Double studwall idea - thoughts?
Last Post 15 Apr 2009 03:10 PM by newbiejohn. 52 Replies.
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richntiffUser is Offline
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01 Apr 2009 02:31 PM
So, I'm continuing on my merry way with house plans and dreams....

Latest idea in my head is a double 2x4 wall, with 12" front to back.  My concept is to put 8.5" of dense pack cellulose on the outer portion of the wall, with the fibernetting holding it in - 3.5" of open cell spray foam to fill up the rest of the cavity.  Note, the open cell is to the inside and will be covered with vapor barrier paint or vapor barrier plastic (PE).  I'm in northern Wisconsin, climate zone 7, -25 degree design temp.  My thought on putting the foam on the inside is to take advantage of the air sealing properties of the foam, while keeping it in a location within the wall that should not cause condensation due to dew point temps, etc. 

This assembly should give me an R43 or so wall, for as near as I can figure, about 5k less (2300sf house) than an 8 1/4" SIP wall at R32.   Predicted air exchange should be very low -  .06  ACH or below.

I'd love to hear some feedback from you guys and gals - what do you think am I overlooking anything, unforeseen pitfalls?  A foam contractor I talked with said installing on top of the netting is no problem. 

Foam contractor said I'm wearing my belt and suspsenders on this - I said I want air sealing, and I want high R - and I want it durable, maintainable, and reasonable.  Not too much to ask, right??

Thanks in advance for your thoughts!
azbuilderUser is Offline
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01 Apr 2009 07:13 PM
OK, I'll bite.

I've been using an R-42 (urethane) SIP that is 5½" thick, and I cannot imagine that your method would save me $$. Especially when taking into account the extra work you will be performing at all window and door penetrations.
Green building across Northern Arizona - VerdeHome.com
RichColoradoUser is Offline
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01 Apr 2009 11:14 PM

Two More Problems:

1) That much insulation will settle and create gaps and air movement.

2) Lost square feet of usable space will add up quickly.

 

I vote for SIPS

 

f7pilotUser is Offline
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02 Apr 2009 06:39 AM
Richntiff, I here you loud and clear. I'm looking into building a 2400 sq. ft. house in the same climate as you are. My design is 2 x 6s in the walls 16" on center and spray 5.5" of open cell foam(talked out of urethane) and put 1" styrofoam on the ouside of the walls to reduce the thermobridging. What I've found in dollars is that the sips(both eps and urethane) with frt. costs are a minimum os $6000.00 more than my method.

The question I have, will the sips save me that much more in energy costs each year than my method?

Remember I'm already $6,000.00 minimum to the good.


Good luck
toddmUser is Offline
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02 Apr 2009 07:12 AM
Hey, richntiff. Search for jklingel on this site. He buiilt a double stud wall in Fairbanks in 1980 and has improvements in mind for his next go round, including drawings he posted here. Also google habitat for humanity's zero energy pilot in Denver. They used plain ole fiberglass for R40 double stud walls, and built a raised truss system that accommodated a full 2 feet of attic insulation extending over the walls.
That would be my only comment on your proposal. Attics are so much more important than walls that what you propose strikes me as overkill, unless you're compensating for lots of glass. Then again, I live in the tropics here in Pa.
My first alternate design is something resembling a stick-built SIP: 2x6 24" OC with reclaimed EPS stuffed in the cavities and sheathing the outside. Use polyurethane foam to seal gaps between foam and studs, and spray a thin shell over the entire interior for ~ R30 in a 7 1/2 inch supertight wall. At your local box store's price for studs and www.insulationdepot.com's price for used foam, it would also be super cheap.
richntiffUser is Offline
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02 Apr 2009 09:48 AM
f7Pilot - my take on SIPs vs. double studwall is that if you detail the air sealing properly, I am starting to believe you can get a stick framed house that just as tight as a SIP house. If not as tight, close enough that I don't think the difference is appreciable.

What's crazy is I'm looking at an 8 1/4" SIP panel saying, well, it's ONLY R-32 LOL! Code is R-19 FG batt - yuk.

I agree urethane foam has large potential drawbacks - #1 in my book is it's rigidity. Lumber moves, if your foam can't move, you are screwed.

AZ - no offense, but I wouldn't use a PU panel for many reasons - and I have to disagree with your cost comparison. I have quotes in hand from a framing component supplier, as well as SIP installers, insulation contractors, etc. After it's all said and done, with me doing none of the labor (saving my limited time for electrical, finish, flooring, cabinetry), I'm quite certain the double studwall will be 5k cheaper, with higher R and equal airtightness to the 8 1/4" SIP. Does PU give you R7.6 per inch??????? Seems very high - I alwasy assume R6 for dense foams.

Construction time frame could be an issue - but since the DSW would be constructed by a wall component builder, I don't figure more than a few days more than for a SIP wall system.

I agree insulation settling and loss of footprint are issues - these are my 2 main concerns with DSW that I have to figure out. Neither one is insurmountable.

toddm - thanks for the info, jklingel has some great things to say. I'm now waiting for The Sipper to come post on this thread :-) I don't have alot of glass. If time were no issue, I agree, I would buy used foam from insulation depot and go that route - I have job, family, etc constraints that don't allow me that luxury - but believe me, I've thought about it!!!
newbiejohnUser is Offline
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02 Apr 2009 11:50 AM
Is there anyone on this site, that I could email back and forth with regarding the specifics of a double wall contructed home? Im going to need advise on the fine details of the construction..

Curious about the following..

Im reading that the best place for the inner vapor barrier is on the OUTSIDE of the INNER wall and cut slices to pump in the cellulose?

Im considering 12" wall with blow in cellulose, how to you ensure the correct compactness and can you avoid settling of the cellulose over the years.

I want to ensure at least a R70 in the cieling

How do you tie the two walls together to avoid thermal transfers

What other tricks to take care of thermal break issues.

Oh and for you guys that were all over me about ICF basement, I DID find a installer that is going to come up and do my basement ICF, but that opens a new set of questions, which block should I use for extreme cold climate in the North East New England.



richntiffUser is Offline
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02 Apr 2009 12:29 PM
I would strongly advise against any operation that compromises the vapor barrier. The right way to do it would be to have the fiber netting in there to dense pack the cellulose, then vapor barrier over that. Yes, you want the VB against the warm side of the wall assembly. With 12" of cellulose, simple blown in will be problematic - I would investigate damp spray cellulose, dense pack cellulose, or blown in fiberglass instead. Dense pack cellulose is a pro job - you need the right equipment, not the POS unit they give you at the home center when you buy so many bags. R70 in the ceiling - I think you are going to have to incorporate some foam in there, otherwise that much cellulose will be an extreme load on your ceiling drywall! I like your thinking though!
f7pilotUser is Offline
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02 Apr 2009 03:51 PM
Hey Richntiff, why don't you just spray 6" of foam in it and be done with it? I've heard from many people that once you get past the mid 20 r value it's a diminishing return. How true this is I don't know but I do know there has to be some one with that magic R vaule number.
Jesse ThompsonUser is Offline
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03 Apr 2009 10:26 AM
There was some very interesting conversation about vapor barriers at the NESEA conference this year. Several Building Science presenters stated very strongly that Vapor Barriers have been greatly oversold in cold climates and are a distant second order issue to Air Barriers (which are critical) as long as you are using some type of sprayed in insulation (cellulose, foam, fiberglass, doesn't matter which as long as it is packed and sprayed).

Here is the reasoning:

A true pressure tested and inspected air barrier on the outside of the wall system will not allow bulk air movement through the wall. Water NEVER condenses in the middle of insulation, only on surfaces (remember all those dew point diagrams showing dew point in the middle of the wall, never happens, only on a cold surface).

So, make absolutely sure your air barrier is air tight, don't use any kind of batt insulation, and you should be fine. Vapor moves in the wall, vapor moves back out of the wall, never condenses. Your ventilation system manages your interior humidity.

The consensus right now is that the Huber Zip wall system is the easiest system in a stick framed building to get an air-tight, visually inspectable air barrier right now. Tape it well, watch your wall / roof junctions, blower door it to confirm, all set.

ICF and SIP fix this air barrier / vapor issue easily, as long as you check your seams and joints, you're all set.

Best book for double wall construction details: EEBA Builders Guide to Cold Climates

Lots of cellulose contractors in Northern New England have experience with double wall installation, just ask them if they've done it before. They have to net the wall into individual cavities to stop it all from moving sideways so they can get it up to pressure.
Jesse Thompson<br>Kaplan Thompson Architects<br>http://www.kaplanthompson.com/<br>Portland, ME<br><br>Beautiful, Sustainable, Attainable
richntiffUser is Offline
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03 Apr 2009 10:55 AM
Jesse -

problem with that is getting the codes and building inspectors changed. I agree - but I don't think that a vapor barrier in a very cold climate is a bad thing, it's probably just not necessary. Some seem to think that vapor barriers are inherently evil. The issues have come from vapor barriers and lack of ventilation. My home build will either have an HRV or a 50-75 cfm continuous exhaust fan coupled with a FanCycler - the Building Science guy's solution, which I like more and more as I read up on it.

Build tight, ventilate right.
richntiffUser is Offline
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03 Apr 2009 10:56 AM
Oh - and the Building Science guys have it right - bullk moisture movement is 10 times greater for air transmission vs. vapor transmission!
Dana1User is Offline
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03 Apr 2009 06:00 PM
Posted By RichColorado on 04/01/2009 11:14 PM

Two More Problems:

1) That much insulation will settle and create gaps and air movement.

2) Lost square feet of usable space will add up quickly.

 

I vote for SIPS

 


Dense pack cellulose just doesn't settle- PERIOD.  At 3lbs/cubic foot & up it's under considerable spring tension (dense-packing can blow out poorly constructed walls) and will remain springy pretty much indefinitely.  (OK if you balloon framed it 4 storys high it might settle- add some structural support every 15' or so and it'll be fine though).  Even 2-hole blown wall cavity cellulose in 9' tall walls doesn't settle unless mis-installed.

Low density open blow in attics, etc, yeah, THAT will settle, but it never goes downright flat. Once it's at 2lbs/cu-ft or so it can't sag any further.

The double-stud-walls with cellulose fill is tried & true. Google (or google images) "Larsen truss" for lots of examples, and recommended stackups when used in conjunction with other insulation types, info on use with/without vapor barriers in different climate zones, etc. The Oak Ridge Nat'l labs has tested whole-wall R-values of Larsen truss designs- it's in their databases if you dig for it.  Exterior retrofitting of Larsen truss superinsulation onto existing buildings is being done- the principle is sound, and can be more flexible in retrofit designs than SIPs.

Not that there's anything wrong with SIPs...  If it can be done with SIPs at least price it out.  Cellulose in double stud-walls has the advantage of creating a huge hygric buffer and surprisingly significant thermal mass.  SIPs are lighter & more space-efficient and generally easy to make air-tight.  R-value per R-value the cellulose may actually win on actual performance (since there's more to heat transfer through a wall than simply R or U-value) but not in such a dramatic way that it's a no-brainer.

For me, if I wanted to retrofit R20-R40 onto an existing building I'd put 3" iso on the outside under the sheathing for R20, and for R40 another 3" iso on the inside under the finish wall.  But for new construction it's hard to beat a Larsen Truss approach on cost and flexibility.  The wall mass makes it much quieter than SIPs or more conventional stick-built- it's not a bad way to go. Be attentive to details like capillary breaks between the top of the foundation wall, particularly if you opt for using polyethylene vapor barriers or 2lb foam anwhere in the stackup.
richntiffUser is Offline
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03 Apr 2009 08:00 PM
Thanks Dana - that's a great post!

What do you mean by a capillary break on top of the foundation wall? I am anticipating building a standard floor deck, 11 7/8" TGI joists, LVL rim board, 2x8 treated mudsill, with caulking at all joints. Also will caulk the bottom plates of all walls and will be specifying caulked drywall at top and bottom plates.

Second guessing the 2lb foam idea - but I still like the air sealing capabilities of it. Is it overkill?
BirdmanUser is Offline
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04 Apr 2009 04:55 PM
I've been playing with a similar idea but for a somewhat warmer area (essentially Cape Cod - about 0F min but high wind and high humidity) My thought was to use 2x4 as the bearing exterior wall then use ripped 2x4's (1 1/2 x 1 3/4) for the interior side with plywood gussets to connect the two. I'd simply build a jig to make these up so they were 9 1/4" out to out and use a 2x10 top and bottom plate. I haven't run the numbers yet so may go down to 2x8 plate. It would be even better to split the plates too to eleminate that thermal bridge.

Here are some points:

This is often called "flash and batt" or "flash and blow" - Interior insulation is fiberglass or cellulose, respectively.

Your open cell foam should be on the outside of the assembly, not the inside. With double wall you can foam the entire stud thus killing the thermal bridge. The idea is that the foam air seals the exterior side of the wall very effectively thus preventing air movement, thus preventing

The foam should be sized so as to prevent the worst case dew point from reaching the inside foam surface. In other words the colder the exterior temp the thicker the foam needs to be so that the when the temperature gradient of the wall is reviewed. This is also related to the overall thickness of the insulation of the wall - ie the foam to total insulation in the wall ratios should remain fairly constant. The building science website has a formula for figuring this.

I was planning on about 2 1/2" of foam on the interior of the sheathing as dense cellulose for the rest of the cavity. I've talked with some cellulose guys and they assure me they can pack it so it won't settle.

This system seems to be the least expensive I can come up with having considered SIP's (which I don't like from a durability aspect), ICF's (a straight R22 wall at high cost and great thickness)

I am not planning on using a vapor barrier anywhere. As I said, I'll have very high humidity in the summer with no a/c. I will use air tight drywall techniques but no actual vapor barrier as I want the wall to be able to dry to both the exterior and the interior (hence open cell foam vs. closed) The cellulose will have the capacity to hold some moisture with out damage and then dry to the interior seasonally.

I haven't discussed this with my building official yet but he's a reasonable guy and will most likely buy into this approach.

The double wall does eat up floor area - but hey, nothing is perfect.

Good luck with your project.
BirdmanUser is Offline
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04 Apr 2009 05:04 PM
Oops
Noticed I left some stuff out

After "thus preventing" I meant to add thus prevent air movement through the wall - the air transporting moisture with it.

Also, I meant to say "when the temperature gradient of the wall is reviewed the dew point temperature should fall somewhere inside the foam."
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04 Apr 2009 05:11 PM
Or make it simple (and better) and do ICFs.
Brad Kvanbek - ICFconstruction.net
richntiffUser is Offline
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04 Apr 2009 05:54 PM
Birdman - my thought was to keep the open cell to the inside - since I'm depending on it to form the air barrier, it should be on the warm side - keep the moisture laden air from even penetrating the wall system.

ICF - the ORNL report says just a 'bit' differently for my climate. Let's not even start that tired argument.
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06 Apr 2009 09:38 AM
Posted By richntiff on 04/03/2009 8:00 PM
Thanks Dana - that's a great post!

What do you mean by a capillary break on top of the foundation wall? I am anticipating building a standard floor deck, 11 7/8" TGI joists, LVL rim board, 2x8 treated mudsill, with caulking at all joints. Also will caulk the bottom plates of all walls and will be specifying caulked drywall at top and bottom plates.

Second guessing the 2lb foam idea - but I still like the air sealing capabilities of it. Is it overkill?

"Capillary break" means a  hydrophobic layer to limit migration of moisture from the (very porous) masonry or stone foundation into the wood framing & cellulosic insulation.  Standard sill-gasket material is the right stuff, but SFAIK it's not normally sold in 2' wide versions.  Using treated wood for the sills isn't a good substitute.  Without the break, ground moisure can wick up into the wood, to be then drawn into the cellulose.  If you have vapor retarders in the wall stackup limiting drying capability in either direction this may lead to trapped moisture in the cellulose building up over time since it dries VERY slowly.  But as long as there is a barrier to keep moisture wicking from the masonry under control this is less of an issue.  That doesn't mean you needn't pay attention to vapor retarder placement in the stackup, however.

In composite polyurethane foam/cellulose wall stackups it can be done with either inward drying (foam on exterior) or outward drying (foam on interior), but if the foam isn't on the "warm" side of the wall (interior, in heating dominated climates, exterior in air conditioning dominated climates) the foam has to represent a large fraction of the total R-value.  This is to keep the temperature profile through the wall such that the anticipated seasonal dew point temperature occurs only in the foam (where there isn't any moisture-laden air that can condense.)  Cellulose is somewhat less sensitive to condensation than other fibrous insulation because the fibers are hollow and will wick & redistribute any condensation that forms uniformly so it re-evaporates within the wall, but it's still something to bear in mind.  (The R-value of fiberglass & rock wool etc. is much diminished on condensation side of the dew-point profile, cellulose less so.)

2lb foam is great stuff, but it's a near-perfect vapor retarder as well as a perfect air barrier- think about where you place it.  Half pound foam is more flexible and also a perfect air barrier, but is semi-permeable.  Unfaced polyisocyanurate is VERY permeable, but also a good air barrier (but isn't foamed in place).  Foil-faced polyisocynaurate board is less permeable than 2lb foam if all seams joints & penetrations are tape & mastic sealed, perhapes the closest thing to a true vapor BARRIER commonly used in construction (not that it's easy to measure the permeability difference between 2" of 2lb foam and a sealed seam aluminum sheet. :-) )
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06 Apr 2009 11:55 AM
So you are saying moisture off my concrete wall will wick all the way through the mudsill, up through the rim joist, through the subfloor, through the bottom plate, and into the cellulose? I can see dampproofing the top of the concrete anyways - but does the moisture really get that far?

With regard to foam in the wall now - I'm still confused as to why, if you provide a good vapor barrier, you need to worry about the location of the dewpoint temp in the wall. The vapor barrier is also acting as an air barrier, so in theory, no moisture laden air should even be able to penetrate the wall cavity, right? Please help me to understand this - I certainly don't want to build a mold incubator.

So, if you can't spray in foam thick enough to ensure the dewpoint temp is met inside the foam, are you better off not spraying any foam? I guess that doesn't make sense to me either. I wouldn't spray 2lb foam in the wall, just 0.5lb.
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