double wall construction or rigid insulation in walls?

[Charcoals]

I am looking to build in Minnesota, so good insulation will be a key consideration.  I've been able to find a bit online about double wall construction, which also appears to be called a Larsen truss.  This construction option interests me quite a bit.  Does anyone have any experience with this, or further resources for investigating it?  If I went this route, I would use 2X4's for the balloon construction (to save on lumber costs) and also for the second "interior" wall.  (BTW, I think the thick walls and window sills look cool and lend an air of solidity, much like a straw bale house.)

On a related note, I am thinking of increasing the overall width of the two-story Universal Cottage to 22 feet so as to have an interior width of 20 feet.  Any downsides to doing so (besides the need to use longer beams for the span)?

FWIW, I've found helpful info about double wall construction at the following two sites:

    http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm

    http://www.greenfret.com/house/larsen.html

In lieu of going the double wall route, I've thought of insulating the walls with rigid insulation (e.g., polyurethane or polyisocyanurate) on account of its much higher R value.  (No need for the rigid insulation in the ceiling, where I could just use mambo amounts of batt insulation.)  I know that using rigid insulation would be pricey, but it would allow me to stick with 2X6 framing.  Thoughts?

As you can tell, my preference is to put the money in up front on quality insulation and then save on heating costs down the road.

Many thanks (and back to lurking).

Charcoals

[MountainDon]

You should check out the Building Science website.

In particular check out section BSP-034: Designs that Work: Cold Climate (Pontiac, MI)

IIRC, they promote the use of wet blown in cellulose in the walls, plus an exterior application of rigid foam along with extra attention paid to sealing against air infiltration.

They have a lot of information available. I like how they divide the buildings technologies into climate groups; a building built for best performance in MN will not be the best design for GA, for example.

[John Raabe]

This is probably the best information on current building science and the details that make it work.

[jr1318]

Charcoals,

Im building in WI, in my opinion you should just go with 2x6 walls with R 21 insulation and stick savings into better windows and doors. The cost for rigid foam would take along time for pay back and most of your heat is lost through ceilings and not walls. Im not sure what part of Mn you are from but I found a place in Blaine that blew menards away on insulation prices. The brand they sell is fibertek and the place  is called CSI supply and their phone # is 763-786-2900 ( Mod please remove phone number if not ok ). The big thing on side walls is to make sure there are no voids in insulation and air infiltration. Once again these are just my opinion.

Jim

[MountainDon]

The big thing on side walls is to make sure there are no voids in insulation and air infiltration.

Jim

Exactly Jim. And those are two of the reasons that the blown wet cellulose insulation is better than any fiberglass batt system, no matter how careful the batts are installed. The blow in stuff literally packs into all the nooks and crannies. It is specialized work and must be done by a properly trained installer. It has to be blown in to the correct density and it has to be allowed to dry properly before drywall or other interior walls are completed.

Fiberglass batts were invented for a couple reasons that mass builders love. 1. It can be installed by nearly anyone for low wages. 2. It requires no specialized equipment. 3. It also allows the building inspectors to easily confirm the factory R-value on the insulation. DIY'ers also love batts for reasons # 1 and 2 as well.

[Squirl]

I disagree with Jim.  You are in a colder climate zone than me and the building code for my place is R-24 for the walls and R-40 for the roof and floors.  I also agree that the cost of rigid foam would take a long time to pay back. The larger studs work better because you can space them 2ft o/c. 

Cellulose has an R-value of 3.2 per inch, but softwood has around 1.2 per inch.  In some thermal images of houses you can see where the studs are because of the heat loss.  The Larsen design seams to over come this by putting space between the two 2x4 and spraying insulation in between.  Another way people overcome this is by staggering the studs in a double studded wall. 

[bobtheengineer]

You've gotta look at more than just R values of the walls.  Just upping the R value of the walls, ends up being plagued by the law of diminishing returns.  Doors and windows make a huge difference.  An entire wall, can lose as much heat thru a single window, as it does the entire wall.  Another big difference, is how much air infiltration, or air exchanges you'll get too.  If you seal the place up too tight, and don't exchange the stale air, you'll have lots of problems with mold and have an unhealthy place to live.  If I was doing my own, I'd put in 2x6 walls, with fiberglass in the walls, and insulated sheeting on the outside (either Urethane, or Styrofoam).  Buy good windows and doors, and pay attention to how they are installed.  If you have the place sealed up tight, you need to get some fresh air in somehow.  Either a forced air fan, or an air to air heat exchanger.  Lots of houses built in the 90's in Northern climates had major indoor health issues.  That Larsen system looks like a lot of extra work, for not much benefit.  I can see where it'd be work to retrofit an existing house, but not very efficient when building new.  Just my 2 cents worth.

[MountainDon]

Another great resource when looking at this is the ResChek tool. Minnesota is one of the states that accepts the ResChek results in verifying that the building in question meets their energy codes. The tool takes the number of windows as well as their ratings, plus floor, wall and attic/roof insulation values, the amount in the wall cavities as well as exterior foam sheets, etc. You print out the result sheet and submit it along with the required plans for approval.

ResChek can be downloaded here. 


I personall feel that the double wall system overly complicates the building process and increases costs beyond being work and cost effective.


One More Thing: I neglected to address the question of increasing the plan width. That might require using engineered trusses rather than 2x materials.

... more... while 2x6 on 24" centers have certain insulation advantages, that also demands heavier drywall (5/8") on all walls... there are a few other concerns as well, but it can work well.

[Charcoals]

Thanks to all who've responded thus far.  I read all the posts (and several times!)

As to going to 22 feet wide rather than 20, I have a follow-up question.  What is the ballpark cost of engineered beams as opposed to 2X beams (for the same length)?  I read somewhere recently that the engineered beams are about 20% more expensive.  Does that sound right?  If so, I'm thinking it might be worth it to "go wider."  It also appears that the engineered stuff is straighter, and less prone to warp or cause floor creaking.  That only stands to reason, considering that with the glue and all, it's no longer lumber in its native state, but is more like plywood.

Charcoals

[MountainDon]

Actually, on reflection, if you use a central beam you could use 2x material for the main floor joists at 22 feet wide. However, the second floor joists might have to be engineered trusses if you are building with interior central supporting walls. For those you could use the ones built from sticks rather than laminated types. That would give room for HVAC ductwork, etc.

[DemianJ]

I agree with those above voting for the rigid insulation with 2x6 studs.  The extra complexity and lost square footage would of the double wall would almost certainty cost far more than the foam.  I think it has made a big difference in the insulation performance of my house (as well as my parent's house in NC with 2x4 and 1/2 iso foam) performance.  This is especially true with small houses which very likely have a lot of windows (and therefore lots of headers, king studs, etc, needed to frame the opening which are often closely flanked by standard spaced studs) to make them feel more spacious.  My house had very few expanses of wall with standard spaced studs, so a very high percentage of the exterior walls would be subject to thermal bridging.  Also, I'm sure you'll pick this up reading those links, but if you use exterior foam, make sure you don't use a vapor barrier on the interior walls that is non-permeable to moisture, such as standard plastic sheeting, etc (lots of posts on this, as well). 

Regarding trusses, my carpenter, who had previously only dealt with 2x joists, was very impressed with the uniformity and ease of installation of engineered trusses.  I can also vouch for the fact that, properly installed, they produce a very flat and solid floor.  Here's a picture of what I used (TJI I-joists with a H-frame of Microlamm LVL beams for each story) based on plans provided by a local building supply house:

[jr1318]

Squirl,

In WI I used rescheck and passed with out any problems, insulation is figured on the whole building not just on component. What state are you building or have you built in that requires r 24 for walls? I still stand by my statement that money is better spent on windows doors and attic insulation, like I said air infiltration is your worst enemy on walls. ( P.S. Squirl I am just very respectively disagreeing with you. )

[Squirl]

Upstate NY, just above the PA/NY border.  The inspectors here require 4" rigid foam insulation on the outside with 2x4 walls.  Another person on this site has a good walkthrough of that from my area, I just don't have there name off the top of my head. I agree with you about good windows. Attic insulation is more important than walls.  2x6 is good in my book too.  Most people forget how much heat can be lost through the studs though.  I would have to do a cost comparison breakdown to see which is more efficiant.  I guess it also depends on how many windows you have.  If you have a lot, by all means, spend more on the windows.

Did you space you space your studs 16" O/C or 24" O/C?

P.S. - Good debate leads to good ideas and a better discussion.

[FrankInWI]

I did pink board on the outside, and wet blow ("Snug Fit") on the inside.  This puppy is tight!  I'd do it the same way if I did it again, except maybe I would have found a way to spray the roof w/foam.  I am afraid of future wet cellulose with my low sloped dormers.  On the upper, the living space, I used tarpaper over the pink board.  On the lower all seams are caulked.

[MountainDon]

I love that interior shot Frank. It looks warm and efficient.   

[DemianJ]

To add to the healthy debate, I'd add that foam sheeting is very inexpensive (ranging from $8-35 for 1/2" to 2") for a small house and is very easy to install yourself (especially the thinner types).  I agree that good windows, doors, and attic insulation are important, but I'd look for other ways to save money than to not use foam.  For example, you could reduce the size (or eliminate) a few North facing windows and pay for enough 1/2" foam to wrap your house.  The cost is further reduced when you subtract the materials and labor of avoiding installing housewrap, assuming you're taping the foam (I used house wrap, as well, but I'm just paranoid).  Of course, if your budget is extremely tight and you are limited to using leaky salvaged single panes, the calculation changes and skipping the foam will allow you to upgrade to 2-3 good double-paned windows from Lowes, etc, then it might be worth it then.  All of this assumes that you can afford to heavily insulate your attic (a very small fraction of the house budget for a small 2-story house).

[Erin]

is very inexpensive (ranging from $8-35 for 1/2" to 2"

$35?? 
I can get 2" for $19...    (Menards)

[MikeOnBike]

Couple of questions.  We are planning a 20x32 1.5 story build this year.  If I used 1"-2" foam board on the outside: 

1. Do I just mark my stud location on the outside and use longer nails to install the HardiPlank/Board?

2. HardiBoard is structural so it doesn't need OSB/plywood under it.  Would the foam require that it is sandwiched between OSB agains the studs and HardiBoard on the exterior?

3. We are in rage fire country, hence the cement board.  Will the foam board melt easily if the house gets scorched but not burned through the cement board.

4. Do you frame your window opening with deeper dimensional lumber so the window still mounts to the wood, ie. use 2x6 window framing with a 2x4 wall with 2" foam?

Thanks!

[jr1318]

Squirl,

I went with 16 OC, one reason was with the loft I did it for shear stength, peak is 22 ft. I used the rescheck and it was really nice, by adding better windows and door I was able to exceed code this way. Another  nice thing was at the end it did heat calc, according ti the calc I am able to heat 950 sr ft with 28000 btu's.

[rwanders]

Have you considered spray foam (closed cell polyurethane preferably)?  It's not cheap and not a DIY method but it does allow very high R values and prevents virtually all air in or exfiltration if properly installed. You can even forgo venting a cathedral ceiling if it is sprayed against the roof sheathing. Trying to jam enough insulation into 2x12 rafters and also provide the recommended 1 1/2 to 2" vent space is almost impossible if you want R-40 or better roof rating. You do not need a separate vapor barrier if you get the proper permeability rating with the spray foam.