloz (Post 545648)
I'm building a 12x10' cottage (slab on grade, vegetated shed roof, 1 story with loft) and weighing the option of whether to stick frame conventionally (2x4 16"OC) or advanced (2x6 24"OC).
I like the idea of added R-value with the thicker walls but without yet doing the stick by stick math, I think conventional framing may actually be less expensive for such a small project. The winter lows in this climate only dip down into the 30s so Im actually not even sure the added insulation is necessary

Advice would be appreciated

Extra insulation is always a good idea. If you will be spending any amount of time in this cottage, any drafts or absence of good insulation will get old in a hurry. You don't need sub zero temps to be miserable.

I'd encourage you to look at total cost for a given R value. Say the 2x4 16"OC is cheaper for just framing-- how would it compare if you added another R10 of 2" XPS sheathing? Using the example of Roxul batts, the 2x6 wall can have RR value about 8 higher than the 2x4 wall. So, you'd need at least 1.5" XPI sheathing to make this up.

So the 2x4 might not be a cost savings when looking at the TOTAL cost-- apples to apples.

If going 24" OC, I'd do a double top plate for sure.

I'd also rule out 2x4 for rafters. I'd rec 2x8 24" OC or 2x6 16" OC.

Plus, the thicker rafters will hold more insulation. Insulating the rafters is really helpful esp for reducing summer heat load.

JMO

jklingel 12-05-2010 10:44 PM

Quote:

Originally Posted by dwoloz (Post 546225)
I'm actually at sea level


I think that I've decided to do a staggered 2x4 stud wall 12"OC and skip the foam. That is a lot of wood, if in both walls. If you double wall, you may be able to get by w/ 16" OC, esp if both walls are load bearing. Engineer time. You don't HAVE to have both walls load bearing. The outer could be 12" OC for load, the inner one 24" OC. Options. Check out the Riversong/Larsen Truss, just for ideas if nothing else. Remember to support both walls to the dirt if they are both load bearing, of course.

... StuccoWrap since it has drainage grooves (Typar does claim it works just fine with stucco though). Greenbuildingadvisor.com has lots of articles on stucco.

c after bullets

dwoloz 12-06-2010 03:27 PM

Quote:

Originally Posted by jklingel (Post 546302)
c after bullets

By 12"OC I meant 12" from one stud to the next closest (staggered). So the studs on the exterior which the plywood sheathing will attach to will be spaced 24" and the studs on the inside the drywall will attach to will be spaced 24".

http://t3.gstatic.com/images?q=tbn:0...udWall.gif&t=1

jklingel 12-06-2010 04:11 PM

OK. I see what you are doing. Are you thinking of one plate at the bottom, or two? Some use a 2x10 plate, but I don't see the reasoning in most cases. I've been told that you need at least 2" of insulation outside a stud to prevent thermal bridging, at that sounds reasonable. That is one good reason for two separate walls; spread 'em as much as you want to.

Hohn 12-06-2010 07:49 PM

Quote:

Originally Posted by dwoloz (Post 546746)
By 12"OC I meant 12" from one stud to the next closest (staggered). So the studs on the exterior which the plywood sheathing will attach to will be spaced 24" and the studs on the inside the drywall will attach to will be spaced 24".

http://t3.gstatic.com/images?q=tbn:0...udWall.gif&t=1

That looks like a good way to split the difference, get most of the AF benefit at the lower cost of 2x4.

I'd think a double top plate would be a must-have for a stagger stud, though.

dwoloz 12-06-2010 08:45 PM

I'd use 2x4 studs with 2x6 plates; that'd give room for 2" of insulation to snake through the studs plus additional fill where space is available

Im still thinking a single top plate would be fine actually if (and a big if), the rafters lined up with the studs. So I could do something like 2x6 rafters every 12" or 2x10 rafters every 24"


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