I have talked, read, discussed, and dreamed about insulation and energy cost until I am dizzy.


I am in N Florida, 3 miles from coast. So hot and humid.  I am in a low lying area and trying to decide on the best insulation for my new home.

I am building 4550 sq feet, 2 story.  I am doing block 1st floor and 2x6 2nd floor.

Here is my question, I am look for a energy effiecienct and quiet home.  I was all set on ICF, but the price is just more than I can justify.  Also, with being in a low area that will flood in a cat 2 hurricane, if I flood the house is ruinded whether ICF, stick or block.

This is how we have the house spec'd now.

1st Floor Concrete Block - Interior walls furred out and 3" of Iceyne CLOSED Cell foam.
2nd Floor - 2x6 - Interior walls sprayed with 5" of Iceynene ClOSED Cell Foam
Roof: Asphalt shingle and underside sprayed with 5" of Iceynen CLOSED Cell Foam.
Window: Are FL code Simonton Energy Star with Vinyl, Low E, dual Pane and a very low SHGC rating.

I feel like this will yield a very effiecient home that will have low cooling cost, quiet, and very consistent tempatures.

The down side is all the foam is very expensive.  Any ideas  or options that will give me the same or better R-Value walls and efficiency, but save me somee money?  I am all ears.

I'd be surprised if an all ICF home is much more expensive than all that icynene. That's a shitload of foam.

I know, but 3 builders all quoted in the 30k more range for the ICF.

I am open to ideas on how to reduce the foam, but maintain the benefits.

Don't know about you, but to me, a 30k upcharge for ICF construction vs concrete block and 2x6 would be a non issue. Considering all the advantages of ICF construction, cut back a little on your finish options and built home that will stand up to the next hurricane that comes through.

5" of 2lb Icynene doesn't even quite meet code for roof R anywhere in FL, and under an unvented asphalt-shingle roof you'd FEEL it.

A better solution for the roof would be to put 3-5" of polyiso rigid board above the roof deck, with a ventilated nailer deck up on furring/purlins, and fill out the R & air seal the roof deck from below with half-pound foam. With this approach you get a full thermal break on the rafters/trusses. (Hunter Panels, Atlas and a few others make vented nailbase panels, but a DIY with standard roofing-iso and furring/purlins works too.) If it's 2x6 rafters rather than trusses, putting some OSB on the interior and doing a rafter-cavity fill of cellulose would be cheaper per unit R than open-cell foam, but you'd still want at least R12-R15 of exterior foam as a thermal break on the rafters, which would bring only slightly above code, but would perform quite a bit better than code due to that thermal break.

Similarly, on the second floor you get more thermal bang per buck going with 2" of exterior rigid foam + spray cellulose cavity fill in a 2x4 studwall than 2x6 framing with 2lb Icycnene cavity fill. The center cavity R of the Icynene would be great- about R25, but the R5-6 thermal bridging of the studs would then dominate the heat loss, reducing your whole-wall R to under R15. With 2" of exterior foam (sprayed closed cell Icynene or rigid XPS/iso) and a 2x4 studwall you'd be close to R20 (whole-wall), since you'd have at least R10 of thermal break over the R3-R4 framing rather than R0 over R5-R6 framing. An R13 thermal bridge passes less than half the heat than an R6 thermal bridge. Unbroken, with a even a 15% framing factor (25% would be more typical), the R6 (2x6) bridges pass more heat than the R25-ish center-cavity of 5" of 2lb Icynene. The 2" rigid + fiber approach may only have an R22-R23 center cavity, but it would have better performance overall due to the R13+ on the framing factor. Air-sealing the sheathing or exterior foam as the primary air-barrier would be on the critical detailing list though. (Beads of caulk under the stud plates, and caulking/gluing the sheathing to the studs as you go helps quite a bit, as does doing the foam as seam-staggered layers of 1" with seams taped both layers rather than 2" of t & g and foam-sealing the edges.)

If your siding has a 1/2-3/4" ventilation gap (aka "rainscreen") between the siding and foam the siding will dry quicker, and if you used foil-faced iso you'd also get some peak-cooling benefit out of it's high infrared reflectivity. (Probably measurable in the cooling bill in FL even with an R20 whole-wall, but not so much in most of the US.)

On the first floor the furring would also be adding bridging if the furring is attached to the CMU block. Putting 2" of t & g XPS (or double layer of 1") between the furring and CMU, and adding an inch or two of 2lb foam to air-seal and further insulate might be a cheaper way to get the same center-cavity R and minimize thermal bridging. Assuming it'll flood eventually someday, stick with XPS (or EPS) rather than iso on the first floor, since iso isn't rated for standing water- it's more hygroscopic than open cell foam in some test conditions (but it's still fine in roofing apps, even with occasional leaks, especially with a ventilation gap between foam and nailer deck.)

Thanks Dana1.

So you rec recommend, on CMU 1st floor, put 2" of rigid board & then attach the furring 2x4 over this and sray the closed cell foam. Giving the thermal break between the CMU & Furring strips and getting the foam for the air sealing quality. On the second floor, put the rigid board on the outside to reduce bridging, then spray a few inches off foam inside for the R factor and air seal.

Posted By WildLines on 12 Mar 2011 08:55 PM
Thanks Dana1.

So you rec recommend, on CMU 1st floor, put 2" of rigid board & then attach the furring 2x4 over this and sray the closed cell foam. Giving the thermal break between the CMU & Furring strips and getting the foam for the air sealing quality. On the second floor, put the rigid board on the outside to reduce bridging, then spray a few inches off foam inside for the R factor and air seal.

On the first floor yes.  You're trading some rigid-foam for some spray, but it's thermally broken and easy to make air-tight.  The installed cost of rigid foam climbs the more features you have to cobble around, and the wastage fraction goes up.  You'd have to price it out- it may be cheaper to do all as spray, but DO set it up with thermal breaks- no stud-edges or furring in direct contact with the CMU. In my neighborhood the cost of XPS per board foot (f.o.b. the distributor's yard) is about 2/3 that of 2lb Icycene (installed). The ultimate installed cost for simple stuff usually works in favor of going with XPS, but sometimes it's a wash.

On the second floor, a cavity fill with spray cellulose rather than foam would be cheaper than spray foam in the cavities.  Tape & foam/caulk seal the rigid foam as the primary air-barrier.  You could use half-pound foam in the cavities, but spray cellulose would be about the same or slightly better performance than half-pound Icynene, usually for less money.  The difference in whole wall R wouldn't be sufficiently different going with 2lb Icynene vs. cellulose or open cell cavity fill to be worth the added cost- the real performance boost is in the R10-12 thermal break over the studs, which will usually cost the same or slightly less than 2" of 2lb Icynene.  You'd get a better performance difference going from XPS to iso on the sheathing insulation than you'd get by going from cellulose to 2lb Icynene in the cavity, usually for a smaller uptick on installed cost.

The greater the fraction of total R that's in the thermal break layer, the better it will perform.  With 2" of  XPS or iso + cellulose-filled 2x4 studwall you're looking at R20-23 whole-wall performance.  If you use 2" of XPS and 2x furring on the CMU wall w 1.5" of 2lb Icynene you're at ~R16, about a 25% boost over what you'd get on 3" of 2lb Icynene between studs/furring without the thermal break.

Thank You Dana.

Mind if I ask your back ground or how you have this knowledge?



One builder said to not use and 2lb/Closed Cell in the house, he said mold will form between the foam and block as the moisture moves through the CMU. Everything I read is the foam is not a food source for mold, but not sure if it will grow on the block?I

They also recommended not using the closed cell in the wall cavaties for the same reason. 

I really think closed cell is fine from a mold standpoint and I would prefer to use the 2ld closed cell on the CMU & Wall Cavaties, & roof so that I get the higher R value and the increased strength.

Thoughts?

WildLines,

Since you are in North Florida, do you plan to have stucco on the exterior of the block and wood stud walls?

I have been designing a home for farther South and the builder there said he would install 2" of EPS and then the furring strips for the drywall.  What I really wanted for this client is to have the foam board outside the concrete block and then stucco but the builder will not budge.  I just think that it would be a better system with the foam board outside provided it does not touch the ground.  I do not want termites tunneling upwards through the foam.  By leaving an open space, the termintes would have to build mud tunnels to get to the foam board.

Alton,

I am doing either real cedar shake or a hari shake exterior. I agree, exterior board seems better from a thermal bridging aspect and I have spoke to a couple builders about it. I have also found them to not like this idea.

We are currently pricing 2 options on our CMUi 1st floor. Either filling the CMU cavities with a foam and then rigid board on the inside or we will use 1" of rigid foam board on the inside with 1 1/2 inches of Closed cell foam sprayed over it. Most likely the later. We will be doing 5" of closed cell in the 2nd floor walls that are 2x6.

Posted By WildLines on 15 Mar 2011 09:22 PM
Thank You Dana.

Mind if I ask your back ground or how you have this knowledge?



One builder said to not use and 2lb/Closed Cell in the house, he said mold will form between the foam and block as the moisture moves through the CMU. Everything I read is the foam is not a food source for mold, but not sure if it will grow on the block?I

They also recommended not using the closed cell in the wall cavaties for the same reason. 

I really think closed cell is fine from a mold standpoint and I would prefer to use the 2ld closed cell on the CMU & Wall Cavaties, & roof so that I get the higher R value and the increased strength.

Thoughts?

Me? I just make it up as I go along! 


My degrees are in math & physics, but my family was in the construction biz for decades. (I'm currently working as an electrical engineer, not building.)  I've followed/studied the moisture and thermal issues related to construction methods in buildings since I was working the family biz back in the 1970s. (I guess I'm not a kid anymore? )  I play around with WUFI and other when optimizing a stackup, but that's only for fine-tuning it.  There are plenty of internet sources for details of why/how stuff works (or doesn't) in a particular climate. (The Building Science Corp has many decently written briefs with pretty pictures and explanations on their website.)  The US National Laboratories also have lots of information and tools that makes quick-estimates easy. (eg: the ORNL Whole Wall R Calculator by which you can determine the effect of exterior foam or stud spacing on whole-wall R, thermal bridging included.)  I'm also a spec-nerd- I look stuff up (and often), particularly when something doesn't sound quite right relative to my experience or physical intuition. (I aced thermodynamics and managed to graduate with honors on the physics degree, not that I've ever resorted to the real math on building-related stuff.  But I have a pretty good handle on how water behaves  and how heat transfer works, etc.)  For issues specifically related to a FL climate searching the Florida Solar Energy Center archives results in a lot of well-documented building-science material.
 
On the interior CMU walls, consider using 2lb Icynene (product name MD-R-200) which has about 2-3x the permeance of generic 2lb spray polyurethane foam (SPF).  (XPS has about 1-1.5x, and Type-II EPS has about 5x the permeance of SPF.)  While it's generally best-practice to provide a ventilation cavity between exterior masonry and a moisture susceptible assembly on the interior, if you have reasonable drying capacity toward the interior and the material in contact with the masonry isn't affected by moisture you should be able to skip it.  If you have stucco, brick or other cladding on the exterior of the CMU, put the ventilation cavity between the cladding & CMU vented to the exterior, which should reduce the average moisture drive from the exterior by about an order of magnitude.  I haven't lived in FL, nor have I dissected any homes there, but there are existence proofs of urea-foam insulation sprayed directly on masonry that have survived since the 1970s. (Maybe there's mold growing on the CMU in those houses?)

On the other hand, if you sprayed 3" of closed cell SPF directly on the exterior of the CMU you've protected it from the exterior moisture as well as air-sealed it, putting the CMU inside the conditioned space, but I'm not sure if that's an approach that would work for you.

In wall cavities, same story.  With a non-permeable exterior foam foil faced iso detailed as an air-barrier,  and a permeable (such as cellulose or open-cell foam) or semi-permeable (2lb Icynene ) cavity fill, an air-conditioned building will have a considerable capacity to dry toward the interior.  But with standard 2lb SPF  you still run the risk of trapping moisture in the outer edges of the studs, should it ever find a way in there.  If using 2lbs SPF cavity fill, use 1" of XPS rather than foil faced iso it's still weather resistant but semi-permeable, and build it with a rainscreen cavity (at least 3/8", vented top & bottom) between the siding and weather resistant barrier layer would allow the assembly to dry toward the exterior to could tolerate the very low permeance of 3-5" of 2lb SPF.  In FL climates it's best to put the lowest-perm stuff on the exterior of the assembly since moisture drives are primarily from the exterior, but with a rainscreen gap on the exterior and an air & vapor impermeable cavity fill like SPF it still works with semi-permeable XPS and a rainscreen gap as the drying path toward the exterior.

IMHO rainscreen gaps & masonry wall cavities should be called out by code any place that gets more than 15-20" of rain/year. FL is good for 40"/year even in the drier parts. See:  http://www.learnnc.org/lp/media/upl...precip.gif  Rainscreen gaps also can be used to good effect to ease winter condensation moisture issues in colder climates, but that's beyond the scope of/irrelevant to a building in FL.  It's  common everywhere in commercial construction and required by code in Canada even on single-family homes, but is still rare in much of the US on residential structures.

On the roof getting away with SPF is tougher, and depends a bit on the roofing material.  If you use composition shingles, they're highly vapor retardent- a secondary nailer deck for the shingles mounted on furring would be best, allowing both the nailer deck and structural deck to dry into the cavity. If it's standing seam steel mounted on purlins, no problem, but mounted on the deck it may trap moisture in spots. For cementitious or ceramic- it depends on the particulars.  From a thermal point of view, exterior foam provides a similar boost in performance as with walls.  A roof is not just a tilted wall, particularly from a weather-driven moisture point of view, but they are similar structures from a thermal point of view.  An R30 center cavity assembly with R5 thermal shorts (5" of SPF between 2x6 rafters) significanly underperforms an assembly with R12 on the exterior (2" of iso above the roof deck) and R18 between the rafters (3" of SPF.)  Here again the higher perm rating of 2lb Icynene might make it a favorable choice for the application.  It takes 4" of the 2lb Icynene to make R20, but it'll have slightly better drying capacity than just 2" of SPF (about 1 perm for 4" of cc Icycene compared to somewhere between 0.5-1perm for cc SPF, depending on vendor and actual installed density.) Then with 2" of XPS on the exterior you'd make code-min at R30, but since the thermal bridges are now up around R14 (a ~3x improvement over R4-5 of unclad rafters without exterior foam) it'll perform at least ~1.25-1.5x better than a code-legal job done with fiberglass batts between rafters or on an attic floor (even better at the temperature extremes.)

It takes about 3" of closed cell SPF  to become structural, but going an inch or two thicker than that doesn't improve it so much that you'd want to jump through hoops in the design to be able to let the water out.  Still, 3" of SPF is highly vapor retardent and you'd have to jump anyway. With the higher perm/lower-K Icynene it's a bit more forgiving, since it still has reasonable drying capacity, and probably a similar structural strength. (I'd have to look that up- if the data exists.) I'm not a big fan of that company's marketing approach- they seem to encourage people to cut corners on code R-value, but FWIW their 2lb foam has some easy-to-deal-with moisture specs, and it's usually a bit cheaper than 2lb SPF at a given R value. It may be the better choice on the roof deck if you want a structural thickness that retains drying capacity, which makes it a no-brainer for the rest of it too. (The more board-feet of material, the better the discount.)  But you'll get a far bigger performance boost out of 2" of exterior XPS than simply adding another 2" between the rafters on the interior.