after months of researching, going back and forth, pricing, etc....I think we finally settled on our walls and house construction.  Let me know what everything things and also offer any suggestions to add/reconsider.

Single story house with basement, bonus room over garage.  Some vaulted spaces (trusses).

Location:  North east Iowa, semi wooded lot to the north, full south exposure (front faces south)

Basement:  4" concrete w/ partial radiant floors over 2" XPS (should I go 2 layers of 1.5" here?  it would cost around $350 more to add another r-5.

below grade Walls:  DIY TF ICF 8" ICF walls.  From the research I have done and my construction experience, it appears that I should be able to DIY the TF walls without much issue unlike horizontal blocks.

above grade walls:  Here is where I have several options.

option 1: (the one I prefer for ease of construction and lowest cost)  2x4 framing w/ 2 layers of 1.5" XPS, dense packed cellulose, exterior strapping for cement board siding, 7/16" osb at corners w/ 1.5 +1" xps at those locations.  face of XPS to align with face of ICF foam.  Floor framing to be recessed in 3" from face of ICF and run XPS cont. over rim boards to meet ICF foundation walls.  Some where around an r- 26-28.

Option2:  Same as above, but cover entire house with 7/16" osb instead of just corners:  adds around $600

Option 3:  Same as above but use ZIP sheathing under foam instead of osb.  Adds about $500 to above price (total 1100 higher then option 1).

Option 4:  Use 2x6 framing instead of 2x4 in any of the above situations.  I like the idea of 2x4 due to ease of construction and weight of standing up walls.  If we used 2x4, we can frame them on the ground, sheath, foam, and strap before tipping up.  2x6 we can do the same, just in much smaller sections.
2x6 also cost about double for wood prices then the 2x4.  However since the depth increases, I could change the XPS so 2 layers of 1" instead to offset some of the cost and still have similar r values. 

4" urethane SIPs (around an r-26 they claim)  would cost about 4-5k more then option 3.

When pricing out 2x6 vs. 2x4 walls, were you planning on 16" OC in both cases? 2x6 24" OC would help to get costs more inline with 2x4 and will reduce thermal bridging.

I have priced both ways, yes.  Also with 2x6 at 24" oc I would need to go with 5/8" gyp instead of 1/2" which adds some more cost and 5/8" is a lot harder to work with I think.  Problem with our design is there are not very many long runs of walls sections, so 24" oc does not save that much framing. Out of 250 studs, it only saved around 20-30. (saves an entire top plate of course however at 180 LF)

Thanks for the figures. I take it that a double 2x4 stud wall filled with fiberglass didn't price out well?

Water level and depth of basement influence the basement floor insulation issue.

i would just use sips. i built with 2/6 walls with osb in the corners and foam sheathing and then sprayed in poly foam and i regret it. the cost was 5k less than just sips but foam sheathing introduces all kinds of undesirable problems. window and siding mounting are just two.

I priced out double stud with cellulose filled.  It actually priced higher then the 2x6 and exterior foam option, but not by much.  However it did have a higher R of around 10.  R per inch is cheaper though with the double stud.  The biggest reason is depth and space it takes up.  If you want to keep your rooms the same size, you need to grow out.  I did not factor in this cost knowing the materials cost alone for the wall was higher, so it was not worth seeing all the additional costs involved (more footings/basement concrete, trusses for roof and floor longer which means more sheathing, subfloor and shingles, etc).

With an exterior foam, it can hang off the outside, thus not really affecting the room size at all.  Plus a big time savings of only framing 1 wall instead of 2.  Air sealing and details are also more complex with double studs I am finding.

For roughly 2000sqft of exterior wall surface, 2x4 with 3" xps and cellulose is coming in at a materials cost of around $3100.

Would be interesting to see how much is saved by using fiberglass insulation vs cellulose. Just use a little more of it (in a double wall) to make up for its lack of performance. But I understand about then having to make the house bigger.

From my estimating, I have found cellulose to be cheaper then fiberglass.

Also, correction, I will be using Hobbs ICF and not TF.  Andy Hobbs, who lives about 1.5 hrs from me, chatted with me a good hr about everything.  Great guy, very helpful.  Great ICF system too.

I have priced both ways,too

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I have not priced it yet, but something I have been detailing is combining EEBA advanced framing technique with 1 1/2" metal hat channels attached to the outside of the stud wall running perpendicular to the studs (24" OC), then OSB sheathing on top of that. Apply spray polyurathane foam from the inside, 3-4" total (R-25 +/-) the hat chanels eliminate all thermal bridging with 1 1/2" of foam. Anyone can frame this so it's a very competitive bid and anything above 2" of polyurathane, continuous spray is all that is needed, zero air infiltration.

I wonder if you will need additional diagonal bracing to prevent collapse as the osb will not be providing a structural membrane like it normally would when attached directly to the studs.

Actually this assembly would provide as much or more lateral stability. Two lbs/cubic ft. polyurethane has a great deal of strength and by the time you get drywall on there, your set.

Posted By Gregw on 30 Dec 2010 09:19 PM
Actually this assembly would provide as much or more lateral stability. Two lbs/cubic ft. polyurethane has a great deal of strength and by the time you get drywall on there, your set.

Your probably right about the foams stability, but I would still leave the bracing (and lots of it) on till the foam is in place. Drywall doesn't count. The trades would be dead in a collapses!

I am now considering running the Hobbs to the roof.  We live in Iowa, and the location we want to build just had an F5 tornado a couple years ago go right through it.  The ICF would be continuous from footing to roof bearing, with the floor suspended with simpson hangers cast into the wall.  THis costs slightly more then SIPs, again assuming DIY.  If contractor paided, we are looking at 2-3x the cost. 

One concern I have is r value.  I know hobbs are slightly higher due to more foam, around r-28-30 range.  However air infiltration is limited to the doors and windows basically.  So how does this truely compare to say an r-40 framed or sip structure?  I understand r-40 framed or sips is still only as good as your joiints and caulking, where as the concrete is basically 100% guaranteed no air infiltration with any extra work.

this option is about $5000 more then 2x4 with exterior xps.

 

Izerarc, You are correct that the framing solution is only as good as your joints and chaulking. What you may be missing is that you due to the conduction of the studs themselves you will need to go back to a double wall to get close to R-40. You can get around it with continuous foam on the outside as I believe you mentioned in a previous post. However, you will need to to with 2x10 or 2x12 walls to get an R-40 with conventional construction.

Consider your walls like an electric circuit where the studs are modeled by a very small resistor in parallel with a very large resistor (the foam). For grins let's say the the small resistor is much smaller than the large one (e.g. wood R-value compared to foam R-value). Now if you try to solve this by making the large resistor much larger your going to be disapointed. Regards.

R value is not the best way to judge a wall assembly. 2" of spray on polyurethane gives you 93% of all you need in a wall. Beyond 4" is completely un-necesary in all but the the coldest areas.

Posted By Gregw on 02 Jan 2011 07:14 PM
R value is not the best way to judge a wall assembly. 2" of spray on polyurethane gives you 93% of all you need in a wall. Beyond 4" is completely un-necesary in all but the the coldest areas.

The marketing BS of spray foam installers seems to be the only place numbers like " ...93% of all you need..." are bandied about.  "All you need" isn't a fixed entity, it will vary by design goal.  A 2" layer 2lb foam is going to run ~ R12-R13 and will thermally perform comparably to 3.5" of cellulose cavity fill at only twice the cost.  Since that 2 inch shot of SPF is " ...93% of all you need..."?,  does that mean the remaining 7% (or ~R1) of what "...you need...", can be made up by 1/4" of fan-fold XPS on the exterior?  Good thing to know that now, so we don't actually "waste" money on that 3" of rigid XPS, eh?  R14 clear-walls, that's all we'll ever need, GREAT!!  (Who knew?)

Clear-wall & whole-wall R-value very much IS a good way to judge a wall assembly,  at least when looking at it from a heat loss and annual energy use point of view.  Whether it's more cost effective to go higher-R than 2" of premium-priced premium product like closed cell foam is another thing.  In many projects 2" of ccSPF can be useful, even worth the freight when properly placed in a high-R assembly, but at 4" you're rarely going to be able to make it work purely on an NPV calc on fuel cost savings.  But by no means is SPF the only way to achieve the air-tightness and vapor retardency needed in a high-R assembly.  (Even 2" of spray foam isn't ever "necessary" in any climate, let alone 4".)  It's good stuff, but it's not the only stuff, and NEVER the best value when going for a true high-R assembly.  As retrofit in thinner walls/roofs where there isn't space to achieve a higher-R by other means, and for structurally beefing up a roof/wall in hurricane territory there may be good reasons for going with more than 2" of closed cell foam though.  One still has to pay close attention to water vapor drives and where the SPF lives in assembly to avoid potential moisture traps when going thicker though.

In this project, with 3" of XPS on the exterior and 3.5" of dense-packed cellulose yields a clear-wall R of about R40-R45, and it WILL thermally outperform 4" of ccSPF (~ R25-ish, center-cavity) by quite a bit.  In terms of installed R/$ it's a reasonable value too.

Consider going with 5.5" of wet-sprayed cellulose in a 2x6 24" o.c. with the same XPS on the exterior to bring it up to R50.  The value of dense-packing just isn't always there if you already have an air-tight R30 outside the studs. Wet-sprayed cellulose won't settle over time the way low-density dry blown can, and 2x6 24" o.c. framing is lower installation labor, fewer boards, yet almost identical amount of board-feet and structural strength of 2x4 16" o.c. framing.

On the radiant slab  insulation, take the same amount of cash you were applying for the 2" XPS, and make it Type-I EPS instead for about double the thickness and ~50% more R.  In IA even an UNheated slab would want more than R10 in a house with R40+ clear-walls to bring the heat loss per square foot into the same ball-park as the above-grade walls.  (The actual heat loss rates will vary by soil type & moisture content.)  A radiant slab will average at least 10F warmer in winter than an unheated slab, which increases the heat-flux out the bottom.  But with 4" of low-density EPS you'll get ~R15-R16 for the same money or less than 2" of medium density XPS.  And with  4" of concrete slab on top you could literally drive a loaded pickup truck on it without running into compressive loading issues.  (Or you could spray down 2.5-3" of 2lb foam there instead. )   Taking it up to R20 or more isn't out of the question if your above-grade clear-wall Rs end up in the R50 range.  In R/$ Type-I or Type-II EPS would be hard to beat, and under a slab the extra thickness just isn't an issue the way it might be on a wall or roof assembly.

EEBA advanced framing technique, 1 1/2" metal hat chanels run perpendicular to studs on exterior, 1/2" OSB, 4" spray foam, zero thermal bridging.
A simple modification to the EEBA framing gives you solid corners with full cross bracing strength and zero thermal bridging. Does not interfere with wiring, great insulation, airtight, no taping or caulking issues, fully thermally broken at all windows and doors. and only 5 1/2" total width. This is for mixed humid climates, increase thickness for northern areas.

hat channels on exterior framing does not provide required shear resistance.  It also adds complexity around openings with flashing, or plywood to span the gap.  Also advanced framing employs 2x6 construction at 24" oc.  How exactly is the total wall thickness only 5.5"?  It is actually more like 7.5", excluding interior gyp. 
Also 4" of spray foam is one of the most expensive possible ways to insulate a house.  Tight and high R?  sure, but i can use ZIP exterior sheathing, 1.5" exterior xps (to match your hat channel), dense spray cellulose in a 2x6 advanced framed wall.  Hit the same R at half the total cost and be a stronger wall.

Posted By lzerarc on 04 Jan 2011 09:52 AM
hat channels on exterior framing does not provide required shear resistance.  It also adds complexity around openings with flashing, or plywood to span the gap.  Also advanced framing employs 2x6 construction at 24" oc.  How exactly is the total wall thickness only 5.5"?  It is actually more like 7.5", excluding interior gyp. 
Also 4" of spray foam is one of the most expensive possible ways to insulate a house.  Tight and high R?  sure, but i can use ZIP exterior sheathing, 1.5" exterior xps (to match your hat channel), dense spray cellulose in a 2x6 advanced framed wall.  Hit the same R at half the total cost and be a stronger wall.

I hope this section is legible, it will help to explain what I'm proposing. I'm modifying the advanced framing to 2x4's and changing the outside corners, top of wall and wall and window frames. The OSB will be screwed to the solid framed corners, top and side for plenty of shear bracing and without bridging. Is this the most cost effective, not sure, but it eliminates taped seams and gaps between rigid foam sheets, no need for house wrap, I think this is important.

This is the wall section for a house I'm building for myself this summer, I have a buddy that's in the spray foam business, so I'm trying to take advantage of that. If you have a better / more cost effective wall, I would love to consider it. I see what you propose above, but like this wall better.