Stick framing and very cut up usually means a higher framing fraction, more thermal bridging, and greater difficulty in air sealing. Even with a SIP roof this would be true, making the argument for putting the pressure & thermal boundary at the attic floor and venting the attic rather than going with a SIP roof or unvented roof approach.

I don't quite understand the rationale for 4-6" of foam with a foot of blown goods above that though. A 2" shot of closed cell is more than enough air-sealing and vapor-retarder, and you won't even need that if you have a crew well versed in air-sealing by other methods. It's hard to beat the low cost & efficacy of blown cellulose in an attic floor situation, and the framing or trusses can be designed to accommodate any arbitrary depth.

To go unvented with fiber on the attic floor, foam on the roof deck splitting the R, in zone 5 you'd have to put at least R20 as foam at the roof deck to approximate code, which would be 3.5" of closed cell foam, or 6" of open cell, which may be what they were talking about with the 4-6" (??).

Hopefully you're using ICF or some other method of insulating poured concrete walls(?). Be sure to add a capillary break between the footings and walls (sheet metal or membrane) to keep ground water from wicking up the wall, as well as at the foundation sill. If going with an insulating sheathing approach, design it such that the exterior foam of the ICF aligns with the exterior sheathing (or SIP), even if it means the foundation sill is cantilevered off the concrete a bit.

In zone 5 it's also cost effective to put 1.5" of XPS or 2" of EPS under the slab too, floating the slab from the foundation walls with 2" of foam as a thermal & capillary break. Once you've poured the slab there's no going back, but with 3000' of basement floor against ~50F subsoil it becomes a very real heat loss. If your geo output is going to be running hydronic radiant floor or other low temp radiation for maximum comfort & efficiency doubling the thickness of the foam is about right. If it's a forced air system (or if you take the ductless mini-split option) going more than 2" of EPS would likely be money wasted.

I've linked to this before, but I will again: To think about the economics of different pieces, read the at least the first chapter of this document, and ponder the values in table 2, p 10

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

The R value recommendations are "whole-assembly" , not center-cavity stick built numbers. To hit the ~R30 wall value recommendation for zone 5 in a stick built would take 2x6 framing with cellulose fill + R15 in exterior foam (2.5" of foil faced iso would do it, and is fairly straightforward to build.) In an OSB clad SIP that would be an 8.5" ish wall thickness before adding the siding and finish interior. Either would be fairly easy to air-seal with bit of diligence.

hometech: "The floor does not need to be SIPs. you can fill the cavity with some fiber, and use bubble foil under the floor joist. " I'm not sure what problem you're addressing with the bubble foil. Unless it has 1" or greater air gaps on both sides it's thermal performance is negligible. Even with the 1" air gaps it's average performance would barely exceed that of filling that 2" of space with cellulose in horizontal installation. It's also a very strong vapor barrier, which is a double-edge sword, which can create as many problems as it solves unless it's placement & installation are very well thought out and implemented. I've yet to see a really good case for bubble-foil in any application, but maybe it exists in some space-constrained apps.

The first ICF bid contractor that got back to me said he wouldn't recommend it due to the number of corners and the likelihood of expansion. He suggested a poured wall and install foam on the outside before backfilling and to stick frame the inside and spray foam it all the way up to the bottom of the subfloor. He's convinced there's no way the walls will stay "straight enough". Still waiting for a second opinion (other ICF builder).

We'll be doing forced air and expect a lot of carpet in the basement eventually so I'm not sure how much the effect of the 54-55 degree dirt will be (that's our well water temp anyway). Guess I've never looked at the R value of carpet pad and carpet. Any thoughts on that? Will definitely be rolling out plastic before the pour to keep moisture/vapor from wicking up. Did that on our current house and our basement (not a walk-out) does not smell "like a basement". I talked about radiant heat with my HVAC contractor. He commented over and over how "nice" of a heat it is. I asked him point blank will I see energy efficiency gains from it over forced air to pay for the difference. He said the benefits of radiant are comfort and that I'd never see a financial benefit.

At this point I'm leaning towards a vented attic with sprayfoam right on top of the drywall for air sealing the ceiling. Seems like it would make more sense than trying to heat/cool the extra cubic feet in the attic. The mixure of foam/fiber is 4-6" of foam on the attic floor with addtional blow in on top of that, nothing in the rafters at all.

In terms of of concerns about ground water wicking up to the point of saturation or even seeing dampness, I'm not sure that's much of a concern. I'm building on top of a ridge that drops 70+ feet over roughly 500 feet to the back and drops 40+ feet over 400 feet to the front (it's in the middle of a corn field). The sides slope away gently. Don't get me wrong, I don't want water probems but the new house will be a walk out where my current one is not. The current one has 2 pits in it of which one has ever seen even a drop of water and that was because I had a poor grade at the time. After some landscaping I've done nothing but vacuum some dead bugs out of them over the past 10 years. The soil drains up here incredibly well. I'm building 1 mile north of our current home and it's the same soil type.

One "new" question I have... I appreciate the value/benefit of a thermal break between the outside and inside. If I end up stick framing/spray foaming the walls, I definitely want to explore the cost of adding rigid foam to the exterior before hanging the siding. One concern I have is how do you deal with the depth of the windows? I don't want the windows to look like they're recessed.


I'll certainly do some reading and appreciate the link.

Thanks again!

I would recommend heading over to www.greenbuildingadvisor.com and reading about things on that site. There is tons and tons of details and explaination on many of the questions/concerns you have. There are many details showing how foam is installed on the exterior of the home as well as around windows. Where at are you in western Iowa? I am in NE Iowa, just over the zone 5/6 line. Where did you get sips quotes from? There is a sips company in Carroll, IA.

I'll check out the site. Thanks! I'm just east of Missouri Valley. The quote I have so far is from Enercept although their dealer has been trying to get them to adjust their price after looking at some math. He really wants to see these take off in this area for obvious reasons. I'm waiting to hear back from EPS out of Graettinger, IA. Who is the company in Carroll? I'd love to get a quote.

I have family that lives around MV, very familiar with that part of Iowa. The company is Iowa Sips, owned by FAC. www.iowasips.com They are a urethane SIPs, which gives you a much higher r per inch then EPS. However I will say your price is slightly high, but not overly high. Quotes I have been getting have been around 5.20-5.50 shipped and taxes for 6" EPS panels, headers, splines, foam, house wrap, etc.

There are both "innie" and "outie" ways of setting up the windows in thick walls, and it makes a difference on how the flashing & weather-resistant barrier are located in the stackup when doing exterior foam. It's worth cruising through the various "how to" links on this blog:

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing

http://www.greenbuildingadvisor.com/category/site-wide-tags/foam-sheathing

If you're putting down carpet in the basement there's an even greater reason for putting at least R5 under the slab beyond mere energy savings (but R7.5-R8 is lifecycle-cost effective just on utility savings.) The carpet and underlayment will be ~ R1-R2, which will make the slab colder, leading to an increased risk of mold on the underside of the carpet. (The risk period is in the summer when the air temps and room humidity/ventilation air are higher- the slab temp will be pretty constant year round.) Putting insulation under the slab keeps the temp at the concrete/underlayment interface well above the dew point of the interior air. Even at 70% relative humidity (at the slab's temperature, not the room's) is sufficient to send the mold risk sky-high. If it's 75F and a comfortable 55% relative humidity in the room the dew point of that air is ~58F. If the slab is even a relatively warm 68F or lower under those conditions the air that migrates through the rug rises to 70%RH+, and mold growth on the underlayment can be pretty rapid. But with R5+ under the slab the slab temp will track the room temp within few degrees even with carpeting above.

It takes at most a couple inches of closed cell foam to air-seal the ceiling gypsum, and at that depth it's a class-II vapor retarder (between 0.5 and 1 perms). Given that it's roughly 17cents/R/sq-ft, vs less than half that for cellulose, going deeper on the cellulose and thinner on the foam seems like a much better value. At 2" closed cell foam begins to be structural too, helping to support the load of deeper cellulose without sag. (I guess I still don't quite understand the rationale for 4-6" of foam here.) When it's between the joists the extra R/inch of the foam is wasted by the thermal bridging too- even if you replaced 2-4" of the foam with cellulose to maintain the exact total depth from gypsum to the top of the insulation, the effect on thermal performance is quite small. Since closed cell is best sprayed in lifts of 2" at a time (to avoid shrinkage & bonding issues, and to avoid self-igniting while curing), limiting the ceiling foam to 2" would reduce the overall installation time too, since it's one pass rather than 2-3. (If your installer claims they can shoot 2lb foam in lifts thicker than 2", run away- find a different foam outfit. The manufacturers don't support that, and the risks of problems go up.)

If they're talking 4-6" of open cell foam, air-sealing isn't as assured with open cell foam, and it provides no structural support. A 2" shot of closed cell with deeper cellulose would still be a better value.

The first ICF bid contractor that got back to me said he wouldn't recommend it due to the number of corners and the likelihood of expansion.

Make sure you have an actual ICF Contractor. That would be one who has experience building with and likes to build with ICF. The first half dozen "ICF Contractors" I sat down with on my build attempted to turn me to conventional construction. That's right, there were 5 or 6. They all said they built with ICF, but it turns out that there was nearly no ICF in any of their backgrounds.

@ICFHybrid - The ICF bid I have is from a guy who has been building ICF foundations for over 10 years. He sold his poured wall forms and does this and flatwork exclusively. Probably not as much experience as some of the guys down in Omaha but a fair amount to say the least. I'm not scared off just yet.

@Dana1 - I was thinking the same thing about just air sealing the attic floor with 2" of closed cell and blown in fiberglass the rest of the way up with energy heel trusses. Here the cost for blown in fiber vs. cellulose seems to be even but the fiber doesn't seem to settle over time like cellulose. Regarding the foam under the floor, I hear you loud and clear and recognize the benefits. Would I still need the poly to deal with the seams or would I get enough vapor barrier from the poly alone? Lastly, what risk do I have with the floor cracking not having as "sturdy" of a base under the concrete?

@Izerarc - Thanks for the lead! I've already called them and sent over my plans. Can't wait to hear back from them. SIPS is still my first choice if I can get the cost in line.

Fiberglass doesn't settle over time like cellulose can, but it suffers a significant loss of R value at the cold temperature extremes due to convection within the insulation layer freely communicating with the attic air above. It's also much easier for the installers to get away with "fluffing" the product- same depth, lower performance and ABYSSMAL loss of performance at the temperature extremes. At -10F attic temp/ 70F at the ceiling, R38 low density fiberglass is performing at around R20, but with cellulose (at any density) you're still getting pretty much the labled R-value performance that it has in the ASTM C 518 test.(Which is performed with 75F center temp, 30F delta-T.) It even slightly outperforms it's ASTM value at higher delta-Ts, even in a cold side up configuration, but I don't know the mechanics of why that's true.

Under a hot summertime roof the fact that fiberglass is somewhat translucent to infra-red radiation causes highest temp in the fiberglass layer to be 1-2" from the top, and that temp is higher than the attic air- you're essentially insulating against a higher temperature with thinner layer. With cellulose the high-temp is at the surface, and only slightly above the attic air, since it convection cools readily.

To hit the labeled R and density cellulose is required to be installed deeper in order meet spec at the settled depth. The manufacturers have guidelines for initial depth & density to achieve that, but it isn't a precision factor. Many things will affect the rate of settling and final settled depth, but the most important is the amount of seasonal humidity cycling, which causes mechanical creepage issues in the hygroscopic material. This factor varies a lot by climate and air tightness of the building. The more air-tight you make the ceiling the less wintertime moisture gets into the cellulose. And by having R12 between the cellulose and the air-conditioned the coldest part of the cellulose layer stays well above the summertime dew points, decreasing summertime moisture cycling.

In an open-blow low density installation the material will settle over time but it slows to a stop as the density approaches that which it hits an elastic level against the seasonal creepage forces. With only the weight of a foot or two to support that doesn't require nearly as high as what you'd need to prevent settling in a wall cavity. Dense-packing walls to the right density for the climate's seasonal moisture cycling works, and works well. In an open blow using "stabilized" cellulose (the same stuff that gets wet-sprayed in open walls) the adhesives in the mix limits and slows settling, but I suspect that in a century it'll hit around the same depth as any standard dry blown. In a windy area in a vented attic it's worth using stabilized formula to keep soffit currents from moving it around on you, if for no other reason. For a synopsis of some of the better work on the settling & densities required for dimensional stability see:

http://www.nordicinnovation.net/nordtestfiler/rep565.pdf

But it's CHEAP STUFF- if you have to top it off with 3" after 25 years to get it back up to the original design spec, do you really care? Even in it's settled-state it outperforms low-density fiberglass goods at the temperature extremes. As a value proposition, blow in 16-18" and LET it settle to 13-14" over the next coupla decades, with a settled R value of ~R50 (but initially higher.)

The exceptional fiberglass blowing wools are superfine versions like Optima and Spider (spider can be wet-sprayed too) when blown at high density. Dense-packed to 1.8lbs density they achieve a similar air retardency to cellulose and consequently shouldn't suffer the same convection related performance losses as the standard fiberglass goods. The manufacturers spec R values at both 1.0lbs and 1.8lbs, but those are 75F center temp/30F delta test values that don't tell the whole story. IMHO only 1.8lbs density or higher is worth it (which would require blowing it in netting to be able to pack it that tight.) It'll take ~10% less depth than open blown cellulose to achieve the same R value, but it'll be significantly more expensive.

Poly is cheap insurance under a slab, even if XPS with taped seams is indeed a sufficient vapor barrier against ground moisture. EPS is usually cheaper, and even low-density type-I EPS used as flat-roof insulation in roofing apps has plenty of compressive strength against residential type loads under a code-minimum slab thickness. But thicker than code-min slabs distribute the weight even better, and adds thermal mass inside the thermal envelope, which helps pare down the heating & cooling peak loads. Somewhat denser Type-II EPS is a big more rugged though, with improved mechanical characteristics and is less easily damaged by handling etc. Most radiant floor guys prefer XPS since you can fix the tubing to XPS with staples, but that's not your issue. EPS also takes on less water over time when submerged, should you be in a high water table situation or have periodic high-water under the slab. It takes it on more quickly than XPS since it's mostly just the interstitial voids between beads that is being filled, but it stops at about 6-8%, and retains most of it's R value even when wet. When XPS takes on water it's absorbed into broken micro-cells, and it never leaves, but it occurs over months/years. As long as the slab is well drained either will work just fine.

Do not use poly vapor barriers at the ceiling, as that will increase the moisture cycling in the cellulose. During the summer there will be at least some drying toward the air conditioned interior through 2" of closed cell foam, and during the winter the cellulose dries readily to the vented attic airspace.

If you put exterior foam on the walls you should not use poly on the walls either. If you put at least the IRC recommended minimum R values in foam for the climate zone you can use good old latex paint as the interior vapor retarder and the sheathing will be BETTER protected by giving it faster drying path. For zone 5 that would be R5 min over 2x4 framing, R7.5 min over 2x6. If you flash-foam the cavities you can safely use less, but going much less than R10 as a thermal break would seem like a crime. R10 foam over 2x4 cellulose-fill works out to ~R20 whole-wall. R10 over 2x6 framing is ~ R24 whole-wall. If foil faced goods are used under the rainscreen you can add R2 to those numbers for approximate annualized performance (but you can't add the R2 to make the IRC min for protecting the sheathing from interior moisture drives. eg, 1" iso really is R5.6 when derated for temp, not R7.6, which would make the code-min for 2x6 framing. You'd still need 1.5" minimum, even though it's average performance would be better.) If you use foil facers on the exterior it's even more important to keep the interior side fairly vapor-open, since the wall has zero capacity to dry toward the exterior.

Got some preliminary numbers back from Iowa SIPs. They use the urethane foam and advertise an R value of 26.9 and 42.3 for a 4" and 6" wall respectively. Is the 2 extra inches worth it when you're already at a 26.9? Their 4" wall can probably fit the budget. The 6" may be a bit of a challenge.

@Dana1 - with cost essentially being equal, cellulose it is. We have it now and have been very pleased. 2" of closed cell to air seal with ~12" of cellulose on top of that seems like a no brainer. Your comment on pushing it to 16"-18" now and let it settle over time is right in line with what I was thinking. The trip charge alone "x" years down the road would likely pay for the extra 3" now. Also, water table won't be an issue at all. I have the pleasure of having to drill over 250' for our well. We're building in the Loess Hills of western Iowa which has incredible drainage.

Last question for all (for now )... Would you do a 4" SIP wall with R26.9 over a 2x6 spray foam wall if cost difference was less than 10% more? 15% more?

Thanks again to all! You've been very helpful!

do you mind sharing your numbers you got from IASips, as in the price per sq ft? I got a quote a year ago, and am curious to see if their prices has changed. For me the 6" was about 20% more then the 4". That was only about a $2k difference. Energy modeling showed the r26 to r40 (assuming air infiltration was the same with both panels) dropped the heat load requirement from 19k to about 17500 btu. This equals around $80 savings a year in electric rates, or a 25 year(ish) repayment. However going with a double stud wall around r40 is approximately the same price as the 4" panels. In my research, exterior foam typically costs more then double stud, materails alone. Labor is slightly higher, but contractors around here would charge roughly the same for simple double stud framing vs 2x6 framing and exterior foam.

Izerarc, their quoted price is ~$5.60/sq. ft. of panel for 6" but will be coming up with a precise quote based on my plans and mentioned a winter discount opportunity. Your comment on dropping the heat load equating to $80/year or a 25 year payback period is EXACTLY the kind of thing I'm looking to address.

I saw your thread from 2010 on debating the same situation. Which route did you end up going?

have not built yet because I keep redesigning the house and can not decide on a building type! One thing to consider is your heating equipment however when looking into the 4" vs 6". For my project I plan on using hyper heat mini splits and not a full furnace or geo system. It just does not make any financial sense for my house. For you you might explore going with the 6" panel and seeing what that does to your heating load requirements. If you are going geo, that could reduce it a ton or more, which that savings would pay for the 6" difference possibly. This is a better route because your home will use even less energy and cost the same amount upfront. The 6" panel will also be stronger. If I chose SIPs, it will be the 6" panel.

Posted By rkinmoval on 17 Jan 2012 08:36 PM
@Dana1 - with cost essentially being equal, cellulose it is. We have it now and have been very pleased. 2" of closed cell to air seal with ~12" of cellulose on top of that seems like a no brainer. Your comment on pushing it to 16"-18" now and let it settle over time is right in line with what I was thinking. The trip charge alone "x" years down the road would likely pay for the extra 3" now. Also, water table won't be an issue at all. I have the pleasure of having to drill over 250' for our well. We're building in the Loess Hills of western Iowa which has incredible drainage.

Last question for all (for now )... Would you do a 4" SIP wall with R26.9 over a 2x6 spray foam wall if cost difference was less than 10% more? 15% more?

Thanks again to all! You've been very helpful!

All else being "in the range", a SIP will take far less detailing to make air-tight than stick-built with exterior foam.  But there's still SOME detailing, and you can't just count on it.  From a building schedule point of view there are advantages to SIPs too.  You have to spend a bit more time up front getting the design right, but once the SIPs are on the truck they go up quick, with far fewer iterations of bringing in different subcontractors. The bigger and more complex the project, the more important streamlining the schedule becomes. (Time is indeed money, and delays==risk.) Performance wise an R27 SIP isn't very different from a 2x6 + 2" of exterior iso, but it goes up quicker, which may in-itself be worth paying a premium.  (This is a good argument for ICF at these R values too.)  There is no simple answer to how much that premium is worth, but know that it's way more than zero.  At a 10% cost delta over the framing & insultion package it's a no-brainer, but I'm not sure where I'd place the crossover.  It just depends- there too many factors.

When evaluating any of this it's good to (at a minimum) keep an updated heating/cooling load calculation. When committed to geo the cost of better insulation & air tightness or high performance windows isn't just about spending another $5000 dollars to save $12 on the utility bill-  as much as anything it's about reducing the size & cost of the geo.  Every installed ton of geothermal is a chunk of change, and if peeling off a ton can be had by say, going to 22" of cellulose instead of 12", it can even cost less up front (and still save you the $12 on the bill.)

But the money might be better spent elsewhere- it takes some careful analysis, but in almost any house there are good/better/best (cheap/cheaper/cheapest?) ways of designing-down the peak loads, and it will OFTEN cost less than the geo tonnage require to support the higher loads.  This differs significantly from homes heated with fossil-burners, where doubling the output of the heating system costs very little. Yes, an envelope upgrade may only save you a hundred bucks per year, but it may save far more up front if you optimize the envelope upgrades on a cost basis. To know where the crossovers lie you have to do the analysis (even get some quotes) because it isn't always obvious.

On custom homes where the owners/architects are optimizing for energy efficiency it's becoming common to model the building for the site factors, climate/location, with all of the relevant R & U values, solar gain factors etc.  There are some pretty good freebies out there, including DOE2 , developed by the US Department of Energy, which is far more sophisticated than a simple Manual-J type heat loss calculator.  The D.O.E. has also developed a optimizing tool which can help decide where the money is best spent on efficiency measures, that has only recently been released to the public (it's been around for a decade or more, IIRC), called BeOpt which is also a freebie. Before breaking ground or committing to any of it, once you have the basic design (and it sounds like you're there), it's worth playing "what if" using these tools.  It can be a lot of data entry on the first pass, but once you have it up to snuff you'll be able to get reasonable answers to the questions of when it's worth going to a higher R SIP rather than ponying up for ultra-high performance windows,  and when peeling back the heating cooling loads is dramatically more expensive than going for more geo output. 

Clearly doing it yourself will cost less than paying your architects to do it, but this doesn't sound like a cheap project, and the optimizations won't always be treatable by "design by web-forum" methods. It takes a much deeper analysis than deciding whether to stick with 4-6" foam vs. 2" and adjusting the thickness of the cellulose to meet the design goals for R value.

BTW: If you have to drill 250' for your geo I'm gonna guess that your cost per ton is going to be well above that of swamp-dwellers.  Running the costs of that against that ductless or hydronic air source heat pumps may be another thing to optimize.  The latter are canned systems where the hard engineering is already done, and in climate zone 5 can hit comparable efficiencies to geo. Air is pretty much the same everywhere, but the heat transfer aspects of soil are much harder to nail down, not at all the same from one site to another, so there's both higher upfront cost and higher design risk when going with geo. The fact that you have a neighbor with a geo success story as your paradigm should be some reassurance though.

BTW: you may find the heat load discussion in this thread of interest:

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/12/aft/79652/afv/topic/Default.aspx

How many tons of geo were you estimating, at what cost?

The 250' is for water. The geo would only be 100-150' closed loops depending on sizing but multiple wells. I'm also considering digging trenches 6' deep since I'll have plenty of land.

I reached out to my HVAC guy about calculating a load sooner than later to help decide with SIPs vs. other options. Initially he was thinking 5 ton but said increased efficiency in insulation could knock it down to 4 or 3.5 but wanted to see what the software said. If I drill holes (1 per ton), I'm looking at $1,650 per hole. This is why I'm also contemplating trench loops instead because I have the room and can rent an excavator.  Rough estimate on the HVAC equipment/install is $12k - $16k including all of the ductwork (plus loops).

That's pretty cheap compared to what it costs in my neighborhood!

12k-16k sounds very cheap for this size of a home. I would question the equipment you are getting. Does it include an HRV? Around here a 2 ton house would be about 20k, including horizontal wells and ducting. High price considering the 12k btu hyperheats are around 2500 each including install.