I have been reading the posts on this site for information for the last year or so, and you all are remarkably helpful.  My wife and I are about a year away from building a home.  We will be building on acreage in south central Kansas (very near Wichita) -- solidly in climate zone 4.  We have not yet decided whether to use a contrator or build ourselves.  My home-building experience consists of assisting my parents building two of their houses (and by "building" I don't mean hiring contractors -- we did all of the work except dirtwork and pouring the foundation).  My father was the brains of those operations and he would be available to assist if we decide to owner-build.  If we go that route, we would likely use contractors for half or so of the construction. I have too much regular work to do to build the whole thing myself.

With that, I think I have narrowed the exterior envelope down to two chioces -- 2x6 advanced framing with exterior foam or double studwall.  Our goal is to have an energy efficient house without breaking the bank.  If we could outperform the efficiency of an "average" home by 40% to 50% without spending an extraordinary amount, that would be our preference. 

The House will be a perfect rectangle, ranch-style with a full basement.  About 3000 feet of exterior wall (which includes a substantial amount of garage and unfinished, future expansion space). 

The advantages of 2x6 with foam are obvious -- it's relatively simple, uses standard building techniques, is relatively expensive, etc.  The downsides are that "green" contractors in my area are hard to find.  I am not confident that we could find an affordable builder who is willing/experienced enough to put in the time to get air sealing and other details right.  This might be the biggest reason that we are seriously considering the owner-builder route.  Plus, I have not checked the wholesale lumber yards, but the big box stores in the area only carry a VERY limited selection of foam board.  Are the many varieties of foam board available by special order in most areas?  How difficult will it be to find options?  Also, this construction method has always concerned me because the primary thermal break (the external foam) will look like a pin cushion by the end of construction.  Between fastening the foam itself and plugging it full of holes for furring and siding, is it really that effective?

double wall SEEMS pretty straight forward.  You simply build a second, internal wall to creat more insulation space. Obviously, the loss of floor space is an issue, but I could add a few inched to the exterior dimensions at minimal cost, I would think.  If I were to DIY this, it shouldn't even be very expensive, I wouldn't think.   Dana1 has suggested that the detailing becomes more complicated?  How so?  My biggest concern about the double wall is that I don't have any experience with it.  I just wouldn't want to dive in and be lost.  Is there a resource available that discusses the details that a DIY'er would want to know before jumping in?  Am I missing something about double stud walls.  I am a little surprised that this system is not more popular, which makes me think that there must be a reason.   

Any help with this is appreciated.

Thanks. 

Whether you choose a standard frame with high performance (foam) insulation or double frame with conventional blown-in insulation is really up to you and what you wind up comfortable doing. Either way, you ought to become highly involved in the process, almost like a co-GC with your contractor. That in turn means you need to find a GC who really understands that you are doing something different and is willing to work with you to get what you want. I don't think you want a GC whose attitude is basically "give me a lot of money and get out of the way, I've been building houses for 30 years." Finding a builder who already knows how to do it right isn't too likely, I think. But there are builders who know which way construction is headed and want to do it right. Don't be surprised to find that your understanding of building science issues, developed from your own research here and on other sites, is greater than your builder's.

My own preference for a well-insulated house naturally is the double frame, blown-in insulation design. We are in the finishing stages of our new house in central NH, zone 6. In our case, we have a 12" exterior wall cavity, with dense-packed cellulose, giving a real R40 wall. Attic is R60. The house is quite tight, with the blower door test giving only 0.65 ACH at 50Pa. The energy model I built myself shows a heat loss of just 21,000 BTU/hr at -3 F, for a house with 4,000 sqft of conditioned space. Actual performance so far seems to be no more than that, and maybe less.

For my house, I have the 2x6 outside wall as bearing the roof and deck loads, with the inner wall nonbearing 2x4, although the front wall on the lower level (walkout) holds up that end of the floor system. The outer wall could have been just 2x4 also, except that I have that and the outer sill overhanging the foundation by 2" to capture the top of the exterior foam. Where there is framing above and below the floor, the outer studs and plate go up to the top of the I-joists, so that the subfloor ties both inner and outer walls together. The rim around the ends of the joists is insulated as part of the lower wall cavity. Where the upper floor wall is supported by the foundation, the outer wall studs go down to the outer sill, while the I-joists and rim rest on the inner sill and the inner wall studs bear on the rim.  In this case, the rim is insulated as part of the upper wall cavity. In general, the 10" foundation wall is waterproofed (not dampproofed) on the outside and has 2" of XPS foam on both inside and outside, from footing to sill.

In your zone 4, you could simplify the double wall construction a bit, using a simple staggered-stud, common 2x8 plate, on an 8" foundation, with cellulose in the cavity, for a wall that is approximately R27, less the much-limited thermal bridging of the framing. Considering termites, you may want to put all the foundation insulation on the inside.  If the inner wall is non-bearing, there is no reason you couldn't pull the studs inward for a wider cavity and more insulation. Don't think of "losing living floor area." Just size the inside according to what you want, and put the walls outside of that. You're only talking inches in either direction.

One issue with wider walls is at the windows. With narrow windows, you get a sort of tunnel effect from the added wall thickness. I did what I saw in the website on the "Hanover house," which is in Hanover, NH, and something you can search on quickly. We flared out the window returns about 20 degrees from straight back from the window. This improves the viewing angle tremendously. Obviously, location of exterior doors has to be considered. In our case, our main entry has two sidelites, although only one on the hinge side would have been sufficient to allow the door to swing at least 45 degrees more than straight out.  Two other doors, also mounted to the outside of the wall cavity, open to a wall partition anyway, so that opening past 90 degrees wasn't possible anyway. Only one door had no partition adjacent to the doorway, so that one we had mounted to the inside of the wall cavity.

Think through the whole house layout and construction yourself. Imagine building it yourself, so you can anticipate problems and design away from them. Get up to speed on every aspect, including framing, continuous air barrier, insulation, mechanical ventilation, and windows. Good luck.

jkieffer,

Since you plan to build with wood, be sure to build a storm shelter in the basement.  Plans for storm shelters are available from FEMA.

A double wall design allows you to increase the R value easily, while a single wall with foam has some built in limitations, ie: 2x6 cellulose with 2" XPS (Styrofoam) will give you about R30. Changing to a foil faced polyiso gives you higher R but substantially decreased moisture permeability, meaning you should increase the R value to more than the interior wall. I think most contractors will feel more comfortable with the 2x6 & exterior foam, but they will need to clearly understand the science behind it. The foam should be held on with 1x3 "strapping", fastened through the foam & sheathing into studs with GRK or similar screws. (NOT long "drywall" screws) The siding is nailed to the strapping so there will not be many holes through the foam.

Buying the foam - in most areas there are still building supply/lumber yards other than the "big box" stores. This is where you'll find most of the material you need to build, they usually are very willing to order things they don't stock and all in all a far easier place to deal with as well as having better quality and lower prices (than HC etc) despite the hype.

S.I.P., E.R.V.,MiniSplit,Condensing water heater with radiant throughout.

Exterior foam can be glued in place with beads of caulk or foam board adhesive near the edges prior to the exterior furring to limit the number of fastener penetration. At any thickness, going with 2 layers with staggered and taped seams increases it's air-barrier tightness. Bonding the two layers with foam board adhesive and caulking the furring to the foam as you install them also increases air-tightness. You may find some of the tips here useful as well: http://www.greenbuildingadvisor.com/video-superinsulating-home-rigid-foam

Distributors that cater to commercial builders (rather than lumberyards) are likely to have a better selection of rigid foam, but even box stores will carry 1" XPS sheathing, and a few different thicknesses of foil faced iso, either of which would be appropriate. Various grades & types of EPS are commonly used in flat-roofed commercial building under torch-downs & membrane roof, and most would be appropriate for sheathing insulation as well.

Using insulated concrete forms for the foundation, and using foam on the sheathing to match thickness with the exterior EPS of the ICF will minimize the thermal bridging at the foundation. In KS you'll probably want to use either metal or copper-clad plastic sill gasket for a termite-resistant capillary break. Extend that to the exterior of the foundation foam lapped downward flashing-style for good measure. An inch of XPS (or 1.5" of EPS, any density) under the slab is also worthwhile in most of KS if you're going with R25+ whole-wall R.

An air-tight (sub 1 ACH/50) 2x6 advanced framing home with cellulose cavity fill and 2" of exterior iso (or 3" of EPS) would have well under half the heat loss per square foot of wall of "typical" or "average" code-minimum batt-insulated KS construction. (The foam alone is about the same as the whole-wal R of any 2x6 framing with perfectly-installed R19 batts once the thermal bridging of the framing is factored in. Batts are never as perfect as sprayed or blown insulation.) Choosing the right amount & type of glazing for each side of the house, and how you insulate/seal the roof or attic would then be make-or-break from a total thermal performance point of view. In general minimizing glazing size on everything but the south side, and designing the size/type & overhangs of the south-facing glass to optimize solar gains during both heating & cooling seasons can buy back an easily measurable fraction of heating & cooling costs.

Double studwalls (as oppose to Larsen Trusses) & staggered studs sharing wider plate are relatively simple to build, but with no exterior foam still have the thermal bridging of the floor & ceiling joists to contend with, and don't protect the structural wood as well as foam-clad buildings. It's primary advantage comes when building to much higher R values (as in DickRussell's example), since fiber insulation is much cheaper than foam. At equivalent "nominal R" center cavity values the foam clad building usually has a slight performance edge once you're at 2" of foam or better.

Key to either approach is air-tightness- a well insulated wind-tunnel is something you really want to avoid. (DickRussell's 0.65ACH/50 is quite good- nearly meeting PassiveHouse spec. Canadian R-2000's 1.5ACH/50 spec would be adequate in KS, but there's no such thing as "too tight".) Designing in air-tightness and insulation between the attached garage & living space areas is also a good idea, whether you insulate the garage or not. It's nearly impossible to fully air-seal between a garage and the outdoors, but it doesn't hurt to try. But design the partition walls with the assumption that the garage will be pretty much the outdoors environment, and it'll perform well even if/when somebody leaves the garage door open 24/7. (You might only go with 2x6" cellulose no-foam for the partition if you're insulating the exterior walls & ceiling of the garage, and using an R10 garage door.)

Wow, thank you all for the responses. 

A couple follow-ups:

On fastening the strapping -- what are "GRK" screws, as opposed to long "drywall" screws?  I googled GRK and discovered that it is a brand of fasteners, but GRK makes many fasterners.  Is there one in particular that you all recommend, or are you just suggesting that a high quality fastener be used?  Also, while we're delving in to the details of strapping, I have to admit that strapping is not typically used in this area.  Siding is simply attached directly to the sheathing.  Most homes use good ol' OSB sheathing, although I am starting to see quite a few houses using Zip sheathing.  In any event, is there a standard (code or otherwise) regarding how the strapping must be attached (length and frequency of fasteners, etc.)?   

On the layers of foam -- the idea of splitting the foam depth in half and staggering the seams makes perfect sense.  But, I intend to sheath the entire house in OSB (or Zip if I don't use ext foam).  I know that you technically only need structural sheathing on the corners, but we experience 100 mph straightwinds once every few years, so I think a little extra structural integrity can't hurt.  If I tape the seams of the OSB, then stagger the seams with a single layer of foam, wouldn't I achieve most of the same benefits without the extra labor of doing a second run of foam around the house?  It seems like staggering the seams a second time would result in marginal returns.  Am I wrong on this?  

Finally, if I do exterior foam, do I still need housewrap?  does it depend on the type of foam?  I had considered using Dow SIS in place of the sheathing, foam,and housewrap, but the R value is not all that high, and you're supposed to fasten it every 4 inches.  I fail to see how that can be an effective insulator when it has that many penetrations in it.  Maybe I am wrong on that, too.  Does anyone have any experience or insight on that?

Thanks, again.

 

GRKs are hardened steel with an excellent galvanized coating, plus they have a "star or torque" head, so they do not break (unlike drywall screws) and the screwdriver stays in the screw. With 2" foam you can use their "standard" 5" #10 screws, spaced about 12" OC vertically. GRKs are carried at most good lumber yards. The strapping provides a "rainscreen" allowing the siding to dry on both sides, prolonging it's life and greatly lessening the chances of paint failure. If you're using plastic siding it isn't as necessary but does help lessen the nail holes through the foam.

You'll need to use either a Tyvek type product on the OSB or ZIP sheathing for your air barrier, under the foam. I'd recommend the ZIP. You can tape the OSB only by prepainting it, otherwise the tape bond will not last. If you use OSB and Tyvek instead of ZIP, I 'd recommend caulking the OSB joints with a good urethane caulk.

My biggest concern with double wall construction is the settling of the insulation over time.
I've done enough remodel work over the last 30 years to see that when you open up a wall
many times the insulation has settled and created a gap at the top.

Just ran across this video about super exterior foam construction from Green Builder Advisor.  Thought you might find it interesting.

http://www.youtube.com/watch?v=dSqp...ure=relmfu

By the way I'm south of Wichita and am planning to build a house in the country.  I'll private msg you.   Dave

A wall with outside foam must be allowed to dry inside,therefore ,no inside vapor barrier. Outside foam should also be thick enough as insulation to prevent inside condensation. You could use wrap as insurance,in case worst happen!

Posted By BadgerBoilerMN on 25 Feb 2011 02:25 PM
S.I.P., E.R.V.,MiniSplit,Condensing water heater with radiant throughout.

Am I missing something?   Why would you do a minisplit AND a water heater with radiant?

Are you proposing two separate heat systems?

A couple of questions that are still outstanding:

Splitting and staggering layers of foam -- is this necessary if the foam is going over OSB (painted and taped seams) or zip sheathing and is staggered relative to the sheathing? I would love to not do a second run of foam around the house if it's not really necessary.

Someone respnded that I Does it matter what type of foam it is? Biob I responded that I should have Tyvek over the OSB, but I just want to make sure that I'm clear that I do not need anything over the foam (other than furring strips). Does the type of foam matter? I am considering going with EPS because it's the cheapest R/$ and I don't need that much R on the exterior in my area. I would probably go with 3 inches, for R-12, but I might be able to get by with 2" at R8. Are there any concerns with that? As I understand, EPS is more vapor permeable than most foams, allowing additional drying ability to the exterior, and it holds its R at high delta T. While the average temps in my area are moderate, we do experience snaps of extreme cold (two weeks with lows below 0 degress F this winter) and extreme heat (a couple weeks near 110F last summer), so the high delta T effectiveness would be nice.

SIP seems to be the most cost effective contruction in the Minneapolis area providing air tight construction and good insulation.

An ERV provides indispensible fress air while controlling heat loss AND humidity or latent energy as well.

A mini-split heat pump will provide heating (I like the back up and quick response for shoulder months or occasional heating), but the mini-split is really without equal for quiet cooling (chilled air) and has the exclusive feature of dehumidifying without cooling (my favorite part).

Posted By jkieffer on 07 Mar 2011 11:23 AM
A couple of questions that are still outstanding:

Splitting and staggering layers of foam -- is this necessary if the foam is going over OSB (painted and taped seams) or zip sheathing and is staggered relative to the sheathing? I would love to not do a second run of foam around the house if it's not really necessary.

Someone respnded that I Does it matter what type of foam it is? Biob I responded that I should have Tyvek over the OSB, but I just want to make sure that I'm clear that I do not need anything over the foam (other than furring strips). Does the type of foam matter? I am considering going with EPS because it's the cheapest R/$ and I don't need that much R on the exterior in my area. I would probably go with 3 inches, for R-12, but I might be able to get by with 2" at R8. Are there any concerns with that? As I understand, EPS is more vapor permeable than most foams, allowing additional drying ability to the exterior, and it holds its R at high delta T. While the average temps in my area are moderate, we do experience snaps of extreme cold (two weeks with lows below 0 degress F this winter) and extreme heat (a couple weeks near 110F last summer), so the high delta T effectiveness would be nice.

Wherever the primary air-barrier is located, staggering and taping the seams creating secondary air-barriers helps maintain the long-term air-tightness of the assembly.  You can't make it too air-tight, and the less air movement between layers, the less potential there is for air-transported moisture ending up in structural wood.

ONLY if the ratio of the exterior foam to total R is high enough for the climate that the mean January temp is at least in the 37-40F range or higher (the dew point of 30-35% RH 68-70F conditioned space air) it doesn't matter what type of foam it is, since you can set it up to dry toward the interior.  At ratios below that you can't use goods low perm ratings below ~0.5-0.75 or so or you risk trapping interior-drive moisture in the sheathing.   With XPS the perm limit comes at 2" (R10), but you could put up to 4" of  Type-II EPS (R16), or up to 5" of Type-I EPS (R18) and still retain exterior-drying.  Any thicker than that, you'd have to increase the vapor permeabilty of the interior to avoid the moisture trap, which could have consequences if the foam wasn't sufficient high R to keep the sheathing from accumulating interior-moisture for weeks on end.

While unfaced EPS  has a relatively high perm rating, any foam with a poly, foil, or vinyl facer will have an extremely LOW perm rating, and would qualify as a vapor BARRIER.  Protective facers on EPS are common- be sure to use the right stuff.

The weather resistant barriers such as housewrap should be over the foam, not between thfoam and sheathing, unless you use a foam rated for WRB applications which typically require using a Z-flashing type taping method the outer lae yer to meet spec as a WRB.  See:

http://www.ujr.ca/EN/PDF_Support_Fi...-guide.pdf

All foams retain R better than low-density fiberglass at high delta-T, but all have drift with temperature (and none have the constant value with delta-T of dense-packed cellulose).  The curves for EPS and XPS are similar, gaining R with falling temp, and an excellent choice for exterior foam in heating dominated climates.  The K-value for Type-I EPS at 75F is around 3.75/inch but that rises to 4.25/inch at 25F.   Polyisocyanurate's K-value falls with temperature, and in climates with January mean temps between 0-25F it should be derated from a design point of view to R5.6/inch (despite a 75F rating of 6-6.5/inch.)  Below 0F you'll get similar or better performance out of XPS at a given thickness. 

The down side to low density EPS is that at high temps that K factor drops to about 3.25/inch or less.  See: 

http://www.transconsteel.com/produc...of_EPS.pdf

So when it's 110F outside and the sun has heated up the siding to 130F, it's probably fully 25% less than it's 75F rating.  But in heating dominated climates the cooling season performance hit is of minor consequence- especially if the house has a decent amount of thermal mass.

Posted By rpatterman on 26 Feb 2011 12:42 PM
My biggest concern with double wall construction is the settling of the insulation over time.
I've done enough remodel work over the last 30 years to see that when you open up a wall
many times the insulation has settled and created a gap at the top.

Settling of wall-cavity celullose is primarly a low-density dry-blown goods issue. If you use wet-sprayed cellulose (with water activated adhesives) or dense-pack it to the proper density for the climate (< hey, it's even in English now!) it'll be stable over many decades.

In a double-wall configuration you have the option of keeping the top of the inter-studwall space open at the top, continuous with the attic/ceiling insulation, which reduces thermal bridging, and allows easy inspection & remediation should it ever sag in the coming decades.

Sounds like alot of trouble to me. This what I did to my own 1921 balloon frame farm house. I think I would build a new one the same way, if not for SIP construction.

2x4 construction, 1" polyisocyanurate.

http://www.jm.com/insulation/building_insulation/products/bid0022_ap_foil-faced.pdf

2# foam on the inside.

http://foamminneapolis.com/spray-foam/spray-foam.shtml?gclid=CNSp6KWYvacCFZFoKgodb3t1Bg


All this made up stuff gives me a headache.

Building with 2x6" 24" o.c. is the same number of board-feet and the same structural strength as 2x4, 16" o.c. construction, but fewer boards to cut, and adds R6-7 to the center-cavity R value, and reduces the framing fraction for less thermal bridging area, and adds ~ R2 to the thermal bridges themselves.

Putting as much a 2" (or even 3") of foam on the exterior isn't dramatically more work than putting an inch, yet putting ONLY an inch of foil-faced goods on 2x4 construction is pushing the margins of good design in MN. The mean January temp in Minneapolis is in the mid-teens, and with only ~30% of the R outside the sheathing you'll be accumulating higher than recommended wintertime moisture in structural wood. With foil-faced iso on 2x4 you'd be better off with at least 1.5" in your climate. If only going an inch, XPS would be a better choice, since you could then reduce the permeance of the interior with vapor-retardent latex, and the assembly could still dry through the foam, and you're only giving up ~ R0.6 of winter-performance. Foil faced goods force the assembly to dry toward the interior only, but when the sheathing temps average below the room-temp's dew point, it can't dry at all, but rather accumulates moisture during those periods. If the stud cavities are blown with cellulose the moisture buffering of the cellulose can be protective, limiting the amount of moisture accumulating in the sheathing and studs by redistributing it in the insulation, released to the interior as the seasonal temps rise.

Foam cavity fill would significantly underperform an exterior foam + fiber fill approach in 2x4 construction due to the severe thermal bridging of 2x4s. Sure, 3-3.5" of closed cell foam would give you a center-cavity R of ~18-21, but with even 15-20% framing factor, with the R3-4 of the studs & plates the heat loss through the framing would dominate the heat loss of the whole assembly. With 2" of iso or XPS on the exterior of an R13 fiber fill you'd have ~R23-24 center cavity, but R13+ on the framing instead of R3-ish, which is a HUGE reduction in heat loss from framing. Even an inch of XPS + R13 fiber in the cavity would outperform a 3" cavity fill of 2lb foam without exterior foam at a fraction of the cost. The center-cavity R would be about the same, but the thermal bridging would be less than half that of the 2# foam fill case.

With 1920s balloon framing, dense-packing cellulose to 3.5lbs density (or noo-skool fiberglass such as JM Spider or Certainteed Optima, to 1.8lbs density goes a long way for reducing infiltration in the assembly. You'd get 10-15% higher center-cavity R out of the fiberglass (a much lower improvement in whole-wall R due to the thermal bridging), but none of the hygric buffering protection that cellulose provides. Going with the higher-R fiberglass would add to the minimum recommended thickness on the foam- center-cavity you'd be at ~ R15, so you'd be looking for at least R12 on the exterior to be protective in Minneapolis.

Double-wall approaches only make sense if going for super-insulation levels. It's possible to hit mid-30s for whole-wall R values with a relatively straigthtforward exterior foam on 2x6 advanced-framing with 3-4" of exterior foam and cheap blown/sprayed fiber cavity fill. Of course you could also get there with a 10-11" SIP, which isn't a bad approach. See Table 16 on p60 (p63 of the .pdf pagination)

http://www.buildingscience.com/documents/reports/rr-0903-building-america-special-research-project-high-r-walls

Hi jkieffer,

If you are still open to other options, there is a LOGIX plant in Haysville. Since you are going to have a basement and may be building the home yourself, you might find that with ICFs you could have a much stronger, safer home for the same cost. I have had owner/builders pay for all the ICFs and concrete for two floors for what they would have been charged for the basement alone (less than 20K). You might do a cost comparison between ICFs and poured basement/framed with foam sheathing.

It's not an option for everyone, but it sounds like you have a lot of good hands on experience. Plus you can get all the technical assistance you need from the local factory. If you are interested, contact Form Systems at 316-522-9285. There are also a lot of ICF homes in the Wichita area if you care to visit any.

Good luck with your future home.

John

jkieffer,

If you want an other alternative to the standard double stud wall, take a look at the build Thorsten Chlupp just build up in Fairbanks.

http://www.greenbuildingadvisor.com/community/forum/general-questions/19942/sunrise-home

Inner load bearing stud frame, dense pack cellulose both inside and to the outside truss area. Plywood air/vapour barrier on outside of 2x4 (or x6 frame).