ICF VS advanced framing and spray foam.
Last Post 12 Jun 2011 07:58 PM by toddm. 44 Replies.
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calvinmlUser is Offline
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02 Jun 2011 04:35 PM
We have talked to several builders in San Antonio Texas area about an ICF home and they all seem to think that for this climate ICF walls are not worth the extra cost.   The attic  insulation is what really matters.   We were told ICF can add as much as 25% to the total cost of the house  

   We are now considering building with an unvented attic with spray foam insulation on the roof.  Then framing using the  advanced wall framing method  with  2x6 24" oc with spray foam insulation and using the EFIS stucco exterior finish

Can anyone comment
Ray GladstoneUser is Offline
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02 Jun 2011 04:48 PM
ICF will maybe add 5% and your new house will not blow down with the next tornado. Why would you build with wood 24" on centers? Did your grandma never tell you the story about the three little pigs? They have a term for that: "Flimsy". Remember Joplin/Tuscaloosa.
Peter JacksonUser is Offline
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02 Jun 2011 05:05 PM
If you're going to sell the house in five to ten years, go ahead and build with sticks and foam. You have to remember though that a wood house these days is built with a lot of OSB which delaminates when it gets wet and stays wet, especially here in Texas. So essentially your wood house will require a lot a vigilance as your outside finish begins to age. If you're going to be in the house for longer than five or ten years, wood = rot, wood = termites, wood = mold. wood = fuel for a fire, etc.
Peter JacksonUser is Offline
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02 Jun 2011 05:06 PM
And yes, wood = tornado chow.
Dana1User is Online
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02 Jun 2011 08:08 PM
An advanced framing house with open cell cavity fill, no exterior foam comes in at a whopping R15 for a typical whole-wall R, which is ~R2 less than the minimal 2" EPS per side ICF. Of course, an inch or two of exterior foam (rigid board or sprayed closed-cell) brings it into the 20s pretty cheaply. It's generally more bang per buck to put the and inch or two of closed cell foam on the exterior, since it air seals more reliably, and you can then fill the stud bays with cheap fiber.

A 2" shot of ccSPF on the exterior, with a wet-spray cellulose or R21 batt cavity fill comes in at around R25 for whole-wall performance, for similar money a full cavity fill of open cell. An inch of exterior rigid XPS taped & sealed, and fiber cavity fill comes in at ~R19. Detailing the structural sheathing and stud plates as a primary air barrier using acoustic sealant costs peanuts, and can be as air tight or better than a cavity fill of spray foam.

With even an inch of exterior closed cell foam (rigid or sprayed) it's R value relative to the cavity fill keeps 100% the structural wood above midwinter dew point of interior air, so moisture drives from the interior are essential zero (assuming you ventilate and air condition the place.) And with the ~1 perm or less rating of 1" of ccSPF and even lower EIFS summertime moisture drives don't touch the structural wood. As long as you don't put up interior vapor retarders (not even kraft face batts) the wood stays nice & air conditioned-dry all summer, and with the exterior R keeps it from gaining moisture in winter. The only other paths to wood for moisture are related to flashing details, and the foundation sill . A good sill gasket makes an excellent capillary break, a copper-flashing capillary break extending beyond the foam on both sides is a termite-proof sill gasket It won't rot from moisture unless you're sloppy.

It's easy to buy a LOT more thermal performance per dollar going stick built. ICF's thermal performance related to it's mass has been oversold, as has it's ease of air sealing, but it doesn't really add up to all that much- the mass works much better when it's full inside the thermal envelope rather than wedged in the middle, and unless the rest of the house is detailed to be air tight, it's not dramatically tighter than any pretty-good stick built with only modest air-sealing details (like taped seam housewrap). The real advantages of ICF are mostly structural, fire resistance, and sound-proofness, all of which are good things, but if thermal performance is your goal it's cheaper to get there with other methods.

BTW: 24" o.c. AF 2x6 construction has the same structural capacity of 2x4 16" o.c. old-school stick build but beats it's thermal performance by ~50% even before enhancements like exterior foam.

Both 5% and 25% are exaggerations of the cost of going ICF compared to stick built of equivalent thermal performance. But an R20 ICF is pretty pricey compared to a 2x6AF fiber-fill + 1" exterior foam approach. Still, price it out.

Beyond ICF vs. stick built...

If you're REALLY serious about thermal performance you'll be very selective about window size, orientation, and type, since in an R20 wall the windows will dominate the heat gain/loss. Going with 25-30% less glazed area taking the minimum-reasonable for daylighting and view is cost-negative (smaller is usually cheaper.) Push-out casements & awnings seal better than double hung, single hungs, & sliders, and offer more egress cross section per square foot of glazing where bedroom egress code must be met. Fixed windows are even tighter. Designing both overhangs and window placements to minimize summer gain, yet allow for some winter gain can also be huge.

Air sealing as you go during construction is the single-most cost effective efficiency measure that can be taken. Define the primary air-barrier for the entire envelope during the planning stage, and make sure it's executed at each step. It's a lot easier to get a good seal with a bead of acoustic sealant under the stud plate BEFORE it's nailed down than anything you do to it after. Spray foam makes air-sealing easier, but like ICF it's not guaranteed.

Attic insulation matters, but designing the mechanicals to be completely inside of the thermal boundary is more important, and makes air-sealing the ceiling boundary & going high-R on the attis insulation much easier. The cost and thermal performance hit of adding a foot of height to the R20 walls to accomodate bringing the ductwork & air handlers indoors is minimal, and the performance gains significant.

First air-seal to under 2 air changes per hour @ 50 pascals pressure (ACH/50), insulate the slab edge to R5 or more (you can skip center-slab in San Antonio unless you're going all PassiveHouse on it), take it up to R15-R20 for whole-wall R on the walls, R50-55 ish in high-density fiber (14" of cellulose, 12" settled depth, not low-density fiberglass at any rated R-value) in the attic, use CRCC rated "cool roof" materials on the roof and keep the roof pitch 4:12 or higher, and be VERY picky about window size, type & placement. That can all be done reasonably affordably with or without ICF, and you'll have half the energy use or less of a typical code-min house.

Spray foam at the roof deck is a ridiculously expensive way to even make code-min, let alone high-R. If going that route consider putting 3-4" of rigid foam (any type) above the roof deck, air-seal the interior of the roof deck with an inch of closed cell foam, and doing the rest as cellulose or high-density new-school fiberglass like JM Spider blown-in-blanket at the rafters. With 3" of EPS you'd have to derate it's R to R10 or so during the summer, the inch of ccSPF would be ~ R6, so you'd then need ~10" of fiber to hit a whole assembly R of R50. That's cheaper than all-open-cell but more complicated to build, and still way more expensive than raising the walls a foot or so & keeping the ducts inside the insulation.

Consider this: Open cell foam on the roof deck R50 would cost you an unaffordable ~$6 per square foot of roof area. With open blown cellulose on the attic floor at R50 would be on the order of $1/foot (often less). Open cell at a code-min R30 would still be 3x the cost of R50 cellulose. Factor that into the performance picture of adding a foot of height to bring all the mechanicals indoors. An extra foot or so of wall height is a tiny fraction of the cost of a truly high performance sealed attic approach (or even a code-min sealed attic.) The primary advantages of spray foam are air sealing, and in some instances adjusting the vapor permeance for better moisture contro, in others, for thermal breaks on timbers, etc.l. Once you have the perfect air seal (attained by any method), it has no further advantage in a high-R assembly- let the cheap stuff do the heavy lifting on thermal control.
robinncUser is Offline
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02 Jun 2011 10:50 PM
Dana......have you ever thought about writing a book?.....seriously.  You know a hellava lot about this stuff!
Peter JacksonUser is Offline
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03 Jun 2011 12:11 AM
BTW: 24" o.c. AF 2x6 construction has the same structural capacity of 2x4 16" o.c. old-school stick build but beats it's thermal performance by ~50% even before enhancements like exterior foam.


50% is a really big number. Why such a big difference?

Ray GladstoneUser is Offline
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03 Jun 2011 08:31 AM
"Wood = Tornado Chow". I love it. I'm going to use that Pete, and I will attribute it to you the first three times. Then it's mine.
ICFHybridUser is Offline
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03 Jun 2011 10:39 AM
50% is a really big number. Why such a big difference?
Doesn't it have something to do with 2 X 4 construction having a 3.5" insulation cavity and 2 X 6 construction having a 5.5" cavity? (5.5-3.5)/3.5 = 57% deeper alone.

Dana1 probably has the details on thermal bridging ratios, etc.
Dana1User is Online
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03 Jun 2011 11:03 AM
Posted By ICFHybrid on 03 Jun 2011 10:39 AM
50% is a really big number. Why such a big difference?
Doesn't it have something to do with 2 X 4 construction having a 3.5" insulation cavity and 2 X 6 construction having a 5.5" cavity? (5.5-3.5)/3.5 = 57% deeper alone.

Dana1 probably has the details on thermal bridging ratios, etc.

The whole-wall R of a perfectly installed R13 2x4 batted (or spray cellulose @ 2.5lbs density) studwall comes in at about R10 after the thermal bridging of the studs, sills, plates headers, and band joists are factored in.  A 2x6 AF perfectly batted or spray cellulose studwall comes in at a bit over R15 after framing factors, a 50% boost in R.

OK, that's really only a 33% improvement in U value, not 50- mea culpa! (Gettin' sloppy in my senescence! ), so you really DO have to add R5 in exterior foam to pull the real 50% inprovement in U value.

A decent comparative study of R values in mid & high-R assemblies (including ICF) can be found here.  See Table 3 on p.9 for the quickie view of whole-wall Rs by assembly type.  (Compare assembly 2a to assembly 1a & 1bii.  Then compare 7a & 7b to 2a.)
Peter JacksonUser is Offline
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03 Jun 2011 04:18 PM
Well 33% is a pretty big number too, and you're saying that's mostly from the difference in thermal bridging. That's a pretty astonishing difference when you isolate it like that. And thanks for the link!
Ray GladstoneUser is Offline
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03 Jun 2011 04:27 PM
Yes, thanks a lot for that link. I have downloaded it and look forward to reading the entire whitepaper.
Dana1User is Online
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03 Jun 2011 05:10 PM
BSC has similar discussions about high-R foundations & roof assemblies on their site as well. They're pretty readable for stuff written largely by nerds with PhDs. ;-) I find this one a useful guide for what might be long-term cost-effective in different areas, depending on the approach taken:

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

They've done a bunch of studies for the US D.O.E. as well, some available on the Oak Ridge Nat'l Labs and Lawrence-Berkeley Nat'l labs sites. Some are readable, others showing the real math, & less digested data targeted toward engineers & scientists.
mike morrisonUser is Offline
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05 Jun 2011 12:36 AM
R values are only part of the picture K value is a bigger one I beleve Dana only quoted icf foam R value not as a system .and we can vent roofs with sray foam on the pitch of the roof with R 50 we also use tech sheild roof sheeting we see a 20 degree diference measured at open framing
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06 Jun 2011 02:56 PM
Not at all related to energy efficiency, but the big problem of advanced framing in my mind is the 24" o/c studs (rather than 16" o/c standard) gives very little support for drywall. If you have any kids/grandkids, they seem to be rather rough in a house and seems like the extra spacing between studs is asking for damaged drywall. I realize this is designed to minimize thermal bridging, but I would hate to be patching drywall every 2 months.
The SipperUser is Offline
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06 Jun 2011 03:59 PM
Of course, there is one other major building system that should be brought into any discussion such as the one that is the subject of this thread. However, there is another forum here that offers a more appropriate place to discuss that system.
The Sipper
Dana1User is Online
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07 Jun 2011 11:30 AM
Posted By mike morrison on 05 Jun 2011 12:36 AM
R values are only part of the picture K value is a bigger one I beleve Dana only quoted icf foam R value not as a system .and we can vent roofs with sray foam on the pitch of the roof with R 50 we also use tech sheild roof sheeting we see a 20 degree diference measured at open framing

As a system ICF is only buying single-digit percentage improvement in thermal performance compared to low mass wall systems of equivalant U value, in most US climates it's a LOW single-digit improvement.

I was talking only  "whole wall" R, not center-cavity R.  The relevance of the K value of the center-cavity insulation compared to that of the framing diminishes rapidly at cavity insulation K values less than 0.25. (R4/inch) due to the miserably high 1-1.25K framing.

Low-E roof decking is only meaningful in places with high sensible-cooling loads.  It can make a real difference in low-moderate R attics, but it's not always commensurate with cost in a higher-R attic.  It makes the most sense when ducts & air handlers are in the attic, outside of the insulation.  Going unvented and thermally breaking with R15-R20 rigid or spray foam on the EXTERIOR will make a bigger difference in your exposed-framing temps.  High-E/low solar absorption roofing materials and keeping roof pitches 4:12 or higher can make as much or more difference than low-E vented decking.
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08 Jun 2011 09:01 AM
I haven't had any problems with 24" (vs the 16" used for interior walls) OC drywall. I wouldn't worry about it even with kids.
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08 Jun 2011 10:00 AM
I haven't had any problems with 24" OC drywall. I wouldn't worry about it even with kids.
I've viewed a large number of houses recently that were built during the "irrational exuberance" of this last decade. A number of them were larger homes than the builders were probably used to building or maybe, they were just forced to use the drywall subs that they could get, but combining large, soaring walls with 24" OC framing can make for some pretty poor results. In most cases, these homes were billed as high-quality construction, but the poor results are visible for all time.

A new homeowner-to-be should at least understand how the quality of the framers and the drywallers go together with big walls and AF design so they aren't surprised when their dream home comes out looking like something less than what they were shooting for.
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08 Jun 2011 10:15 AM
24" OC drywall is a cheap and cheesy way to build. I do not understand why anyone would want to live in a cheap cheesy house. Lots of floor space, but flimsy construction that will fall down in a high wind. Plastic hollow-core doors, cabinets that are stapled together with nary a dovetail in sight, molded EPS trim, vinyl siding, bouncy floors, yech! I made a pact with myself 25 years ago: No more cheap sh*t. I decided that if I couldn't afford high quality, I would do without. It's a policy that has served me well.
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