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Double stud wall construction-anybody seen it/done it?

Post in 'The Green Room' started by Badfish740, Oct 7, 2010.

  1. Badfish740

    Badfish740 Minister of Fire

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    Since researching superinsulated home building techniques has developed into somewhat of a hobby I ran across a new (to me) method that seems pretty straightforward-the double stud wall:

    Double stud wall with spray foam

    R-35 or so walls sound pretty good to me. The interesting thing is that they only use 2" of spray foam (to keep the cost down I'd imagine) and fill the rest with cellulose. I wonder what a wall full of spray foam would be rated at (and what it would cost).

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  2. dougstove

    dougstove Member

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    In my area most of the better contractors are doing double wall 2 x 4", more or less as described in the building science article.
  3. lukem

    lukem Minister of Fire

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    Closed cell foam R value is about 50% higher than cellulose.
  4. Badfish740

    Badfish740 Minister of Fire

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    Wow-so it would probably be safe to say that R-45/R-50 would be possible will all closed cell foam.
  5. Burn-1

    Burn-1 Feeling the Heat

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    The modified Larsen Truss is also a great way to make a larger insulation cavity.

    Superinsulated home using Larsen Truss

    The 2,000 SF house in the article above only needs 4/5 of a cord for a winter in Western Massachusetts.
  6. Reggie Dunlap

    Reggie Dunlap Feeling the Heat

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    That's a good way to build walls, but very expensive. In a mild climate like NJ I'm not sure it's worth the expense. The first inch of urethane foam does 90% of the insulating, every additional inch helps to a point of diminishing returns, somewhere around 2-3 inches in walls. I'd concentrate on making the walls airtight with minimal foam and putting the savings into roof insulation.

    There are less expensive ways of creating a thermal break so the framing members don't conduct heat outside:
    -Install 3/4" strapping on the inside of a 2x4 or 2x6 wall and fill the entire cavity with foam and/or cellulose.
    -Install rigid foam on the exterior of the wall with siding installed over the foam.

    Depending on wall thickness you may need extension jambs on doors and windows if your framing isn't l isn't 6 9/16" deep.
  7. btuser

    btuser Minister of Fire

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    You only lose about 10% through the walls. It's not worth the money. There's an argument that your wasting money when it comes to 2x4 vs 2x6. The whole point was to have 2x6 walls 24" on center but everyone builds 16" on center so you're gaining very little.
  8. nate379

    nate379 Guest

    Most houses around here are built with at least 2x6 walls and if not that, then 2x8. I have even seen some up toward the interior with double 2x6 and 2x8 walls.
  9. vvvv

    vvvv New Member

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  10. northernontario

    northernontario Member

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    My sister is currently building a straw-bale wall house using a larson truss wall setup... Truss' are the load-bearing component, bales are in-fill, then plaster over top of it all. Easily R40+ in the walls.

    Combined with an insulated concrete floor, insulated concrete foundation, well insulated attic (I believe she's doing atleast R50), designed for south-facing solar gain... it's gonna be a toasty little house!

    Of course, spray-foam has an aged R-value of ~R6... so on a 2x6 stud wall (5.5" depth) you're looking at roughly R33 in the walls. But the huge advantage to sprayfoam is that it seals all air movement... so you achieve a true insulative value... doesn't matter if the wind is blowing or not. The problem with fiberglass batts is air can move through them... so if you haven't sealed off all air movement, their insulative value drops.

    And yes, you have to remember that insulation follows the laws of diminishing returns.
  11. vvvv

    vvvv New Member

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    wood expands & contracts with humidity, so will the sprayfoam develop cracks eventually?
  12. Badfish740

    Badfish740 Minister of Fire

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    Good points-thanks. I will take issue with one of them though-NJ's climate is indeed mild compared to Vermont...if you're talking about winter ;) I'm really less concerned about about losing heat in the winter than I am with gaining heat in the summer. Our summers are humid and nasty and it seems like we get more 90+ days every year. The home we're planning is not going to be huge (just under 3000 sq/ft) and I plan on heating with a gasification boiler. In terms of property, a woodlot capable of producing at least a cord per year will be a requirement, so a supply of free fuel + the efficiency of a gasifier (DHW needs will be mostly taken care of by flat plate solar collectors) = heating cost isn't that big of a concern. Sure, I don't want to be burning 5 cords a year like I am now, but if I could get down to three for a house that size I think I'd be happy. Anyway, back to our brutal summers. Right now in our 1960s era ranch with fiberglass in the walls and ceiling and a fair amount of air leakage, air conditioning will add anywhere from $100-150 a month on top of our normal electric bill. That's not with keeping the house at 68 degrees either-we're lucky if it gets down to 74. The point is that I would love to be able to keep the house at 68 in the summer and not spend hundreds per month on electricity. I'd have to find some way to run the numbers, but I'm betting that R-40 or so walls would help with that a great deal. I'd also love to either install photovoltaics during construction or later down the road, and super-insulating the house along with other cost savers such as LED lighting and no resistance heating loads (wood heat and hot water + propane for cooking and clothes drying) would probably allow me to have a negative electric bill for the life of the home. Upfront costs would be certainly be higher, but if we stay in the home 30 years (or more) the payoff would be huge.
  13. btuser

    btuser Minister of Fire

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    Sprayfoam sticks like glue. It would take a large earthquake. Eliminating the thermal bridging in conventional structures is a serious upgrade with the double-wall construction. I'd say maybe 10 grand for an average house for the walls, then you're looking at jamb/trim extentions.

    I bet we'll see this as a retro-fit to McMansions in the future. You'll lose some square footage but what's the big deal in a house 3x bigger than you need?
  14. vvvv

    vvvv New Member

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    equake is sudden impact. wood expand/contract by humidity involves many cycles of little impact. if foam gets rigid, it should crack where its "glued" to the wood.........ithink
  15. Badfish740

    Badfish740 Minister of Fire

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    I should add that we'll be DIYing it. Another reason we're looking at double stud construction vs. something like SIPs is because double stud is more labor, but no more complicated than conventional framing. We're going for a rustic look so jamb and trim extensions will hopefully be rough sawn lumber from trees on-site. Plus, let's say that we stay in this home 30 years (hopefully more) and that electricity rates DON'T go up :lol: yeah right...but let's assume that to make the math easy. Last year I spent about $1800 on electricity. $1800 x 30 years = $54,000-remember, this number is not adjusted for inflation, rate hikes, or anything else. When you factor in rebates, tax credits, etc...that might cover the extra cost of double stud walls, LED lighting, and photovoltaics, but when you also factor in the power that would be sold back to the grid over that time AND the sale of the SRECs over the life of the system (currently trading around $600/Mwh and only showing signs of going up) the home could possibly pay for itself in 30 years.
  16. Reggie Dunlap

    Reggie Dunlap Feeling the Heat

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    If your set on eliminating the thermal bridging it's going to be easier and cheaper to sheath the exterior in rigid foam. In your climate I don't think any method of eliminating bridging is going to add up to much savings over the long term.

    Another option is to build standard 2x6 walls, install one inch of spray foam against the plywood sheathing and put fiberglass batts in the remainder of the stud bay. That would give you an airtight wall and accomplish 99% of what the double wall would, at a fraction of the cost.
  17. btuser

    btuser Minister of Fire

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    There's no way for the wood to swell when its encased in foam.
  18. Reggie Dunlap

    Reggie Dunlap Feeling the Heat

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    Foam can crack if it's installed improperly. I've seen it happen when they mixed the chemicals wrong and when it was applied in temps that were too cold. Both times the cracks showed up with a day or so. I always wait at least a week before covering the foam up just to make sure.

    Other than that foam is pretty rugged. Seasonal movement of framing members doesn't cause any problems.
  19. vvvv

    vvvv New Member

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    encased is front, back,sides, top,& bottom?
  20. semipro

    semipro Minister of Fire

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    Some foam is more flexible than others. The canned type sold for use around windows and doors is more flexible than the regular type.
  21. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    Here is a nicely done home that uses double stud walls and cellulose:
    http://www.builditsolar.com/Projects/SolarHomes/MAZeroEnergy/MAZeroEnergy.htm

    Some more double stud wall material here:
    http://www.builditsolar.com/Projects/SolarHomes/constructionps.htm#Double

    If you want the lowest cost way to get wall R values up in the mid 20's with not thermal bridging, I think the horizontally strapped or Mooney wall is a good way to go:
    http://www.builditsolar.com/Projects/SolarHomes/constructionps.htm#Stick
    The first "gimmie shelter" example is very good, and the Mooney wall entry is quite detailed.


    The Larsen Truss is another not so expensive way to get high R values with minimal thermal bridging:
    http://www.builditsolar.com/Projects/SolarHomes/LarsenTruss/LarsenTruss.htm

    You can use my Home Heat Loss calculator to play with heat loss from each componetnt (walls, ceilings, windows, infiltration) separately -- just plug in the degrees days for an area near you and enter the R values for the kind of construction you are interested in. It will give the loss through each component and the cost of the energy you are using for each component -- you can play around with improving each component independently. With respect, statements like only 10% of the heat goes out the walls are kind of misleading in that they assume some typical construction of the other components.
    It literally only takes 5 easy minutes to see the actual heat loss and dollar numbers for your the house construction you are interested in in your area.
    http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm
    The calculator is free -- if you have any trouble with the inputs, drop me an email.

    When you do the wall R values, bear in mind that the the thermal bridging of the studs in conventional walls cuts the R value by a fair bit. A wall insulated with R 19 insulation ends up more like R15. The whole wall calculator will give you actual R values for all the common wall constructions that include thermal bridging.
    http://www.ornl.gov/sci/roofs walls/AWT/InteractiveCalculators/rvalueinfo.htm

    I'm personally not convinced that the spray foam is needed as a component of a double stud wall. Many many double stud walls have been built with just dense pack cellulose insulation, and I've not heard bad things about these. The foam (especially if closed cell) is nice, but substantially increases the cost of the wall. If you put a few bucks into being careful not to build in water vapor and condensation problems, its probably not needed.


    Gary
  22. benjamin

    benjamin Minister of Fire

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    Gary's a hard act to follow, well done!

    The foam article seemed very leery of moisture issues. I don't like to hear that everything has to be detailed just right or you'll end up with problems.

    Two vapor barriers (inside and outside) is not a good idea. Does the magic of spray foam change this? Reggie Dunlap may be right here, exterior foam has a pretty good record despite being less than ideal regarding the "2/3 rule" (vapor barrier on the inside third of the wall), that's assuming there is no interior vapor barrier.
  23. StackedLumber

    StackedLumber New Member

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    We've had great success w/ the "flash and batt" way of insulating w/ 2 inches of spray foam that gave us 7.1-8.1 R value per inch and then stuffed the rest of the cavity w/ R-19 batts. We did have one spot that did crack and we ended up cleaning it out and giving it a second application of foam. Not sure what caused it to crack, but my guess would have been the temperature b/c it was pretty cold when we sprayed.

    Also, the rigidity of the spray foam is amazing, way more than the stuff from a can.
  24. precaud

    precaud Minister of Fire

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    Hi Gary, I'm glad to see you here at the Hearth.com forums. Just in case folks didn't put 2+2 together, builditsolar.com is Gary's website. And in my opinion he has done more to make DIY solar building accessible to more people than just about anyone. The solar air heaters I built onto the side of my home are a combination of one of Gary's designs, Nick Pine's concepts, and early work done in the Taos, NM area using low-mass passive solar collectors. If you're thinking of adding solar of any type to your home, spend some time perusing Gary's website, it's the best resource there is.

    Sorry to blow your cover Gary, but you deserve the thanks and respect! :)
  25. nate379

    nate379 Guest

    Had I been around when my house was built (I bought a "model" home) I would have paid extra to have 1/2" "R Board" put on the sheathing before the siding went up. My walls are R21, but the R board would help "decouple" the studs. Right now sometimes when the frost is just right or it's very cold like (-30*) there is enough heat transfer from the studs that it will show where they are on the siding even!

    Some of the other houses that were built they put a thin foam between the sheathing and studs. I don't really like that idea because the OSB to stud should be tight against each other for a good shear wall. With the foam the OSB is floating on the studs a bit.

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