My wife and I will be building a new house this spring in a climate that gets below zero in the winter and abover 100 in the summer.  We've received one material quote for our house that works out to $52.04 per lineal foot (9 foot wall) for SIPs.  Granted this includes all the cutouts etc. but can't seem to find a framer that doesn't provide a price break in labor with SIPs vs. conventional.  Conversely I've mocked up a 12' wall section (also 9' high) with 2 x 6 walls and 7/16" OSB and 4" of spray foam to land at $27.93 per lineal foot.  I know this doesn't account for window/door headers but can't possibly imagine that will eat up roughly $25 per lineal foot.

Yes, I know there are benefits to the thermal break with SIPs and I do love the concept but am desperately struggling with justifying the cost difference (~$16,000 in materials for the whole house).  We will be installing a geothermal heat pump.  Our neighbor has a 4,800 sq. ft. stick framed house with fiberglass insulation and a geothermal heat pump.  They experience between $40 and $60 per month for heating and cooling.  Our power company offers separate meters around here with a discounted rate for electric heating and cooling which makes it very easy to measure the HVAC/water heating expense.  Our new house will be approximately 6,500 sq. ft. and would guess to see around $100 - $110 per month avg for the heating and cooling if we used fiberglass and hopefully much less with spray foam.  What I'm struggling with is how can SIPs be so much better that spray foam knowing it will cost so much more?  Will I ever get my money back?

As a side note, we currently pay on average $150 per month on a 4,800 sq. ft. home with an air-to-air heat pump to put things into perspective (includes water heating).

Would greatly appreciate your thoughts/suggestions.  I'm still waiting for one more SIP bid but am not getting my hopes up too high that it will be much better.

Thanks in advance!

A better insulated and lower cost stud wall would be to put 1" to 2" of rigid foam board on the outside of the studs/OSB, air seal that boundary, then fill the space between the studs with sprayed cellulose insulation. This provides a thermal break over the wood framing and will get you a true R20+ wall. What you have proposed with no thermal break over the framing will be around R15 for the whole wall at best, assuming you are using closed cell foam (even less if what you proposed is using open cell spray foam). And it will be more expensive because spray foam is far more costly per R than other forms of cavity insulation.

If I were to do 1" of rigid foam on the exterior, how do you fasten it and would it pose any challenges with attaching vinyl siding? Thanks!

I think you would need to install vertical furring strips and then install the vinyl siding to the strips.  Ask Dana1, but you also might need house wrap over the EPS or XPS to make a better rain screen.  Be sure to remember not to set the nails or screws that attach the vinyl too deeply.  Nailing too tightly will bind the vinyl and cause loud popping noises when it does finally move.  Believe me, with vinyl's large coefficient of expansion, it will move with temperature changes - so let it float freely.

By installing furring strips, you can install thicker foam board.  Ideally, at least two layers of foam board should be installed with staggered seams.  In my opinion, installing foam board beyond the studs, is very important to conserve energy.

Posted By rkinmoval on 09 Jan 2012 10:30 PM
If I were to do 1" of rigid foam on the exterior, how do you fasten it and would it pose any challenges with attaching vinyl siding? Thanks!

http://www.greenbuildingadvisor.com...-sheathing

http://www.greenbuildingadvisor.com...side-house

With through-screwed furring you can mount just about any siding. With very thick foam and heavy siding the fastener spaceing needs to be tightened up, but for an inch or two 24" o.c. is good enough for almost any siding, with the low mass of vinyl, even with very fat foam you'd still only be at 24" o.c..

Be sure to air-seal both the structural sheathing and the rigid foam, lapping the seams of each to reach SIP-type air tightness levels.

One of the best things about vinyl siding is that it pretty much creates its own rain screen. I dont see enough benefit to still do the furring strips.

Having grown up in Iowa, I completely understand the need for extra insulation. It seemed to me that it cost about the same to either heat or cool your house,summer or winter, and reducing those costs and increasing your in-home comfort level is very important. I was looking at your numbers for stick and spray-in foam, and I must have missed something. Around here, spray-in foam is pretty pricey. Ok, say the spray-in foam, material and labor, runs at $5.00 a sq/ft, at a 9 foot tall wall, that comes to $45 a linear ft. Even at $3.00 a sq/ft, that's still $27 a linear foot. That's not including the cost of the studs, sheathing, labor to cut, frame, assemble and put up the wall. Something else to think about is on a 9 ft wall, you would have to use 10 ft studs and cut off a foot of each one, creating a lot of scrap.

Steve
GrandCountySIPs.com

Posted By trigem1 on 10 Jan 2012 12:11 PM
...Something else to think about is on a 9 ft wall, you would have to use 10 ft studs and cut off a foot of each one, creating a lot of scrap.

You can buy pre-cut 9' studs that are 104 5/8" long just about anywhere because 9' ceilings are so common today.

Lowes pre-cut 9' studs

I would not use spray foam and use 2" of taped rigid foam plus 2x6 studs with stabilized cellulose. Plus a second air barrier like Certainteed Membrain on the interior.

Posted By Springtime on 10 Jan 2012 11:31 AM
One of the best things about vinyl siding is that it pretty much creates its own rain screen. I dont see enough benefit to still do the furring strips.

If you poke 10,000 nails through the foam the thermal bridging of the fasteners will be an order of magnitude higher than screws 24" o.c., and your air-barrier aspect goes to hell.  Even at 1" of foam depth the odds of nails sagging /bending over time with wind load & vibe is high.

Yes, vinyl siding is inherently backventilated/rainscreened,  but if you have 2" of XPS sheathing rainscreens don't provide much additional drying capacity toward the exterior, and the stackup would be better if designed for drying toward the interior.  For exterior drying stackups stick to unfaced EPS (~R8 @ 2" rather than ~R10 @ 2" for XPS), as it's far more vapor permeable. 

If you use foil faced iso you get a radiant-barrier benefit that very much CAN use the extra depth of the furring and will add another R2-3 to the average performance.  It's also somewhat higher R/inch than XPS, but not by as much as the 75F ASTM C 518 test rating would indicate.  Derate to ~R5.6/inch at 0F outdoor temps.  With foil facers you then MUST set it up to dry toward the interior, since foil is a strong vapor barrier.  Fiber faced roofing iso is similar in vapor permeance to 1.5" XPS, give or take, and provides a modest drying path into the furring cavity, but has no r.b. benefit.

Most of IA is in US climate zone 5- as long as you have R7.5 or more (2" EPS would do) on the exterior of 2x6 framed wood-sheathed wall you don't need an interior vapor retarder.  If it's on  the northern fringe of IA in US zone 6 you'd want more exterior R (R11.25 min, per IRC 2009), but that's also achievable at 2" with polyiso.

A cellulose filled studwall at typical framing fractions comes in at ~R14 for a "whole wall" R value, after all thermal bridging is factored in.  The R value of the exterior foam can be added to that number at full value, so 2" of EPS would give you an R22 whole-wall,  XPS at the same depth would deliver ~ R24, and 2" of iso delivers ~R25.   This is a decent range of R values, but there may still be an economic argument for more.  R30 whole-wall when one on the cheap this way can make sense in zone 5, (R35 for zone 6).  See table 2, p10 as a rough guide.  Building with 3" iso on the exterior isn't all that tough or expensive. Over 4" gets awkward, and the cost of the fasteners for the furring starts to climb, but 6"+ is possible for the dedicated. (At some point the cost & risk crossover will point you to SIPs or double-studwalls, Larsen Trusses, etc.)  

If you're using an expensive heating fuel like oil or propane there's a rationale for even higher R, but at R40+ you might do the economic analysis comparisons of stopping at R30 and heating with ductless air source heat pumps (mini-splits/multi-splits) rather than fossil fuels, putting the rest of the money into photovoltaic solar on the roof. That first R10 (whole wall- like a cellulose-filled 2x4 framed house) buys a lot in comfort over a sub-code uninsulated shell, and the economics of taking it to R20 is pretty easy- after that it's worth doing cost/benefit on every component of the building to figure out where the money is better spent.  At R20+ whole walls the heat loss/gain of the windows is usually greater than that of the walls, but whether better windows or fatter walls are the better deal are design-specific.   In IA doing the site & building analysis for passive solar is worthwhile, and it can provide a large fraction of the total heating energy with very marginal upfront cost if going with R30+ whole-wall construction.

There are some high-density rock wool rigid insulating sheathing products coming out soon (may already available in some markets) with excellent drying capacity, yet air-retardent enough to not need an exterior air-barrier for thermal performance.  But that makes detailing the structural sheathing as an air barrier even more critical, since you can't fully air seal the rock wool the way you can rigid foam.

If you care about the greenhouse gas emissions of differing foam processes, use EPS or iso- they use much more benign pentane as the blowing agent, whereas XPS is usually blown with HFC143 with 100x or more greenhouse potential.

Thanks for all of the thoughts.

Dana1 - Regarding the location I'm in climate zone 5. The callout on nails sagging/bending is a HUGE risk for me as we'll be on top of a hill in the middle of nowhere and the wind really knows how to blow around here. You asked if I will be using an expensive heating fuel. As I mentioned in the initial post, we'll be doing geothermal. If it's not too much to ask, please take a look at what the energy costs are in my first post for a neighbor's 4,800 sq. ft. house with fiberglass. Sure, mine will be more due to square footage, but I need to be careful not to be "spending $10 in building costs to save $8 in energy". Would love your thoughts on your recommendations all things considered.

Regarding the cost of spray foam. I've received a quote for 1 lb. foam at 4" for $2.40 sq. ft. installed. They claim an R value of 24. Any thoughts on that? Too good to be true?

Thanks again,
Ryan

One pound foam should be open cell.  If so, the aged R-value will be less than closed cell.

Is usually takes 2lb foam to hit R6/inch or a bit higher. I have no idea who makes a mid-density foam that would be R24 @ 4"- do you know the name of the product?

At $2.40/foot for R24 it's in the same 10 cents/R-foot range as rigid EPS or iso, and well under the pricing of 2lbs foam which tends to run closer to 17cents/R-foot.

The value proposition:

In a stick built structure anything more than a air-sealing skim coat of closed cell in the stud bays is largely wasted, due to the thermal bridging of the framing. If the framing fraction is 20% of the total area (on the low end of typical, employing some advance-framing techniques) spraying only 4" for a center-cavity R24 a 2x6 (5.5" deep) stud bay only delivers about R13 for whole-wall, which is LESS than a much cheaper full cavity fill of R3.5/inch sprayed cellulose, which comes in at R14. For less total money doing cellulose in the cavities and R12 (2"- literally half the foam) on the exterior would air seal better, and deliver a whole wall R of about R26, which is TWICE the thermal performance of 4" of cavity foam only.

Half the foam, 2x the R performance, more reliable & complete air sealing for about 2/3 the total cost of 4" of foam in the cavities.

If your framing fraction is more like 25% the whole-wall R with 4" of cavity-only foam runs ~R11.5, but the cellulose fill still runs ~R13, making a 2" exterior spray + cellulose fill good for R25 whole-wall, which is MORE than 2x the performance of cavity-foam-only.

To have some place to hang the siding, pre-installing the furring using 2" XPS chunks as spacers between the furring and structural sheathing allows the spray foam installer (and builder/inspector) to use the furring as a depth gauge. If going this route it's worth using 2x furring for the better rigidity yielding a flatter exterior surface. When using rigid foam the dimensional tolarances of the foam are pretty good, and 1x4 furring is complaint enough to keep it pretty flat with 24" o.c. spacing, but without the support of the rigid foam 1x goods may have too many dips & twists, etc.

If for some reason you or the contractor can't stand cellulose, doing the cavity fill as high density (1.8 lbs per cubic foot) Certainteed Optima, or JM Spider will deliver another ~R1 whole-wall compared to 2.5lbs cellulose. The advantages of the super-fine new-school fiberglass over cellulose is that it dries quicker should it ever become wet and it installs a bit more consistently & quickly. Cellulose has the advantage of being able to buffer seasonal moisture, making it somewhat more protective of the structural sheathing & studs over the winter when going semi-permeble on the interior side of the assembly, and it's cheaper. With R12 in exterior foam the sheathing is well protected without interior vapor retarders, so it really isn't a deciding factor if you go that high for exterior R. But if you the exterior foam or only put an inch out there, cellulose would be the better choice.

The other value proposition:

Price out the geo, then decide how much the loads can be reduced by going with a higher-R/lower-U envelope design that takes passive solar gains into account. That first coupla tons of geo can be pretty pricey, and if you can get peak load under 3 tons for less than the price difference between a variable compressor-speed air source heat pump and geo you'd likely achieve the same or lower operating costs for less initial investment. It's unlikely that you'd be able to use ductless multi-splits/mini-splits in a house that size, but if you could such systems can operate at average efficiencies comparable to geo (after the air-handler & pumping power is added onto system-COP) in most of zone 5. With ducted air-source heat pump with variable speed compressors and air handlers you'd still average a COP of around 2 or even a bit better during the heating season, whereas with geo you'd get maybe 3-3.5 (with all system-power factored in.) If you're intending to have radiant floors, the Daikin Altherma air-source to hydronic heat pump is still far cheaper than geo, and runs comparable COPs to mini-splits. In zone 5 it would really perform well, comparable to geo.

In zones 4 & 5 geo may not be able to compete against the new-school ductless air source heat pumps without significant subsidy, once you have a heat load within the output range of these systems. Geo systems are all custom designs, increasing cost & complexity, and there is a very real design risk, whereas the ductless air source systems are fully modulating cookie-cutter systems where all the engineering was done in the product development stage. Air is air, doesn't change much, but the performance of geo wells doesn't always meet the design assumptions.

If you cut the heat load of the building by something like 40-50% with a better envelope the operating cost of even a ducted variable speed air source system with a COP of 2 would be about the same as the lower-R higher-U structure with geo or split-systems. Wall-R is only part of the equation- air-tightness is key, as is thermally breaking all structural elements. A 2" overshot of exterior foam is a good start, as is using ICF for the foundation (with exterior foam of the ICF aligned in the same plane as the spray foam over the sheathing), and insulating under the basement slab, floating it inside the interior foam of the ICF, etc. For air-tightness use only casement & awning windows for the operable windows, and fixed windows wherever you can. Where you can skimp on window sizing, you should, since a U0.3 window is an R3.3 hole in your otherwise R25 ish wall, with nearly 8x the heat loss per square foot. Design & size the south facing windows for optimal passive gain. A few grand in design-optimization for energy use up front can save on both initial costs and operating costs. (If you're an energy nerd and want to do the simulation & optimization yourself, DOE2 and BeOpt are free downloads from the US DOE, and pretty good.) Cutting the heat loss of the walls by half or more at relatively low cost is pretty straightforward, as outlined above, and it doesn't take rocket-science math to similarly upgrade the rest.

Gear in mind that designing and building it for air-tightness is critical to performance- a well insulated wind tunnel still costs a lot to heat. Have the architects design a continuous primary air barrier, and make sure the builders understand & follow that design. Aim for 1 air change per hour @ 50 pascals in a blower door test, and TEST & rectify as soon as the windows doors & shell/exterior-foam are up. Don't let electricians & plumbers randomly drill everywhere without air sealing behind them. If you get it down to under 2 ACH 50 you'll be doing pretty well, but if you're at 5+ it's worth chasing pretty hard before signing off on it. These aren't a pie-in-the-sky numbers- many homes are built to the Canadian R-2000 standard of 1.5ACH/50 tightness every year, and the PassiveHouse 0.6ACH/50 has proved to be not all that extreme either (but only with very tight windows & doors, and a job site "air sealing Nazi" keeping track, taking names, and sometimes kicking butts. :-) )

I'm not sure who the manufacturer is on the foam yet. I'll still need to get those details when making the final decision. Have to admit I've never even heard of spray foam on the outside of the house. Seems like the labor and materials for the firring strips may challenge the cost to do so, not to mention you'd need a real straight shooter on the foam. I'm guessing you'd need a "no wind" day to do that. We only get 3 of those a year.

What's your thoughts on rigid foam on the outside to establish the thermal break and spray foam on the inside to air seal the house? Could you avoid the firring strips that way?

Btw, the quote I got on 2 lb foam is $1.90/sq. ft at 2". Is that good?

We are exploring ICFs for the foundation. Still waiting to hear back on the bids for that.

On the windows front we'll be casement or fixed. There are virtually no south facing windows though to help with passive solar (view just isn't there )

I love the idea on the blower door test. We'll be sure to do that.

Posted By rkinmoval on 11 Jan 2012 08:57 PM

Btw, the quote I got on 2 lb foam is $1.90/sq. ft at 2". Is that good?

That sounds pretty typical.  Rule of thumb around here is $1 per board foot (1 square foot at 1" thick).  Your quote is 5% less than that.

Posted By rkinmoval on 11 Jan 2012 08:57 PM
I'm not sure who the manufacturer is on the foam yet. I'll still need to get those details when making the final decision. Have to admit I've never even heard of spray foam on the outside of the house. Seems like the labor and materials for the firring strips may challenge the cost to do so, not to mention you'd need a real straight shooter on the foam. I'm guessing you'd need a "no wind" day to do that. We only get 3 of those a year.

What's your thoughts on rigid foam on the outside to establish the thermal break and spray foam on the inside to air seal the house? Could you avoid the firring strips that way?

Btw, the quote I got on 2 lb foam is $1.90/sq. ft at 2". Is that good?

We are exploring ICFs for the foundation. Still waiting to hear back on the bids for that.

On the windows front we'll be casement or fixed. There are virtually no south facing windows though to help with passive solar (view just isn't there )

I love the idea on the blower door test. We'll be sure to do that.

With R12 exterior foam (2" of foil faced iso) you'll still need furring to avoid the thermal bridging and pulling/leaking potential of 10,000 nails, but it does mean you can drop back to 1x4s for the furring.  It's really the "right" way to do it, and it's not very complicated or expensive (see the "how to" link in my first post of this thread.)  The biggest material  cost adder to  exterior foam at 3+" thickness tends to be the fasteners, but at 24" o.c. and shorter depths even that won't add up to much with 2" goods.  (There's no advantage to going with 1x2s or 1x3s either, in fact quite the contrary-  narrower goods tend to be more twisted & crooked, and will be more likely to split.)

Buck a board foot (1" x 1 square foot) is the typical range for a mid-sized job, but a 6k' house is probably 1.5-2x the material so you're getting a bit of a discount.

Spraying closed cell goods on the exterior really isn't all that tough- it's done all the time in commercial construction.

If you went with 1-2" of 2lb foam in the cavities as the air seal, it doesn't seal between the plates and subfloor nor does it air-seal the foundation sill, etc. and you still need to fill the rest of that cavity completely up with cheap fiber to get the full benefit and avoid thermal bypass channels (which are also fire spreaders) behind the gypum.  The inch of closed cell + fiber is sometimes referred to as  "Flash'n'Fill" is a very reasonable approach for making a tigher-than average house.  It doesn't matter that it's difficult to get consistent 1" depth- the air sealing doesn't take much and ANY amount of closed cell on the interior of the sheathing forms non-wicking condensing surface that is protective of the sheathing when the sheathing falls below the dew point of the interior air.   Even though the foam adds significantly to the center-cavity R values, the thermal bridging of the studs robs it blind performance-wise, so going fatter than the minimum necessary to air seal the cavites is pretty much wasted. Save the budget for the exterior foam (rigid or sprayed), where you it adds 99% of it's rated R value to the whole-wall performance rather than 50-75%.  

You'll pay more per board-foot for a flash'n'fill, but it's far fewer board feet. Odds are  a 1" nominal air sealing flash shot will run you less than a $1.10/ft^2 if they're quoting you $1.90 for 2".  The difference in center cavity R value between 2" and 1" with cellulose on the rest is only R2.5, and the difference in whole-wall R is a bit under R1. Going thicker on the exterior foam is far better bang/buck.

So, back to the original question. Given all the ways and techniques one can manage to achieve comparable air tightness and thermal efficiencies using stick built framing, do SIPs warrant the hefty premium in cost that the original poster seems to be struggling with?

Echo that 3cityblue. Even if it cost $100 more every month to heat and with stick frame/spray foam I'd be looking at roughly a 13 year pay back period. Reality tells me it would far longer. Need to find a cheaper SIP option.

I made the decision you are asking and now will need to make it again. With a construction loan time is almost more important than maney. You need to get it done quickly.
I don't know if your home will be a single or two story home. So this comment is for a single story home. Using SIPs for the walls is almost a no brainer. What you are trying to do is make an hamberger without using any beef. You can achieve a SIP like wall using stick framing, spray foam and outer foam board. This will cost less in materials but the time and labor is high and you have to do it right.
For most homes most of the cost for SIPs are in the roof and floors. These are large areas to cover. The floor does not need to be SIPs. you can fill the cavity with some fiber, and use bubble foil under the floor joist.

The roof is the big question. I decided to do a stick frame roof to save money and that was a big mistake. It ended costing about $15,000 more. Why because it was a high pitch and high roof. Too many components. There are less surprises with SIPs. It is easier to figure out the budget and stay in it.

My next home I build needs to be energy efficient, simple to build and under budget. Which I will post for some ideas onexterior costs and flooring

Our house will be a 1 1/2 story with 3,086 on the basement (partially excavated garage), 2,414 on the main, and 1,152 on the second. The basement is a walkout with 109 lineal feet of SIP. Main has 268' and the second floor has 190'. The SIP manufacturer I've got a bid from so far looked at my roof and told me you'd be better off stick framing it since there are no cathedral ceilings on the plan and it's fairly cut up. The SIP builder I've been working with thus far has suggested 4-6 inches of foam on the ceiling with a foot of blow in on top of that. My roof is a combination of 6/12 and 8/12 for pitch. That being said, it would likely take more foam to spray the underside instead of going flat on the ceiling. Anyone ever heard of that?