Double-Stud Walls

Extra Thick Walls Make Room for Lots of Insulation

UPDATED November 1, 2013

Bird's-Eye View

An easy way to build a thick wall is with alternating studs

Double 2x4 walls are built in the same way as conventional 2x4 walls. Instead of a single exterior wall, however, the house has two parallel exterior walls, set about 5 inches apart. Door and window openings must, of course, line up. Advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. techniques can minimize material use and maximize insulation.


Key Materials

No special skills or tools required

One advantage of double 2x4 walls is that the necessary materials and techniques are familiar to all framers. The learning curve for double 2x4 walls is much smaller than other innovative techniques, and no special equipment is required.

Double 2x4 walls are usually tied together with wide top and bottom plates (2x10s, 2x12s, or, if permitted by code requirements for fire-blocking, 8-foot rips of plywood or OSB). It's always important to include an air-sealing detail — either caulk or a gasket — between the bottom plate and the subfloor.


Design Notes

Thick walls don't have to mean dark rooms

The deeper the wall, the less light a standard window opening will let in. One alternative to more glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. is angling the window jambs. The beveled edges will also soften the visual transition between bright windows and dark walls.

Thick walls can create useful nooks. Don't let deeper window and door openings go to waste. Well-lit window seats, work surfaces, and built-in shelves or cabinets are just a few of the details that could put these serendipitous spaces to work.


Builder Tips

Windows may be the tricky spot

The most common construction problem with double 2x4 walls involves layout errors that result in window or door rough openings that don't line up. It pays to double-check the rough openings before the walls are built.

However, making the inner frame an inch bigger on each side makes it easier to fasten through the backs of deep extension jambs into the framing with pocket screws for a solid joint.

For good-looking wood trimmed windows, use deep extension jambs. Don't bother trying to keep the extension jambs flush with the window's frame. Instead, offset biscuit slots 3/16 in. to make a decorative (and consistent) reveal. This reveal can be seen on the top jamb. The stool should be flush, so it's best to remove the factory stool and make a custom one.


The Code

2x10 plates can help with fire-blocking requirements

There are no specific code sections dealing with double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. framing, although all the provisions in Section 602 ("Wood Wall Framing") still apply. Fire-blocking (Section 602.8) can be more difficult in double-stud walls than in conventional framing because the two stud layers can provide a chase that spreads or contributes oxygen to a fire. A simple, cost-effective way to deal with the vertical fire-blocking requirement is to use 2x10 or 2x12 plates that receive both 2x4 stud layers. Horizontal fire-blocking is also required and must occur at intervals not exceeding 10 feet. While glass-fiber or mineral-wool insulation batts that are securely retained in place are acceptable fire blocks, loose-fill insulation is not, unless specifically tested and approved to retard the spread of fire and hot gases.

OTHER CONSIDERATIONS

Net-zero-energy success story
The first house in the country to document 12 months of net-zero-energy use was a Habitat for Humanity house built in Wheat Ridge, Colorado. The home has double 2x4 walls secured to 2x12 top and bottom plates. The walls are insulated with three layers of R-13 fiberglass batts.

USING I-JOISTS AS STUDS

Instead of building double 2x4 walls, some builders of energy-efficient homes find it easier to frame walls using 12-in. I-joists as studs. That's the way builder Ed Sindelar framed the walls of a home he built for architect Katrin Klingenberg of Urbana, Illinois.

The manufacturer of the I-joists that Sindelar used, Trus Joist/Weyerhaeuser, has developed details allowing their TJI floor joists to be used as studs. For structural integrity, walls framed with I-joist studs must be sheathed on both sides with plywood or OSB. Visit the Passive House Institute US website to see some examples of houses constructed with I-joist walls.

GREEN POINTS

LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -H Points available in MR2.2 (Materials & Resources) for environmentally preferable wall framing materials, e.g., FSCNonprofit organization that promotes forestry practices that are sustainable from environmental and social standpoints; FSC certification on a wood product is an indicator that the wood came from a well-managed forest. lumber.

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 6 — Resource Efficiency: 3 pts. for each advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. technique employed, up to 9 pts. (601.2).

ABOUT DOUBLE 2x4 WALLS

Getting to R-40

Builders of energy-efficient or near-zero-energy homes often aim for R-40 walls. Although a variety of methods have been proposed to meet this goal — for example, SIPs, Larsen truss walls, thick exterior foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , or the use of I-joists as studs — most builders settle on the most affordable option, which is to use double 2x4 walls with a total wall thickness of 9 to 14 inches. The space between the double walls is usually insulated with cellulose.

The spacing between the double walls depends upon the desired wall R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. . A 5-inch gap between 2x4 walls provides room for 12 inches of cellulose, yielding an R-value of about 40.

If the walls are being insulated with cellulose, you need to choose between several possible installation methods. Usually the damp-spray method is not used because very thick walls dry slowly.

The cellulose can be blown behind air-permeable netting, or the cellulose can be installed after the drywall is hung. If the latter method is chosen, drywallers are usually instructed to leave a continuous horizontal 4-inch gap between the lower drywall and the upper drywall so that the gap between the sheets is 4 feet off the subfloor. The gap is used by the cellulose installer and is later patched.

Include a ventilated rainscreen gap and permeable sheathing

Because the exterior sheathing on a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. is colder in winter than the sheathing on a thinner wall, double-stud walls are at risk of moisture accumulation. Research has shown that the risk can be lowered by including a ventilated rainscreen gap between the siding and the sheathing.

Ventilated rainscreen gaps have multiple benefits. While rainscreen gaps may be optional for conventional walls, they are mandatory for double-stud walls.

Building scientists also advise builders to avoid OSB when sheathing double-stud walls, since OSB is moisture-sensitive. Instead, choose a more durable and vapor-permeable sheathing like plywood, diagonal boards, structural fiberboard, or DensGlass Gold.

For more information on these issues, see Monitoring Moisture Levels in Double-Stud Walls.

DISADVANTAGES AND ALTERNATIVE APPROACHES

The inside just got smaller

There are two potential disadvantages to using double 2x4-wall construction. The first is that the thick walls can rob a floorplan of interior space. Secondly, building double walls is time-consuming.

Make the foundation wider. If nothing but wall thickness is changed, a smaller interior will be the result. Designers need to keep this in mind and make the foundation larger to compensate.

Consider a different approach. If the size of a home's foundation is fixed, a home with 2x6 walls and thick foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. will take up less interior space than a home with double 2x4 walls.

If the work schedule is tight, SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. walls go up much faster than double 2x4 walls. But the total cost for SIP walls will be much higher than for double 2x4 walls, even when insulation costs are included in the calculations.

THICK WALL RETROFIT: LARSEN TRUSSES

A double row of 2x4 studs creates an affordable thick wall, but there are other ways to build thick walls. In some cases, especially retrofit situations, it makes sense to consider Larsen trusses. (For more information on Larsen trusses, see All About Larsen Trusses.)

A Larsen truss is a lightweight vertical truss attached to the exterior of a building's wall framing. Larsen trusses are ladder-like elements built from pairs of vertical 2x2s connected by intermittent gussets made from 3/8-in. plywood. The depth of a Larsen truss is usually 8 to 12 inches. Positioned like auxiliary studs, Larsen trusses have only one purpose: to make a wall thick enough to install additional insulation.

The Larsen truss system was developed in 1981 as a superinsulation retrofit technique by a Canadian builder, John Larsen of Edmonton, Alberta.

When installed on an existing house, Larsen trusses can be attached to the existing siding. In new construction, they are usually fastened to the exterior face of the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . The bottoms of the trusses are connected with a wide plywood bottom plate that cantilevers from the foundation. Larsen trusses are usually insulated with cellulose, although fiberglass batts can also be used.

FURTHER RESOURCES:

Monitoring Moisture Levels in Double-Stud Walls

How Risky Is Cold OSB Wall Sheathing?

Riverdale NetZero Deep Wall System

All About Larsen Trusses

For more information on Larsen trusses, see "Retrofit Superinsulation" by John Hughes (Fine Homebuilding No. 20, April/May 1984), and "High Efficiency At Low Cost" by Jim Young (Fine Homebuilding No. 87, Spring 1994).


Image Credits:

  1. Bob La Pointe/Fine Homebuilding SIP #15
  2. Martin Holladay
  3. Gary Williamson/Fine Homebuilding #150
  4. Dan Morrison/Fine Homebuilding #189
Tags: , , , , , ,
27.
Nov 8, 2013 6:58 AM ET

Response to Dennis McQuerry
by Martin Holladay, GBA Advisor

Dennis,
I suggest that you consult an engineer. There are too many variables to answer your question over the Internet.


26.
Nov 7, 2013 6:34 PM ET

2-story double stud walls with 2X4s
by Dennis McQuerry

Our architect is telling us that we'll have to use 2X6s as the load bearing exterior wall and 2X4s for the interior wall, since our house will be two stories with 10 ft ceilings. I'm wondering if there's any way to make both the interior and exterior walls load bearing, and if so, would it then be possible to use 2X4s for both the interior and exterior wall.


25.
May 28, 2012 4:55 AM ET

Response to Jerry Liebler
by Martin Holladay, GBA Advisor

Jerry,
With standard wood-frame construction (walls with double top-plates), there is no need for roof trusses or rafters to align with the studs. So 16-inch-on-center stud spacing is fine.


24.
May 27, 2012 9:13 PM ET

Edited May 28, 2012 6:34 AM ET.

Fiber board sheating issues
by Jerry Liebler

After much thought I'm thinking of switching from OSB to fiberboard for the sheathing on my double stud walls filled with dense packed cellulose. Outside the sheathing will be a highly permeable house wrap (Green guard c2000) and an xps rain screen (Green guard dc14) which has very low permeability. The exterior siding will be 'adhered' brick using the Ambrico "E Z brick" system. The problem is the sheathing requires studs 16" OC but my roof & floor trusses are 24" oc. I see 5 possibilities, which should I chose? 1, build the exterior wall 16" oc and align every 1 out of 3 studs & depend on the, now double, top plates and subfloor to carry every other truss. 2, build the outer wall 16" oc with every 24" grid line 4" away from a stud center-line & use double top plates. 3, same as 4 but stick with single top plates & metal ties, 4,build the outer wall with 12" OC studs, this would still allow a single top plate. 5, build 12" oc but use 2x3 for the non load bearing studs.
What should I do?
Also, since the rain screen and fiber board are better insulators than OSB and building paper I'm now considering going to a 2x4 inner wall with fiberboard on it's exterior giving me a 3 1/2" "service cavity" which is much easier to air seal. I'd still fill the service cavity with UN-faced cellulose bats to stay close to r40 for my 12 1/2" deep wall, not counting the brick,.


23.
May 27, 2012 5:06 AM ET

Response to Jerry Liebler
by Martin Holladay, GBA Advisor

Jerry,
Probably not. But the only person who can provide a useful answer to your question is an engineer who can review your plans.


22.
May 27, 2012 2:40 AM ET

Double stud walls
by Jerry Liebler

My dream house will use double stud wall. It is a single floor over unheated but insulated basement. I plan on using "advanced framing" 24" oc aligned. My outer wall is load bearing. Do I need to use 2x6 for the outer wall?


21.
Nov 8, 2011 3:31 PM ET

Edited Nov 8, 2011 3:31 PM ET.

For Joseph: advice on air sealing
by Martin Holladay, GBA Advisor

Joseph,
I suggest you visit this GBA page: How To Do Everything.

Scroll down that page to the "Air sealing" section. You will find links to 9 useful articles on air sealing.


20.
Nov 8, 2011 3:11 PM ET

Interior partition walls (non-loadbearing).
by Joseph Barry

Martin, this has been excellent help. If I had to use poly, at all, I could place it either before or after the insulation in the interior wall's cavities, given the 2/3rd rule, and use the #15 building paper after the furring and before the backboard.

Can you recommend an article (or discussion) for good green air sealing measures, please?

My first concern at this point is how to handle the pair of junctions between each of two interior partition walls and the exterior wall (the to-be-installed doubled wall). These partitions are non-loadbearing so there is some flexibility in placement under/between joists (ceiling and floor). The bathroom is stipped down to the studs now.

Cheers.


19.
Nov 8, 2011 2:57 PM ET

Response to Joseph
by Martin Holladay, GBA Advisor

Joesph,
The old rough-and-ready Canadian rule of thumb for those installing poly in the center of a deep wall: you can do it, as long as at least 2/3 of the wall's total R-value is on the exterior side of the poly. So do the math.

If your calculations lead you to conclude that you can put the poly in the center of the wall, don't put a second layer of poly under the backerboard. Stick with asphalt felt (or nothing besides good air sealing measures).


18.
Nov 8, 2011 2:50 PM ET

New insulation in both walls.
by Joseph Barry

Martin, the plan (so far) is to use new installation on both walls (and roof) -- for the very reasons you raised.

(We already have newly installed shingles and full length ridge vent across the bungalow; and the soffits look very good; insulation is the next step in the attic.)

What would you recommend as the better options for the position of poly (or #15) within the layers of this bathroom's exterior wall/ceiling?

Here are the planned layers:

Insulation in outer wall's stud cavities, that wall's interior sheathing, insulation between walls, insulation in inner wall's cavities, furring, backerboard, moisture resistant panelling.

I had planned on #15 between the furring and the backerboard. But what if I were to use ply? What would be the better options for its position?


17.
Nov 8, 2011 2:26 PM ET

Edited Nov 8, 2011 2:52 PM ET.

Response to Joseph
by Martin Holladay, GBA Advisor

Joseph,
I think your plan is risky. Unless you know the R-value of the existing 2x4 wall, it's best to assume that the old insulation was poorly installed, is damaged by rodents, is thin, and is poorly supported. Its performance might also be degraded by air leaks.

That means that most of your dependable R-value will be in your new inner wall, and it also means that if you put the poly where you propose, it might be cold in winter and therefore might be a potential condensing surface.


16.
Nov 8, 2011 2:20 PM ET

Junctions with exterior double wall.
by Joseph Barry

Thanks, again, Martin. My house is in the moderate area of N. Ontario along the shore of Lake Superior.

If I use poly, what are the best options for its position in the layers? What do you think of placing the poly after the first layer of insulation -- right on the first wall's interior sheathing?

This position would help with air tightness for the ceiling since I could run the poly continuously across the sheathing on the interior side of the roof trusses/joists. On the other hand, if I place the ceiling vapour barrier between furring and backerboard/wallboard (poly or #15) I would need to block out the gaps in the double wall as well, I think.

Cheers, Joseph.


15.
Nov 6, 2011 3:52 PM ET

Response to Joseph
by Martin Holladay, GBA Advisor

Joseph,
Unless you are in the prairie provinces of Canada or northern Quebec or northern Ontario, I don't recommend the use of interior polyethylene. I suggest that you remove the drywall and the existing poly on the wall before building your second wall.

The #15 asphalt felt that you plan to install is a perfectly adequate vapor retarder. In fact, it is a "smart" retarder with variable permeance. Don't forget to pay careful attention to airtightness when building your new wall.


14.
Nov 6, 2011 11:19 AM ET

Edited Nov 6, 2011 11:24 AM ET.

Vapour barrier.
by Joseph Barry

Thank you Martin. Rigidity is an important consideration.

This bathroom will be well-vented to the eaves in the wide overhang. However, given that the outside wall will not be altered and the double wall added, what would be the best options for location of the vapour barrier?

I was planning on #15 building paper between the furring and the backerboard on the interior double wall. A mould resistant panelling is planned for the finished wall which will adhere directly to the backerboard. This system would include the cieling which already has a 0.5 inch layer of sheetrock still intact. The interior sheathing, furring, #15, backerboard, and panelling would be layered as on the walls.

The old exterior wall had a plastic vapour barrier (untaped) on the interior sheathing between it and the sheetrock. The cieling had/has no vapour barrier between it and the insulated attic.

In light of the double wall, if I used plastic vapour barrier in lieu of builing paper, what are the best options for its location in the layers of materials on walls and on cieling?

Thanks.


13.
Nov 6, 2011 10:13 AM ET

Response to Joseph
by Martin Holladay, GBA Advisor

Joseph,
Your plan will probably work. But I should point out that the studs will be more likely to bend when someone leans on the wall, because they won't be as strong in the 1.5 inch dimension as they are in the 3.5 inch dimension. It's safer to install the studs the usual way.


12.
Nov 6, 2011 10:10 AM ET

Edited Nov 6, 2011 10:15 AM ET.

Bathroom and adding second interior wall.
by Joseph Barry

Building a new bathroom in 60 year old house. The exterior wall is 2x4 with brick. If I add a second wall, on the inside, studs offset from the existing wall, what are the pros and cons of my turning the studs face-edge toward the interior?

Why would I consider this? Less thermal bridging because of the narrower interior stud wall (that will hold up backerboard); and so more room (3.5-inch gap between bottom plates) for additional insulation between the double walls; but it would mean less insulation between the face studs of the interior wall. Would there be a net gain in efficiency or would it be a wash? Either way the wall would go from 3.5 inches to 10 inches in depth; the gap between double walls would either be 3 inches (with regular edged studs) or 5 inches (with face-edged studs).

Thanks


11.
Jun 28, 2010 6:46 AM ET

Response to Daniel Graham
by Martin Holladay, GBA Advisor

Daniel,
Your plan will work fine. If your 2x4 wall meets the building's structural requirements, then the purpose of the I-joists is simply to hold insulation and provide nailing for wall sheathing and siding.

Remember to stagger the 2x4s with respect to the I-joists to minimize thermal bridging.


10.
Jun 27, 2010 7:18 PM ET

2x4 wall plus I-Joist?
by Daniel Graham

I purchased a truckload of salvage I-Joists (BCIs) and want to build thick walls for my house with them. (Local -40 F. winter temps.)

I get comments from various sources that I-Joists are not designed for use on a vertical axis (i.e. as 'studs'). Comments come back like "get manufacturer's approval or spec" and "have an engineer calculate".

What about building a thin (2x4) wall to meet code, and add I-Joists for insulation carrier? Effectively this is a double wall, but meets code and gets me the wall thickness I want. I hate to buy that many 2x4s just to meet a checklist item, but seems that may be the least expensive way to go.

Any input is appreciated!


9.
Jan 12, 2010 6:07 AM ET

Basement windows
by Martin Holladay, GBA Advisor

David,
If your basement windows are "partially above grade" and partially below grade, the exterior of your basement walls cannot be insulated with Larsen trusses, since Larsen trusses include wood components that shouldn't be used below grade.

I recommend that you transition to a different method of exterior insulation for the below-grade portions of your walls. Most builders use XPS foam to insulate the exterior of basement walls. The above-grade portions of the the XPS need to be protected from damage by UV light and weed-whackers with cement backerboard, fiber-cement siding panels, pressure-treated plywood, metal flashing, stucco, or proprietary plastic panels.


8.
Jan 12, 2010 2:16 AM ET

Basement windows with Larsen truss retrofit
by David Argilla

How do you detail basement windows with a Larsen truss retrofit. We need to remove siding on our house, so Larsen truss looks interesting for insulation retrofit. I can see how to deal with above ground windows, but how do you incorporate basement windows that are partially above grade?


7.
Sep 4, 2009 8:58 AM ET

Excellent suggestions
by Martin Holladay, GBA Advisor

Jamie,
Thanks for your excellent suggestions. We'll do our best to address these questions in the months ahead. Stay tuned.


6.
Sep 4, 2009 8:52 AM ET

...and the openings?
by Jamie Wolf

Martin has covered the innie-outie debate regarding placement of windows in thick walls. I find thinking about the opening details - including doors - related to these important decisions to be an often glossed over part of this discussion. The wall section alone is the easy part. I'd love to see a series on thick walls and roofs that digs into the harder parts, i.e. the openings with flashings in three dimensions, the foundation/floor/ceiling/roof interfaces, the thermal bridge issues, and maintaining the continuity of air, vapor, weather, and thermal boundaries in all of the above.

... oh, and doing all of the above durably, simply and efficiently with as little foam as possible!


5.
Aug 21, 2009 12:51 PM ET

Translation
by Anonymous

translate.google.com did an acceptable job of translating the german PDF. Thanks!


4.
Aug 21, 2009 10:25 AM ET

The document is no longer on line
by Martin Holladay, GBA Advisor

TrusJoist Macmillan developed engineering details for using their TJIs as joists; however, the method was promoted only in Germany. The German document containing the necessary details had a curious title: "Balloon und Platform Framing Details" — with all of the title's words in English except "und."

When I wrote an article that referenced the document for the May 2004 issue of Energy Design Update, the German document was posted online. It has since been taken off-line.

Fortunately the document can still be accessed through the miracle of the Wayback machine. Try this link:
http://web.archive.org/web/20051111012117/http://www.trusjoist.com/PDFFi...

Of course, to read the document, it helps to be fluent in German. I'm not.


3.
Aug 21, 2009 10:10 AM ET

TJI as studs
by Anonymous

"The manufacturer of the I-joists that Sindelar used, Trus Joist/Weyerhaeuser, has developed details allowing their TJI floor joists to be used as studs."

Can you point to a source for this? A link in one of the PDF files at the Passive House US site detailing the use of TJI's as studs is no longer functioning. A search of ILevel.com site turned up nothing as well.


2.
May 8, 2009 7:42 AM ET

double wall double plate
by Robert Swinburne

I have done a number of double wall houses. my preferred method is to frame the house as a normal 2 x 4 exterior wall house then space what is basically an interior wall 1 1 /2 or 3 1/2" inside the exterior wall. stud alignment is for sheetrock and only has to line up at windows and doors. It helps to think of it as just another interior wall. Flash and batt (spray 1" to 2" foam then cellulose) works very well for this system.


1.
Apr 15, 2009 7:25 PM ET

Double framed walls vs. SIPS
by Michael Shubat

Great idea for using a 'standard' process to increase U-values, but I have to disagree on the cost comparison to SIPS. In particular when the cost of insulation is factored in, I have found 6" SIPS panels to be barely more than single wall 2x6 with batts. The labor savings of SIPS in the framing and sheathing stages (remember, every wall is framed AND sheathed when you stand it up...) combined with the low thermal bridging and 'super insulating' qualities inherent in SIPS easily offsets the tried and true balloon framed shell.


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