Please help! We are building a new home in Texas. We have a very large attic (9,000 sq ft of decking). I've spoken with several installers of foam insulation and received multiple quotes. It seems the most energy efficient application that is cost effective is to spray the rafters with 3" of closed cell foam to "seal" the attic and put 11" of cellulose on top of the drywall in the attic. I'm a bit concerned about moisture build up in the attic. Any opinions would be appreciated! Thanks.

Forget about the cellulose just seal the rafters and let the house breath
Go to www.demilecusa.com
They can answer questions and provide quotes
FOAM the 21 st Century evolution in insulation and waterproofing

You will save at least 30% once completed

Thanks so much for the response. Would it be acceptable to seal the rafters and do cellulose in the exterior walls? When you say "save 30%" are you referring to energy cost?

If this is new construction have the walls and rafters sprayed if you do that and install gas and on demand water heater you could save as much as 50% that has been documented here in Florida.
Sealing off the attic reduces heat there from average of 120 degrees down to around 85 degrees all year.
Reducing this heat buildup save energy and by not adding cellulose to the ceiling allows the home to breathe
Ck foam web sites for more details
Its the future

Do you have any info on doing cellulose in the walls and open cell foam on the rafters? How many inches of open cell on the rafters? Our house faces N & S and most doors & windows are covered by porches. We are going with the tankless gas hot water heaters. I will check out the website. Thanks again for all you info!!

buck3647, you're all wet, and you're steering him dead-wrong on 3" of cc. foam being sufficient insulation for TX. You're advocating a ~R20 solution over an R50+ solution- there WILL be a substantial performance & comfort difference. Seal the attic yes, skip the cellulose, no WAY.

And "...save at least 30%" "..as much as 50%" is bogus in reference to new construction, bordering on outright fraud. (30% or 50% elative to what?) 30% relative to a leaky house with 5+ air changes/hour natural ventilation rates, and R19 fiberglass batting, maybe, but highly dependent on how tight the house is (and in FL, the slope & solar absorption & IR emissivity of the roofing,etc.) But for new construction with 1ACH natural ventilation & 11" of cellulose on the attic floor, with roof pitches greater than 2:12 and cool-roofing material, you're gonna have to show me. It's simply not credible. (The laws of physics can only be violated in the marketing spin-meister's dreams.)

And "...let the house breathe" is the polar opposite of 3" of closed cell foam, with a vapor retardency of less than 1.0 perms, making it a not-so-breathable semi-impermeable solution (but fine, in this instance.) "Let the house breathe" is an old-school wrong headed concept. "Built it tight, and ventilate right" results in fewer humidity issues, better indoor air quality, and protects the building better. Foam insulation, properly applied can be a large part of that approach, but the where & how it's "proper" is highly dependent on climate zone, with few universal prescriptions. Where & how you use vapor retardent layers in the stackup depends on summertime dew points, and wintertime outdoor temps.

In TX there are NO regions where 3" of cc foam on roof deck + 11" of cellulose on the attic floor would cause a moisture problem. On the gulf coast the vapor retardency of the foam helps reject the high inward vapor drives of sun on a dew or rain-soaked roof, and the hygric buffering of the cellulose moderates the attic humidity. In the panhandle it's not cold enough long enough in winter to end up with condensing conditions in the attic based on the R value balance. (The average OUTDOOR temps in Lubbock in January are above the dew point of 68F 30% relative humidity indoor air, so the average attic temps won't be either.)

As long as you don't VAPOR seal the attic floor (air-seal, yes, but use no kraft, foil or poly vapor retarders or vapor retardent ceiling paints) you won't have a moisture problem in the attic. Cellulose can absorb & release significant amounts of humidity without damage- it'll PROTECT the framing timbers by wicking moisture away under the rare hours of condensing conditions, and won't just accumulate moisture in any TX climate zone.

Cellulose in the walls can also a "right" solution. On the humid gulf coast it may require an exterior-side vapor retarder. Foil radiant barrier under the siding, with at least a 3/8" rainscreen gap would be ideal there, but 2" of closed cell foam/foam board on the outside of the sheathing would also work. In the drier parts of TX vapor retarders aren't necessary or desirable anywhere within a wall stackup, but air-barriers YES. Foam can be your friend here too, but a full-cavity fill is a more expensive route to air-tightness than some other solutions, and doesn't have the hygric buffering or thermal mass boost you get with cellulose. (But if the house is in a flood zone, foam would be preferable to cellulose.) "Flash & fill" with an inch of closed-cell on the interior side of the sheathing of a 2x6 stud bay to achieve high air-tightness (with a vapor retardency of only ~2 perms) with cellulose to bring up the total R is becoming a common solution. In colder regions that stackup may require interior-side vapor retarders, but not anywhere in TX.

What is not considered when calculating R factors is the air leaks by eliminating air leaks the R factor is greatly increased therefore even though Closed Cell 2 LB foam is rated at a 6 per inch by eliminating the air leaks bumps the actual R factor up to R 30 or higher by adding another inch.
Again, would suggest reading the history and documentation on closed cell foam
You don't know what you don't know
With bat insulation and cellulose you will always have air leaks

Posted By buck3647 on 20 Apr 2010 11:05 AM
What is not considered when calculating R factors is the air leaks by eliminating air leaks the R factor is greatly increased therefore even though Closed Cell 2 LB foam is rated at a 6 per inch by eliminating the air leaks bumps the actual R factor up to R 30 or higher by adding another inch.
Again, would suggest reading the history and documentation on closed cell foam
You don't know what you don't know
With bat insulation and cellulose you will always have air leaks

I've read plenty. Air sealing the roof deck is not sufficient.  (Methinks you need to take your own reading prescription.)

"Bumps the actual R value to R30 or higher" is completely inaccurate, would not pass at the ASTM C 518  test required to make that claim, and the FTC could come down on you for fraud for MAKING that unsubstantiated claim. The R6/inch performance has nothing to do with it's stopping air infiltration, and everything to do with how samples perform in an ASTM C 518 conducted heat transfer test at modest delta-Ts.  The whole house infiltration argument you're making is beyond the scope of that test, but valid.

The performance of the cellulose is somewhat enhanced somewhat by going with an un-vented attic, especially if the air-sealing at the ceiling was sloppy or ignored.  The R-value of cellulose does not vary much with delta-T due to convection within the fiber the way it does with low density fiberglass, but even the convective R-value loss with fiberglass also decreases with the depth of the material (its dT/dx with depth that drive internal convective issues with fiber insulation.) Cellulose at 1.5-2lbs density (typical open-blow) has VERY little convective loss due to these factors, but isn't good air-barrier against whole-house stack-effect losses.  A sealed attic fixes this.  In walls, dense-packed to 3lbs density cellulose is pretty good against stack-effect issue too (not quite as good as foam, but WAY better than batts.)  Even at 2-2.5lbs density (typical 2 hole method) it's considerably better against convective losses than any batts.

I've used foam, I LIKE foam, I'm also all for going with an unvented-attic approach using foam, but your completely off base with these wild assertions about it's performance, or how it performs in combination with cellulose in an unvented attic approach.  Like I say, I've read lots- if you have something other than bald claims to back it up, let's see the evidence- I'm not averse to reading more.  What I don't know , I don't know, but what I DO know about heat transfer issues is quite a bit.

I really appreciate all the information. I've spoken with one of the foam manufactures as well as several installers to get multiple bids. From what I've seen (cost being the biggest) I think we are planning to do 5.5" of open cell on the roof rafters to seal the attic and do cellulose in all exterior walls as well as not vent the attic. We plan to do 5" open cell on the ceiling of the garage because we have a bonus space above that may be used as living space. It looks like the cost to do this is around $24,000 (9,090 sq ft of decking & 4200+ ext walls). Please let me know if this sounds effective for NE Texas along with the 2 gas tankless hot water heaters and what to expect on the ROI. One more question ~ what is the biggest cost savings benefit on going with a two stage heat pump over a single stage heat pump when using zones? Is it worth the price difference? Thanks again!

Thanks so much for forwarding on the info to Demilec. The reason we are considering cellulose in the walls is two reason 1. COST 2. we were concerned with how easy it is to access wires for electrical & alarm system if we have foam in the walls. Any input? My understanding is the cost is much greater for foam vs. cellulose. Thanks.

Again I am in Florida so what goes in Texas is unknoiwn I would call or email Demilec and at least converse with them perhaps they have installers for more estimates, Min of 3 I say

buck3647: Back to large fonts again eh? That makes for a more convincing argument! I'm sold, you're absolutely right, it'll save 30-50%.

texansteph: Got a zip code? (Useful, for weather data, etc.)

5.5" of half-pound open cell on the rafters without cellulose on the floor might meet minimum code, but it's on the skimpy side at ~ R20-ish. Cellulose tends to be cheaper per unit-R than half-pound foam, but the difference in cost is sometimes pretty small. (Closed cell is typically 2x the cost of cellulose at equivalent R.) Once you have it perfectly air sealed, its all about R value. Cellulose has a more stable R value over temperature than either half-pound foam or fiberglass, and there is no moisture-trap hazard putting some on the attic floor. Open cell on the roof deck is nicer to the roof deck than 3" of closed cell, since it affords the roof deck more inward drying capacity, and leaks can be more readily located & fixed. The limited downside of that is that the latent load on the AC is slightly higher (but not enough to care.)

Tankless HW heaters are rarely cost-effective on fuel-savings alone if natural gas, but may be if propane, or you're looking to fill some monster-sized soaking tubs and would otherwise need 100gallon tank heaters. If you're in a low freeze risk zone you might get more bang for buck out of a low-cost simple pumpless batch solar installation. If your family is primarily a showering vs. tub bathing crew, drainwater heat recovery heat exchangers A: more than double the showering time you get out of a tank heater. In warm-water TX it makes a typical tank heater into an "endless shower" situation at 2.5gpm shower flows, and reduces the fuel used for showering by over 40% (performance depends on the size of the heat exchanger.) You need at least 4-5' of vertical drainpipe downstream of the shower to work though- first-floor slab-on-grade units don't exist. It only works for simultatnous hot-water and drain flows though- for tub fills that drain later the return is zero. But if the shower is being uses 30minutes+ per day, it can be cost effective- a bigger average efficiency boost than going from a 0.60EF tank to a 0.82EF (or even 0.90EF) tankless for a similar difference in installed cost. If you go with a tank, going with force-draft and electronic ignition rathter than atmospheric drafted reduced the whole-house air infiltration due to open flues,etc. If you go tankless, models that are sealed-combustion, with combustion air ducted in from the exterior eliminates all air infiltration related to hot water heating. (Many can be made sealed-combustion with kits.)

Going 2-stage on the AC reduces short-cycling on zone calls. (By how much is highly design-dependent.) Reduced short-cycling is both an efficiency & maintenance issue. Whether it's "worth it" from a cost-effectiveness POV depends on a number of factors that you're not likely to be able to easily sort out on a web-forum, but you WILL be more comfortable for lower operating cost during the cooling season going 2-stage. Getting the AC right-sized, and not oversized is the first step though. A Manual-J type heat gain/loss analysis for every room in the house needs to be done to get both the zoning & compressor size right, and should be provided by the HVAC designers. (If they're giving you an X tons per Y square feet number, they're not doing the math.) How much insulation, and what type of windows & roofing you use makes a difference in AC sizing as well- they can't get rea on the AC design until that work is already etched in stone, or at least "pretty good".

The zip code is 75503. When I spoke with a rep at Demelic she recommended 5.5" open cell in roof rafters and 3.5" in ext walls to envelope the house. The rep was out of Dallas. I'm waiting on a few more bids but it doesn't seem like we will save that much to do cellulose instead of open cell in the ext walls. There are so many different was it seems to use the different products either alone or as a hybrid system. I just want the biggest bang for the buck without having a diminishing return on investment. We plan to be in the home about 10 years and would like the energy efficient investment to pay for itself before then so we aren't wasting money and future homeowners have all the savings.

Posted By texansteph on 20 Apr 2010 11:12 PM
The zip code is 75503. When I spoke with a rep at Demelic she recommended 5.5" open cell in roof rafters and 3.5" in ext walls to envelope the house. The rep was out of Dallas. I'm waiting on a few more bids but it doesn't seem like we will save that much to do cellulose instead of open cell in the ext walls. There are so many different was it seems to use the different products either alone or as a hybrid system. I just want the biggest bang for the buck without having a diminishing return on investment. We plan to be in the home about 10 years and would like the energy efficient investment to pay for itself before then so we aren't wasting money and future homeowners have all the savings.

The thermal mass of cellulose gives it a slight performance boost in mixed climates like Texarkana's, making it roughly equivalent to Demilec in the walls despite a ~R2-R3 center-cavity disadvantage under steady-state conditions.

Texarkana is close enough to the gulf-coast heat & humidity that an exterior vapor retarder on the walls may be a good idea.  A 1/2-3/4" layer of foil-faced iso sheathing on the exterior, with a ~3/8" air gap between the housewrap & siding (built-out with furring/ripped ply through-screwed to the studs, or using a purpose-built rainscreen product) would roughly double the clear-wall R-value at the studs, and provide a more significant boost than the difference between cellulose/foam for the cavity fill. FSK-taping the seams and foaming/caulking the edges turns it into a very effective air/vapor/radiant barrier.  From a raw heat gain/loss point of view that is about a 25-30% reduction in conducted heat transfer (and HUGE reduction in radiated heat transfer on the sun-baked sides of the house.)

A cheaper approach would be to just use exterior foil radiant barrier as a vapor retarder/ radiant barrier/air barrier outside the sheathing, and a 1/4-3/8" rainscreen/air gap between the RB & siding, but that approach provides no thermal break at the framing, and the conducted performance suffers both summer & winter.

Cellulose cavity fill & exterior iso sheathing gives about the same conducted R-value as if you did a full closed-cell foam cavity fill (better than a Demilec open-cell fill) due to the thermal break at the framing, and has better radiated heat rejection.  This is probably the better bang/buck. at the walls.

As for the roof, "cool roof" roofing materials buy you something in your neighborhood, particularly (but not exclusively) for roofs with low slow (2:12 or lower).  If applied at the recommended density, 5.5" of  Demilec gives you ~ R24.5 (it's 0.7 lb foam not half-pound-  ASTM C 518 tests yield ~R4.45/inch of thickness), which may meet code-minimum in your neighborhood. (R30 seems to be the code trend for most gulf-coast states though.)  Open blow cellulose is much cheaper per R than wall installations-  doubling the R by adding R20-25 in cellulose (6-7") at the attic floor may in combination with the Demilec may still be cost effective in a longer-term NPV analysis, but probably not at 10 years.  The Oak Ridge Nat'l Lab R-value table recommendations for are based on very conservative cost & discount rate assumptions, for a 25-year maximum for going positive in a net-present-value calc.  For Texarkana R30-R60 is recommended for attics.  (For walls, R13-R15 + R0-5 sheathing insulation is recommended.  Half-inch iso is ~R3.)

Demanding 100% payback in utility costs at a 10 year NPV is somewhat silly- you'd have to go below code-minimums to make that work.  (You may well make 10 year payback in a simple-terms though.)  Resale on a house may/may-not be affected by it's historical utility bills, but if codes get tightened or utility prices skyrocket as part of a state/national energy or carbon policy, it could.  Under CA Title 24 sale of existing houses are now require to meet an increasing building efficiency standard. (Exisiting stock doesn't have to meet new-construction levels, but there is a step-function minimum that ratchets up annually.)  If you start out at bare minimum (or below) code you could be caught short at time of sale if similar policies get implemented in TX. 

Thank you so much for the info. It does seem crazy now to worry about the 10 year roi. I would assume in 10-15 years the required standards in TX will get much more rigid. One more question. We are doing the foil backed foam board on the "knee walls" in the attic to give them something to spray the foam to b/c we have a 10/12 roof pitch and about 1000 sq ft of porch covering 3 sides of the house. We didn't want to pay the $ to enclose all of that area in the "envelope". Since the area over the porches will not be insulated and it will be closed off from the attic should we put vents in the soffits (siding under our eaves)? As far as the exterior of the house we are doing all brick & stone so the air pocket we will have better the brick and the tyvek/plywood sheathing I'm assuming will be enough if we are doing the cellulose or Demilec on the exterior walls. Is that correct?

Posted By texansteph on 01 May 2010 10:11 AM
Thank you so much for the info. It does seem crazy now to worry about the 10 year roi. I would assume in 10-15 years the required standards in TX will get much more rigid. One more question. We are doing the foil backed foam board on the "knee walls" in the attic to give them something to spray the foam to b/c we have a 10/12 roof pitch and about 1000 sq ft of porch covering 3 sides of the house. We didn't want to pay the $ to enclose all of that area in the "envelope". Since the area over the porches will not be insulated and it will be closed off from the attic should we put vents in the soffits (siding under our eaves)? As far as the exterior of the house we are doing all brick & stone so the air pocket we will have better the brick and the tyvek/plywood sheathing I'm assuming will be enough if we are doing the cellulose or Demilec on the exterior walls. Is that correct?

Venting the uninsulated dead-space over the porch should be fine, but may/may not be necessary for keeping the roof deck in that area from building moisture.  I'm assuming it's completely inaccessable from conditioned space?

The air gap between the masonry and studwall behaves as something of a vapor retarder against peak vapor drives (any moisture that condenses drips to the bottom of the cavity rather than soaking/diffusing into the wall structure )  A semi-permeable to semi-impermeable element on the exterior of the sheathing (the side facing the air pocket) would protect the sheathing better.  Vapor retardent latex on the exterior of the sheathing wouldn't be a bad idea, but it may not be necessary, as long as the cavity fill and interior wall finish are vapor permeable (cellulose & half-pound foam, interior latex are good, closed cell foam & foil/vinyl wallpapers are not.)  A 1-2" layer of closed cell foam or XPS sheet goods on the exterior would be great, but required deeper masonry ties, and adds cost/complexity.  Adding a thermal break like that over the studs is a good performance boost though.