A builder received these bids for full encapsulation with spray foam (3.5" of foam in the 5.5" walls and 5.5" in roof rafters),
foam rafters + flash & spray? (2" foam and 3.5" cellulose) walls, or foam rafters + cellulose walls.

Keep in mind that these are the prices after being credited 5.5" of fiberglass in the walls and 5.5" of cellulose in the attic (beside the point but yep, they will make up the remaining R-value with cellulose on the attic floor).

Do these numbers look right to anyone?? How is it possible that flash & spray walls (total 5.5" of insulation) is more expensive than 3.5" of foam????


Thanks!


Quote information:

1.) FULL ENCAPSULATION: $11,310.00

2.) OPEN CELL FOAM ENCAPSULATION SYSTEM WITH 2 INCH OPEN CELL WALLS AND 3.5 INCH CELLULOSE : $11,695.00 (Hybrid - Roof encapsulation with 2" spray foam and 3.5" cellulose blown on top of that)

3.) OPEN CELL FOAM ENCAPSULATION SYSTEM WITH CELLULOSE WALL SPRAY (R-19) TOTAL $8,615.00 (roof encapsulation with all cellulose walls)

>>How is it possible that flash & spray walls (total 5.5" of insulation) is more expensive than 3.5" of foam????<<

With flash foam + spray cellulose you pay more for the foam since it's lower volume, and you add labor since it's a multiple step process. It's a 43% reduction in foam volume, but you're adding 3.5" of cellulose, which isn't free. The total center-cavity R is increased by ~60% for a modest 3.5% increase in overall price.

But since you've increased the thermal bridging to 5.5" of wood instead of 3.5" the whole-wall R scales too. With 3.5" of o.c. foam @ R3.5/inch, with 3.5" depth and a framing factor of 20% you've gone from about R10 to about R14.5 a 45% increase in whole-wall performance for that 3.5% boost in price. Only partially filling a stud or rafter bay is false economy since it increased the heat loss through the studs. At ANY R/inch with even the crummiest & cheapest insulation type, the performance boost you get with a full cavity is worth it.

FWIW, air sealing rafter/stud bays with open cell foam usually isn't as perfect as with closed cell foam, and although it's air-impermeable, open cell foam is VERY permeable to water vapor, making it less suitable for the underside unvented roof decks & sheathing flash-foam in US climate zones 4 & higher. Even a sloppy uneven inch-ish of closed cell will seal better and provide better moisture protection for the sheathing materials than 2-6" of open cell.

Thanks for the post, I hadn't considered the thermal bridging across the sides of the exposed 2x6's when only 3.5" of foam is installed. I guess I should have also mentioned a little more information...the home is 2800 sqft. 2 story, barrell tile roof, all brick exterior, taped R-3 foam wall sheathing, and advanced framing in zone 3.

All very good points! I will probably go with 2" of foam in the walls + cellulose to totally fill the cavity especially for the price difference however, in reality is 2" of foam sufficient to air seal the walls close to the same degree that 3.5" would? One of my goals is to reduce the ACH50 enough to where the HVA/C tonnage would require only one unit thus saving me the cost of the second unit. Is it possible to A/C a two story with a single unit?

With closed cell foam you don't even need 2" to get a good air seal, and with open cell even 3.5" isn't assured. It's just easier to get a good seal with closed cell. Caulking the sheathing to the studs ( and the bottom plate to the subfloor) or detailing the exterior foam as your primary air barrier can be tighter first-pass than counting on a foam cavity fill. The very high expansion ratios of open cell seem to make it less tightly bound to the studs, with more likelihood of gaps in the air seal long the sides of the studs, and small cavities/bubbles at the corner where the stud meets the sheathing. A single splinter protruding from the wood can be enough to create a tiny void, where the initial expansion of the foam on the tip of the splinter blocks the rest of the foam from filling in. This doesn't seem to happen with the lower & slower expansion of closed cell foam.

With the brick cladding and only half-inch rigid foam on the exterior, the type of rigid really matters, since the vapor drives from the exterior are quite high. Masonry wicks up a lot of rain & dew moisture, and releases it at high intensity when warmed by the sun. In zone 3 you don't really need to worry much about wintertime moisture accumulation, and with a moisture-reservoir type cladding putting a low-perm foam on the exterior is better. Half inch XPS or 3/4" EPS are orders of magnitude more permeable than foil-faced iso, which is probably the way to go here. With taped seams and foamed edges it can also serve as the primary air barrier. If you already have XPS/EPS in there, putting any more than an inch of closed cell foam in the stud bays may reduce it's drying capacity to the interior too much. (Higher-perm variants such as Icynene MD-R-200 would be OK though- that stuff is still about 1.5-2perms at 2".) If there's any doubt, use only open cell foam (or just cellulose and caulk the sheathing/stud interfaces), for the higher vapor permeance.

On the AC front, whether you can air condition the place with a single unit depends on what you mean by "unit". A single compressor/condenser unit and an evaporator coil in a ducted air system, sure! One window-shaker, probably not. A single-head mini-split, also not likely unless it's a very open floor plan with a lot of air communication between the upper & lower floors. Managing solar gains (and designing them in or out, as appropriate) and getting a high-R attic are probably bigger factors on the AC loads once you're under 5ACH/50 (which isn't super-tight.) The high-mass roofing material is also a huge help. In the eastern half of US zone 3 lower ACH50 numbers make a big difference in the relatively high latent-loads found there, even when the sensible loads are low or non-existent. ( San Diego CA doesn't have even 1/4 the latent load of Charleston SC, but both are US zone3.)

I would look at other alternatives. First off spray foam does not prevent thermal bridging. Lumber can account for 25% or more of a wall surface. Thus 25% of a 2 x 6 wall will have an R value of about 5 not including drywall and cladding. This significantly drags down the whole wall performance.

I would consider a dense pack insualtion in the walls with the appropriate amount of rigid insualtion on the exterior. I take it you are insulating the attic floor and not the underside of the roof. The attic floor should be air sealed. This can be done with either one part canned foam or sprayed OC or CC. The only places that need to be sprayed are areas that leak. That is top plates, plumbing stacks etc. Your drywall ceiling is already an air barrier so no need to spray it. Then pile on the celluose to above code minium.

Joe L at BSC talk highly of dense pack insualtion in retrofits as comes close to an air barrier as you can get. Though this is not a retrofit useing a dense pack compliments your air sealing measures. No air barrier is perfect and if you knew where the imperfections where you could fix them. Since you dont then use an insulation that is a backup. Dense pack cellulose fits the bill. A few of the new dense pack fiberlgass test out as tight.

For the attic you want raiser heel trusses.

Roberth: In advanced framing as mentioned by Chris you're talking ~15% for a framing-fraction, not 25%. The whole-wall R of cellulose or open cell foamed 2x6 AF wall is about R15, to which he has added R3 of rigid foam, so he's really looking at ~R17-R18, give or take. If Chris's R3 rigid sheathing is taped & edge-sealed, it'll beat the air tightness of any dense-pack solution, since dense-pack doesn't block air under the studwall plates and band joists, etc. (Wet sprayed standard density is probably the better bang/buck if the primary air-barrier is done right.) Dense-packing makes more economic sense in retrofits, where the air-barriers are undefined and can't fully remediated without a lot of house-surgery (like re-siding or all new interior walls.)

I'm assuming the framing is already up and the wall thickness pre-determined by it's placement on the foundation- you can't cantilever brick off the foundation, so thicker exterior foam probably isn't in the cards, but yet, that would be a MUCH more appropriate use of the foam budget. If the framing is NOT up, you get more bang per buck out of going with a 2x4 16" o.c. wall (with advanced-framing type corners) and 2.5" of exterior foam (same wall thickness). With 2.5" of EPS and detailing the sheathing as the primary air barrier (or using foil/vinyl faced EPS + appropriate tape for the facer) you get R10 on the exterior for about buck a square foot. The 16" o.c. 2x4 wall will have about the same load capacity as the 2x6 AF wall, and about the same amount of board feet. With a cellulose fill and some AF features it'll come in at ~ R10-R11 for a whole-wall R, and with foam would be at R20+ for comparable or less money as the ~R18 wall under discussion. The PITA issue on thicker foam in his stackup becomes sourcing masonry ties that are reasonably priced and easy to install.

To me a home that is in the bid stage is not under construction.

I never said the dense pack was the air barrier. They would be a couple of different ways to detail an air barrier. That air barrier is not perfect plus over the lifer of the house someone may puncture it. The BIB system does not stop air flow in the same manor a dense pack or sprayed cellulose would. A couple of the new dense pack fiberglass insualtions are on par with cellulose. So it acts as a secondary barrier.

How many builders install the air barrier and then test for leaks before preceeding. Energy Star has presriptive standards that allows a visual inspection without testing. I see a lot of opportunity for mistakes to be made that would not be found until a blower door test is completed.

On custom builds the details of cavity-fill & other insulation are often left as a late-breaking detail AFTER the shell is already up all too often, but hopefully you're right, this is in a design phase with all details open. Many insulation posed on this forum already have the roofing shingles up and the siding on, so I've begun to presume that to be the case ( but perhaps I shouldn't.)

You can dense-pack cellulose in a BIB just as easily as with it already rocked-in, and the results are pretty similar if you roll the BIB flat before adding the sheet rock to minimize the potential thermal bypass gap between the insulation & wallboard. (The walls are flatter that way too.) The wet-spray low density stud-scrubber approach isn't quite as air-retardent as dense pack, but it's still pretty good compared to low density fiberglass. Spider or Optima at 1.8lbs (in a BIB system or already rocked in) are similar to 3-3.5lb cellulose in air retardency, and are slightly higher-R. But no dense-pack in a stud cavity can block the a leaks under studwall plates, band joists & subfloors etc, whereas detailing the exterior rigid foam or structural sheathing as an air-barrier can. (A bead of caulk under the stud plates etc, as the framing goes up is the cheapest & easiest way to treat that, and there's something to be said for caulking the sheathing to the framing too.)

Detailing the structural sheathing as the primary air barrier is the most-preferred option IMHO, since it's the most durable and most-protected, but that isn't going to be possible to do perfectly if the exterior foam already up and they're discussing what to stuff in to the stud bays.

Very few builders are blower-testing the closed in shell prior to insulating & finishing, but that's the time when it's easiest & cheapest to rectify many leaks. The Energy Star levels of air-tightness are pretty lax by R-2000 standard, and R-2000 is more than 2x as leaky as a minimally tight PassiveHouse. The IRC levels are pretty breezy too. In a house I cared about I wouldn't be taking a whole lot from the Energy Star playlist, even though it's way better than the typical code-min approach.

Sometimes I fall back to my experience and assume that is the way it is being done. In my appraisal days I did many appraisals based on plans and specs before a shovel of dirt was turned. The builder wants to know the buyer qualifies for the loan and the home will appraise. During construction the appraiser is asked to do inspections for construction draws, during the inspection he checks to see if it is being built to plans and specs.

Since the loan is locked in there are usualy not changes that would force the buyer to dip into their pockets for $10,000 unless is was a very high end home. When the home is complete the appraiser is asked if the home was completed per plans and specs. If the appraiser is asked for a new value that is a new appraisal.

When I hear BIB I think lose fill at low desnity and not comparable to dense pack for air tightness. No one in our area intalls cellulose in new constrction.

...and yet, dense-packing into mesh is done all the time by some contractors (to avoid having to drill into sheet rock or sheathing compromizing air-barriers or finish-quality.) It's much more inspect-able too. But it requires rolling the bowed out mesh flat as part of the process. eg:

http://www.advancedhomeenergy.c...pdated.pdf

I'd hazard that most high R homes under discussion on this forum qualify as "high end" in terms of budget flexibility on thermal performance items, if not necessarily the luxuriousness of the interiors. (cood b rong offan am)