Helpful? 0

Double Wall

I am planning on building a new house and am looking at using a double wall. Outside wall will be 2x6 studs on 16 centers with ½ plywood sheeting and rain screen. Roxul R24 in cavity’s. Inside wall I will use 2x4 on 16 centers. I will leave a 1 inch gap between both walls. Is this enough or two much? In the 2x4 wall cavities I will install Roxul R14. I have been doing some research hear on the internet and I have read that sometimes the 2x4 studs warp because of no sheeting on the opposite side of the drywall. Can this be a problem and if so can I brace the inside of the stud with strapping or something of that nature? Also I live in Newfoundland Canada so can I use poly as a vapor barrier or one of those new vapor retarders. Also what kind of R value should I expect to get with this set up? Also any problems you know of with this type of wall or precautions I should take. Thanks

Asked by Sam Fowler
Posted Tue, 01/22/2013 - 00:08
Edited Tue, 01/22/2013 - 05:16

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13 Answers

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1.
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I will leave it to the pro,
but i am unsure that your setup will be much more efficient than let's say 2X6 with roxul + 2-4" of foam on the exterior ..which would be probably easier to built also.

You will gain more in using exterior insulation boards than in doubling the wall.

I believe you would like to double the wall to get a thermal break between studs ?

I will leave pro fellows here discuss about the use of a vapor barrier in your situation...

What kind of exterior finish will you be using ?

2X4 always warp, if sheathing is there and screwed properly, i guess it helps everything hold a bit more straight, but don't set your hopes too high with the green wood studs they sell lately :p

2X6 is usually a little more stable, and with sheathing and good craftmanship it can be made very straight as a wall unit.

I will refrain from commenting on the double wall design because my lack of knowledge on the matter. !! :)

Also have you planned your air sealing strategy ?
i believe that double walls might be a little trickier to seal, unless all is done at the exterior
(if so..i think u need to seal also at the interior because any air leaks would end up on a "cold" seal surface ?? )

Answered by Jin Kazama
Posted Tue, 01/22/2013 - 02:35

2.
Helpful? 1

Sam: If your loads will not crush them, you may want to consider 2x4 inside and out, with plywood on the outside of the inner wall for the air barrier. Tape and/or goo the plywood edges, etc. Then you can run 2x2 strips on the outside of the inner wall, atop the plywood, that hold plywood gussets, which then hold an outer 2x4 wall. A solid weather resistant barrier goes outside the outer wall, then stuff in Roxul or, perhaps easier, dense pack cellulose. A guy up here used to build this way, and I believe he still does. Search here for the Sunrise House, google Thorsten Chlupp, or Larsen or Riversong truss. Use 24" centers if the loads allow; less wood, better wall R value. When I built long ago w/ double 2x4 walls, I stacked batts between the walls like hay bales. That killed the thermal bridging. Anyway, there are 100 variations of double walls, and several folks here will give you plenty of options. Membrain may be a better option for you than poly, but I guess it costs money; your call. I think the gap between the walls will encourage convective air loops and degrade your R value. Why do you want the gap?

Answered by John Klingel
Posted Tue, 01/22/2013 - 02:54
Edited Tue, 01/22/2013 - 02:57.

3.
Helpful? 1

Sam,
The GBA website has many articles on double stud walls. See, for example:

GBA Encyclopedia: Double-Stud Walls

Is Double Stud-Wall Construction the Path to Efficiency on a Budget?

Building Plans for Double-Stud Wall Construction

Q&A: Airtight Sheathing & Thermally Isolated Double-Stud Walls?

Q&A: Double Stud Wall section

Q&A: TJI stud bays vs double stud wall

You might want to consider using blown-in insulation (cellulose or blown-in fiberglass), which has many advantages over batts. If you do decide to go ahead with batts, I would advise you to increase the width of the gap between the two walls from 1 inch to 3.5 inches, so that you have enough room between the two walls for a layer of batts (installed horizontally).

Answered by Martin Holladay, GBA Advisor
Posted Tue, 01/22/2013 - 05:15
Edited Tue, 01/22/2013 - 05:19.

4.
Helpful? 1

I wouldn't put Roxul on the interior of any home until they start selling a formaldehyde free version. Their MSDS shows 1% - 6% of their product is a known carcinogen.

http://www.roxul.com/files/RX-NA-EN/pdf/Roxul%20Material%20Safety%20Data...

Some type of blown in, formaldehyde free insulation would be at the top of my list.

Answered by Jesse Thompson
Posted Tue, 01/22/2013 - 07:52

5.
Helpful? 1

Sam,
I think truss walls (Larsen or Riversong types as mentioned above) are better than "double walls".
Partly because they are generally more "buildable" than typical "double walls" but also because the sheathing can be located closer to the interior (where it will be warm and dry in winter).

My own house uses a modified Riversong truss that puts all the insulation to the exterior of the plywood sheathing.
The plywood sheathing in my case is made "airtight" using high-quality tape.
The plywood sheathing also functions as a vapour retarder.
I agree with Martin that if you're going to put a space between the studs, it should be wide enough to accomodate insulation (if using Roxul batts, 3.5" is a good number).

Also, Jesse Thompson makes a good point.
Having handled my share of Roxul, I can tell you that it has a smell that I wouldn't want in my house.
But this shouldn't be a problem if the Roxul is used outboard of whatever is used as an air barrier (in my case the plywood sheathing).

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Answered by Lucas Durand - 7A
Posted Tue, 01/22/2013 - 12:00

6.
Helpful? 0

Jin, john, Martain and Jesse, thanks for the reply. I wasn’t aware of the formaldehyde in the Roxul. Anyways I had my plan in place for outside walls or so I thought. 2x6, plywood sheeting, building wrap, 2 inches of rigid insulation, 2x3 strapping for rain screen and spruce clapboard. I started to get worried about a cold wall because I wasn’t sure if the 2 inches would be enough on the outside wall. Here in Newfoundland it’s really hard to get an average winter temperature because well we are known four our erratic weather, sometimes we have cold winters sometimes we have wet winters, never know. Also we have plenty of rain and high winds something else to maybe make the wall wet. Trying to keep a wall dry hear is tricky. My house now is 8 years old, had spruce clapboard rain screen building wrap and then ½ plywood, no ridged foam. Trying to keep water out of this building is a full time job. Last hurricane that blew up the east coast blew water in through the vent holes in my front windows and past the gaskets and ended up on the window ledge inside. Anyways the reason I am exploring the double wall construction is hopefully it will lead to a dryer outside wall. It looked fairly simple to construct or that is what I thought, nothing is ever simple I guess. Also I’m not looking for a super insulated wall but something around R24. I still have some time yet for I won’t start building till maybe July so any information I can get is appreciated

Answered by Sam Fowler
Posted Tue, 01/22/2013 - 23:28

7.
Helpful? 1

Hi Sam. We did a staggered stud wall, 2X4's 16" on center inside and outside on 9.25" plate and will spray cellulose (R32) and added 2" rock wool board (roxul comfortboard IS R 8.4) on outside of house wrap / plywood sheathing layer, then 1x4 strapping then the siding. Some folks use plywood for their plates. But even a 2x6 wall with 4" of some kind of rigid insulation and then a rainscreen, would help a lot with keeping the water out. Our details and photos we are gradually posting at http://agreenhearth.com/staggered-studs-and-wall-panels/ and http://agreenhearth.com/stone-wool-to-warm-the-heart-and-the-home/. The window details are definitely more work with the exterior insulation but its worth it - will post about window flashing soon.

Answered by Patrick Walshe
Posted Wed, 01/23/2013 - 00:35

8.
Helpful? 0

Patrick
Thanks for the info. A picture is worth a thousand words as they say. I like this design and it is easy for me to build. I am building this house myself with no help so i want to keep things simple. If you are getting R32 with this set up thats good enough for me. I wont need to add any rigid to the outside, not saying it's good or bad idea just that i'm not really comfortable with it right now . Two questions. Are you adding some kind of sheeting to the inner 2x4 studs in case they warp and how to get cellulose into cavity, is their something special for keeping this in place.
Sam

Answered by Sam Fowler
Posted Wed, 01/23/2013 - 02:48

9.
Helpful? 0

Okay i have decided on a wall. I will build pretty much as Patrick did. 2x10 top and bottom plates, 2x4 studs. Cellulose for insulation Inside part of the wall will be sheatid by 1/4 or 7/16 OSB and try to get this as tight as possible. I will nail some 2x2 or 2x3 to this inside wall to run electrical and whatever. Sheetrock on the 2x3. Outside 3/4 plywood, building wrap, strapping, rain screen and spruce clapboard. Could anyone out their give me a rough idea of what kind of R value i might get with this set up. Took Martins advice and will go with Cellulose. No vapor barrier.
Thanks Sam

Answered by Sam Fowler
Posted Fri, 01/25/2013 - 03:11

10.
Helpful? 1

Sam,
If you build a wall on 2x10 plates (that is, a wall that is 9 1/4 inches thick) with staggered studs, and you fill the wall with dense-packed cellulose, you'll end up with R-34 insulation.

You'll get a little bit of R-value from the air films, sheathing, service cavity, and drywall. But you'll also get some thermal bridging at the plates and window rough openings, and this thermal bridging will reduce your whole-wall R-value. Calling it "an R-34 wall" is close enough, as far as I'm concerned.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/25/2013 - 08:39

11.
Helpful? 0

Martin
Thanks for your input. Don’t know where I would be without you guys on hear. Standard 2x6 wall is still king where I live, no need to change I’ve been told. R34 is just great.
Paul

Answered by Sam Fowler
Posted Fri, 01/25/2013 - 08:51

12.
Helpful? 1

Double 2x4 wall.....Hi all. I'm currently building a house with a double stud wall system. I just want to chime in on the idea of using 2x10's or 2x12's as top and bottom plates. The interior and exterior layers of 2x4's don't need to be connected by wide top and bottom plates. There is no need for this at all. I just built conventional 2x4 walls, stood them up, and got the roof on. After the roof was on I went back and built the inside layer of 2x4 wall. Less expensive. Easy. Uses less big timber. I was even able to use 2x3's for some of my interior layer of wall, and saved a few more bucks. Also, I would have concerns about the dimensional stability of a 2x12 used a top or bottom plate.

Answered by jake fleming
Posted Sat, 01/26/2013 - 09:06

13.
Helpful? 0

Jake
Yep you will more than likely save lumber. In my case my house is a 2 story and i wanted to add 2 or 3 inches of foam between the outside sill joice and the floor joice's to help with the bridging. The 2 x 10 gives me lots of room for that. Also i want the seal the whole wall unit and the 2x10 on top does this for me with sheets of osb in the inside, some caulk and no wires inside it should be pretty air tight.

Answered by Paul Fowler
Posted Tue, 01/29/2013 - 02:20

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