I am going to use icynene as a base layer and then put the cheaper cellulose on top. The question I have is how much of each should I do? I know I need to cover all of the wood, but what is recommended by you all? Thanks

It depends on where you are located. It also depends on your truss or ceiling joist spacing (deep cellulose is heavy and can bow gypsum board if support spacing is far apart).

My current plan is to have ~1" of closed cell foam sprayed over the top of the ceiling board for strength and air sealing, and then apply ~12"+ of blown cellulose over that for a total R value of ~R50.

i am located in KS. and I have no idea how far apart the joist spacing will be. but if 1" of foam is all that is needed to get a good air seal, then that is what i'll do.

how much cellulose should cover the joists to prevent heat loss/gain?

There's no magic number for how much of a thermal break you'd over the joists, since there is no such thing as prevention, only reduction. But consider that 3" of cellulose over the joist top cuts the conducted heat transfer though a 2x12 joist roughly in half, and the radiated heat gain of the stud edge (under a hot roof deck) by nearly 100%:

The R-value of typical density timber used in framing runs ~ R0.85/inch, so the 11.25" of a 2x12 is ~ R9.5. At typical attic open blow densities you get at least R3.4 per inch out of cellulose, so 3" over the top edge is an additional R10+, increasing the R-value at that point to R20, a reduction by about half, maybe a bit more. The between-joist R of that much cellulose (14.25") would then be R45-R50, but the R20 thermal bridging brings that down some (by how much depends on the joist spacing). With 2x10s and 14.25" (settled depth) of cellulose you'd be at about 6"/R20 over the joist tops for R30-ish total for that fraction, and you could pretty much ignore the thermal-bridging in your performance estimates.

It's impossible to use 1" of open-cell Icynene as an air-barrier, since it's nearly impossible to get anything like a uniform 1". But with practice most installers can shoot 1" nominal closed cell, and it's still an excellent air-barrier even if the thin-spots get to be 1/2".

The "...no idea how far apart the joist spacing will be..." implies new construction, not retrofit. In KS (US climate zone 4/5 depending on location) there's an economic rationale for going to R60-65 ish if done with low-cost cellulose. That would take ~ 16-18" (settled depth- increase initial blow by 1-2" to account for that), and would weigh about 2-2.5lbs per square foot. Be sure the joists are rated for the dead-loading (no 2x4s over long spans), and use a layer of 1/4-3/8" OSB screwed to the underside of the joists as structural support rather than trying to support that weight on 1/2" gypsum or it will sag over time. With an OSB underlayment the OSB can be detailed as a primary air-barrier using far less foam than 1" of cc foam over the whole shebang. If this is to be a trussed roof, build with "energy heel" trusses to be able to bring the full R60 out to the exterior edge of the walls over the top plate of the studwall, and design the bottom chord element with some margin over the 2.5lb/foot static loading. If joists & rafters, same story- design for the static load an provide sufficient depth at the eaves, even if it means a stubby studwall element at the ends to support the rafters/roof.

The fewer the holes you make in it, the less air-sealing detailing is required, and the more reliable the seal. That means keeping all ducts etc inside the conditioned space an minimizing the use of recessed lighting cans (and use only gasketed air-tight insulation-contact rated fixtures, if you must.) Sometimes it's worth adding a foot or two the framing of the top floor to accommodate ducts, etc. but that's a lower cost solution than a sealed-attic with insulated roof deck to deal with placing mechanicals in the attic. With 2x10 joists with 18" of 1.5lbs nominal density cellulose you'd have more than R30 over the the joist tops, and R60+ for a total whole-assembly R.

If this is an all new design, consider designing & building to the R-values for zones 4 & 5 (thermal bridging included, not center-cavity R on stick-built) in table 0.2 on p10 in this document:

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

Mind you, this may be close to 2x code min , even more on some elements (is there a sub-slab R specified in KS code? I doubt it!) Those can still be cost-effective in a longer term financial analysis if lower-cost methods & materials (such as cellulose rather than open cell foam) are used wherever possible. The comfort levels go up, and the cost (& operational cost) of HVAC mechanicals fall too.

use a layer of 1/4-3/8" OSB screwed to the underside of the joists as structural support

If you have joists and clear areas even in some places, screw it to the top, caulk it and it will act as an air barrier, eliminate stud and light fixture thermal bridging and provide structural support for cellulose. The enclosed area would be "conditioned", so if really necessary, could be used for some duct work.

Around here, 7/16" OSB costs less than 1/4" or 3/8" OSB, and is obviously stronger.

how does 5/8" gyp compare to weight of the cellulose?  Code is 5/8" on ceilings.  Also with insulation this thick, are people building elevated rat runs I assume above the insulation?

All good points to take in to consideration. Key is to design for it, and not to try to support R60+ cellulose with gypsum. Even with the additional cost of the wood it's far cheaper than a high-R foam solution.

Posted By Dana1 on 21 Jun 2011 09:58 AM
All good points to take in to consideration. Key is to design for it, and not to try to support R60+ cellulose with gypsum. Even with the additional cost of the wood it's far cheaper than a high-R foam solution.

You are right! even if there's additional cost of wood,it will be still be cheaper than high-R foam!

Posted By jumpingspidermedia on 22 Jun 2011 06:52 PM

Posted By Dana1 on 21 Jun 2011 09:58 AM
All good points to take in to consideration. Key is to design for it, and not to try to support R60+ cellulose with gypsum. Even with the additional cost of the wood it's far cheaper than a high-R foam solution.

You are right! even if there's additional cost of wood,it will be still be cheaper than high-R foam!

Not only is it cheaper, it's a lot greener than high-R spray polyurethane foam too: 

http://www.greenbuildingadvisor.com...insulation

(The other reason to limit use of polyurethane foam or XPS to where it's doing the most good.)

Thanks for the replies. A lot of information that has gone to good use.

How does the HVAC duct work incorporate itself into the insulation in an attic? Does it lay on top of the insulation? Under neath it? What is the solution to avoid the unwanted heat loss/gain from that point of view?

Thanks a million.

Best not to put duct work in the attic at all.

Please also keep in mind that the use of Radiant barriers in the attic is also very effective. Study at Texas A&M showed that adding radiant barrier to R 19 was more effective than upgrading to R 30. Good luck and hope you use Green Insulation products

Posted By jonr on 23 Jun 2011 12:48 PM
Best not to put duct work in the attic at all.

If I do a concrete main floor, can the duct work still be located in the main floor's trusses?

The builder I am talking to now says that he usually uses 8" trusses in the attic? Earlier in this thread it was stated that 10" or 12" trusses are the norm. Is 8" OK?

How much should the 'rasied heel' or 'energy heel' cost me more than the standard. He said it would cost less than a grand on most homes.

And for the third post in a row......

Regarding the support for the cellulose. 5/8 gypsum will or will not bow?

and if there was 1/4" or 1/2" OSB as a base, would you want to spray closed cell foam along where it is nailed/screwed to the joist as well as the creases between sheets? or just dump 16-18 inches of cellulose and call it good?


thanks for all the help guys, i hope im not asking the same question over and over. it is alot to process and im doing my best to build a great home.

Posted By radiantbarrier on 23 Jun 2011 12:54 PM
Please also keep in mind that the use of Radiant barriers in the attic is also very effective. Study at Texas A&M showed that adding radiant barrier to R 19 was more effective than upgrading to R 30. Good luck and hope you use Green Insulation products

Show me the study.

Texas A & M did a LOT of study of careful study of radiant barriers back in the 1980s.  There were studies by Texas A & M that also did studies that demonstrated R19 in low density fiberglass underperformed higher density goods in both cooling and heating applications.

R19 doesn't meet code anywhere that actually HAS codes for attic insulation (it  met code in some places back in 1980 though.)  R30 is a pathetically low standard, even for Zone-2, and more is clearly cost effective.  The cost/benefit of RB vs. going from R30 to R40  (or R19 to R40) doesn't generally favor RB unless there are uninsulated ducts & air handlers in the attic, and doesn't reduce the benefit of higher-R at all.

See: http://www.ornl.gov/sci/ees/etsd/bt...et2010.pdf

In particular study the calculated savings charts p5.  Even starting with R19 the savings of adding RB alone are dramatically below bring it up to code-minimum + RB, and the savings of adding RB to a current code-min roof are quite small. Comparing the RB savings to the RB + insulation savings, it's possible to derive a rough estimate of the fractional savings attributable to the fiber as compared to the RB.

blavis: 5/8 or even 3/4" gypsum won't support R50+ of cellulose without bowing unless the truss/joist spacing is quite narrow, but 7/16"  or 1/2" OSB would handle spans in excess of 24".

Making the OSB an air barrier is still a good idea- cellulose by itself is not sufficient.  Using OSB as an air barrier you can caulk the seams that are screwed to the joists, and either use mesh tape & mastic, or spray foam spanning the exposed seams on the top (insulation) side.  Alternatively you could stagger the seams of the gypsum and use acoustic sealant on the seam, between the gypsum & OSB. There are many ways to get there with some detailing. Foam-sealing the big holes- all electrical & plumbing penetrations of the air-barrier is also important, of course.

Special considerations need to be made around combustion flues, etc. but there is always a way. Using sheet steel as the air-barrier component in contact with the flues/chimneys works, as does wrapping with rockwool batting to a thickness & height above the cellulose layer to meet code clearances to combustibles is also tried & true.  (While treated with fire retardents, cellulose still can't be in direct contact.)

Dana, Thanks for your detailed replies. I truly do appreciate it.

Could somebody please reply to my last three posts? They are questions I need answered.

Thanks



Edit: My third question was answered by Dana1, can I get some help on the first two?

1 story or 2? With a concrete slab could you use open web floor trusses to run your mech? Sheet the top with plywood or osb. If its a one story spray foam 1" and top with 15"+ cellulose.
Truss size would be dependent on span, spacing and weight.