Double stud or 2x6 with exterior foam?

[Alan Gage]

I was all set to build with 2x6 walls (wet blown) and 2" of exterior XPS but during the past day or two started thinking about a double stud wall (dense packed) instead. The house will be a little under 1000 sq. ft. (single story) in a climate similar to Minneapolis.

Reasons for NOT going with foam are:

1: Not having to use so much foam in construction (environmental)
2: Cost - foam is expensive
3: Ease of construction? Seems like considerably more work attaching windows and furring strips
4: A wall that breathes better to the outside

I think the money I'd save by not using foam would allow me to upgrade my sheathing from OSB to plywood (would hopefully handle moisture better) and pay for the extra framing materials. Having never framed up a whole house before it also seems like it would be easier to construct the 2nd interior (non bearing) wall as opposed to installing and detailing the exterior foam.

Either way I'd end up with a highly insulated wall with limited thermal bridging. The double stud wall would mean colder exterior sheathing that could be prone to condensation in winter. But I'm thinking a wall that could better dry to the exterior and plywood instead of OSB would help mitigate that potential problem.

Any thoughts from those with experience would be greatly appreciated. Is there a big advantage of one over the other that I'm missing or is it a bit of a wash? Since this is new construction I'm thinking the loss of interior space with a wider wall wouldn't be a problem as I could just make the foundation a few inches bigger.

And while we're on the subject of double stud walls: What is the best way to frame them? It seems like it would be easier and more energy efficient to start by building a standard 2x4 wall and adding the 2nd, interior wall, later on in the game. If you used a 2x12 header and sill plate and did both walls at the same time you'd have thermal bridging from both plates and a heavier (more awkward?) wall to lift. If you started with a simple 2x4 wall and added the interior wall later you'd have a thermal break (about 6") in the top and bottom plates that could be filled with foam.

Alan

[rocking23nf]

I dont see the benifit of this, houses have been built with a single wall 2x6 for a long time, with no issues.

I would stick with that simply because its easy.  As for plywood, I dont know what the current cost is like, but a few years back there wasnt much different in cost, so i used 1/2 inch plywood on my walls, and 5/8s on the roof.

I proper built home with 2x6 walls isnt going anywhere.

[Alan Gage]

My problem with a regular 2x6 wall is the thermal bridging from the studs and the relatively small amount of insulation that can be installed. I'm going for a very energy efficient home and with the cold weather we get (-20 is fairly common) I'd like the extra insulation. The calculations I've done show about 2000 BTU/hour difference between a standard 2x6 wall and the other two options I mentioned with an inside temp of 68 and outside of -20. If the outside temp is 0 degrees the difference is about 1500 BTU/hour.

A standard wall isn't completely out of the question. With this small of a house the super insulation doesn't seem to provide quite as much benefit. There just isn't that much outside wall space and a larger percentage is made up of windows, more insulation doesn't help there.

It's a constant battle trying to decide where to draw the line.

Alan

[Squirl]

I can understand your point of view Alan.  I don't worry as much about moisture, but I do about the insulation value and cost.  The code for the climate of the area I am building in in NY has a minimum requirement of R-19 for fossil fuel houses.  They say that a 2x6 cavity filled is acceptable, but a 2x6 cavity is only 5.5 inches thick which, in reality, drops the value of fiberglass and blown.  For electric R-24 is the minimum.  Minneapolis is 1-2 climate zones colder than the area I am building in.  Good insulation and tight building can give the greatest dollar return over the life cycle of a house when spent at the time of building, especially if it is your future house.  Energy costs seem to far outpace inflation.  I understand the concern of thermal bridging, but you are already reducing that by going with 2x6 construction.  The studs become a much smaller percentage of the exposed area and the area that is exposed to studs is twice as insulated as a 2x4.  I am sorry I do not have experience building with a double stud wall.  I am still debating whether to go with double stud or just 2x10's.  For me, the thought is that the inspector would be more familiar with the design, but I don't know if you would have that problem.  I did remember a story of that here:
http://www.countryplans.com/nicolaisen.html
I have seen others build with the studs offset from the other wall.

rocking23nf, OSB around me is $7 for wall sheathing vs. $14 for plywood per sheet.

[MountainDon]

OSB, plywood, moisture....
To me the issue with moisture in the wall is more the concern of mold than any issue with the sheathing materials degrading. I'd use the OSB. Also remember that depending on the structure there are 9 and 10 foot sheets available that permit covering walls and rim joists with one sheet.

Also, speaking for myself again, moisture will not condense in the wall if there is no surface as cold as, or colder than, the dew point. Two layers of external foam with staggered seams will eliminate that.

I agree the exterior finish work is more complicated, but in the long run it may be very worth the effort.

Wet blown cellulose would be my choice for wall cavity insulation.  My $0.02 worth

[Alan Gage]

Thanks for the thoughts.

To me the issue with moisture in the wall is more the concern of mold than any issue with the sheathing materials degrading. I'd use the OSB.

I'm not so much worried about the OSB degrading but from what I've read it sounds like plywood is much better at redistributing (for lack of a better word) moisture. If I didn't use exterior foam and had to worry about the inside of the sheathing getting wet from condensation the plywood would do a better job of transferring that moisture to the outside so it could dry. I like the argument that if the wall is allowed to breath to the exterior (which it wants to do in winter anyway but can't really do with exterior foam) then the small amount of moisture that might accumulate in the walls from condensation isn't of any major concern. Also that if the interior air barrier (drywall) is detailed well enough moisture condensation inside the walls will be minimal anyway.

It will be nice to get some practical experience under my belt instead of trying to draw conclusions from what everyone else says. It can be tough figuring out if they know what they're really talking about or if there's a hidden agenda.

Thanks again,

Alan

[Alan Gage]

OK, ran some numbers tonight and I find the results kind of interesting. With either a 2x6 wall and 2" exterior foam or a 9.5" double stud wall I come up with a heat loss of just under 11,000 BTU/hour. This is figuring a 57 degree temp difference between inside and outside temps, which should be about average for Dec., Jan, and Feb around here. Our average highs for that time of year are low 20's and average lows are low single digits.

But what I found interesting was that if I figured a standard 2x6 wall with just cavity insulation the heat loss is 11,816 BTU/hour, only about 875 btu/hour worse than the other two higher insulation options.

875 BTU works out to 256 watts. So if I was heating with 100% electric that would be an extra 256 watts/hour. Over the 3 month period of Dec., Jan., and Feb. that would work out to an additional 553 KWh, which at $0.11/KWh amounts to only $60.83 over the 3 months. Not as dramatic of a difference as I expected.

This is a fairly small house though (under 1000 sq. ft) and I'm sure the benefits of a thicker wall would go up dramatically as the house size increased. There are also probably more windows in the house than would be recommended for efficiency but I decided I'm not giving up my views.

What made the most difference in my calculation was upgrading from .3 U-factor windows to .2, that was an 1100 Btu/hour improvement for any of the wall assemblies.

Someone please correct me if my numbers seem off. I used Rescheck to come up with the UA number for the assembly and multiplied that by the inside/outside temperature differential to come up with BTU/hour heat loss.

Also, a nifty calculator I came across for converting different energy types as well as calculating them for different time periods (type in 875 btu/hour, tell it to calculate over a 3 month period, and it tells you it would take 553 KWh, among other things).

http://www.mhi-inc.com/Converter/watt_calculator.htm

And BTW, I won't be heating with 100% electric. It will be 98% wood that costs me nothing more than chainsaw gas and time. Electric will be on low if I leave the house for a few days

Alan

[Alan Gage]

Double checked some numbers this morning after thinking about it last night. I'd been using the Building Science "Research project for high R walls" to come up with the whole wall R value for double stud walls but had been using REScheck to calculate the others. Turns out REScheck is giving something like R18 for a standard 2x6 wall where Building Science gives it a whole wall R value of R15.

So when I changed the 2x6 wall to R15 in Rescheck I come up with a 1312 BTU/hour difference between a regular 2x6 wall and something like a 9.5" double stud wall. That's 384 watts/hour, or 830 KWh over the same 3 months listed above. $91.30 difference over 3 months with 100% electric heat (at $0.11/KWh).

Alan

[rocking23nf]

shouldnt it be r20? thats what i put in my walls.

[Alan Gage]

shouldnt it be r20? thats what i put in my walls.

I'm trying to use the "whole wall" R value, which takes into account framing members and thermal bridging. So yes, R20 will be installed in the cavities but the actual R value of the wall will be closer to R15. Supposedly, in a normally framed 2x6 wall, framing can take up 20-25% of the space. Which means only 75-80% of the wall has insulation in it.

I'm figuring it the same way for the double stud walls but the penalty isn't as steep because of limited thermal bridging.

Alan

[rocking23nf]

does it also count a vapour barrier, i noticed a huge difference in tempurater the second i put the plastic barrier on the wall.

[Squirl]

What kind of insulation is R-20 for a 2x6 wall cavity?  I normally see R-19, but as mountaindon has pointed out on a few threads that this decreases when you stuff it into a 5.5 in. cavity.

[MikeOnBike]

You can get an R-21 batt that is 5-1/2" that doesn't need to be compressed.  It doesn't seem to be as readily available as the R-19 and I think it is quite a bit more expensive.

Insulation compression chart:
http://numsum.com/spreadsheet/show/21111

[Alan Gage]

does it also count a vapour barrier, i noticed a huge difference in tempurater the second i put the plastic barrier on the wall.

No experience here but I'd think the fact that a vapor barrier is also an air barrier would be the biggest reason for the improvement. Everyone says that controlling air leaks is more important than insulation.

Alan

[rocking23nf]

I agree, we taped every spot we could with tuck tape, it made a huge difference in the climite inside the cabin.

I went from no insulation, having 3 electric portable heaters going full blast, -5C outside, and around +15 inside

to

2 heaters going half settings, and +21 inside.

And my insulation is only 80% done in the cabin.

[Triathlete]

Building 2x6 walls is standard practice in Canada, where temperatures are significantly colder than just about anywhere in the USA with the exception of Alaska.  A house needs to breathe somewhat to keep all indoor waste gasses from reaching high levels.