Dana1,

One insulator came back with 3" of closed spray for about same price as 5.5" of open cell, and leave an air gap between that and drywall. At first I thought that might be ok, but then I remembered the whole moisture build-up in the OSB/condensing plane and ability of OSB to dry. So I thought I'd better ask if I should even consider that?

Posted By Boontucky-girl on 08 Nov 2012 11:50 AM
Dana1,

One insulator came back with 3" of closed spray for about same price as 5.5" of open cell, and leave an air gap between that and drywall. At first I thought that might be ok, but then I remembered the whole moisture build-up in the OSB/condensing plane and ability of OSB to dry. So I thought I'd better ask if I should even consider that?

Not only does that limit the ability of the OSB to dry, it's a lower performance solution overall!

With a full cavity fill of open cell foam the center cavity R of the foam is about R18-R19, and the thermal bridging of the other 20-25% of the area is 5.5" of wood, for about R5.5.

With 3" of closed cell the center cavity R of the foam layer is still R18-R19, but the thermal bridging at the stud is now only 3" of wood,  or about R3.

At a 25% framing fraction at R5.5 about half the heat transfer is through the studs.  In undimensioned units: 

25%/R5.5= 4.5    75%/R18= 4.2 

So it's at rough parity.

But in the closed cell case with R3 bridging:

25%/R3= 8.3   75%/R18= 4.2

The bridging losses nearly doubled, resulting in a total loss that is ~44% higher.

You can quibble that the average wood-to exterior depth is really closer to 3.2" or something but the principle is the same.

Running with 2" of closed cell and installing R15 rock wool or compressing unfaced R19 fiberglass batts into the 3.5" cavity in the would provide a net improvement in both drying capacity and thermal performance over a 3" closed cell solution.  If going with 3" of closed cell, only  the Icycene MD-R-200 product has sufficient drying capacity- 3" of generic 2lb foam has less drying capacity than a kraft-facer on a batt. 

But don't leave any stud cavity unfilled- stuff it with SOMETHING, since it's otherwise a flue for spreading fire quickly from the bottom to the top of the stud bay, while making a shorter thermal path through the studs.  If batts going with high-density rock wool at R4/inch has pretty good air retardency and R-value, but SFAIK box stores don't carry 2.5" rock wool designed for 2x3 cavities, but most would carry R15 designed for 2x4 stud cavities (another reason to stop at 2" on the cc foam.)

Thanks Dana1. Hadn't even considered the thermal bridging.
I've never seen rock-wool insulation at the box stores around here, but I can look, but right now DIY is not an option. I am going to predict that this particular insulation contractor will ask if maybe I'd want to fill the rest of the cavity with open cell, since he's a foam guy. Assuming pricewise we're close, should I consider it?
I would guess in this instance we wouldn't want a poly vapor barrier since we'd be sandwiching the open cell between two vapor barriers, and that probably is a no-no. Would this be another option? 2" cc + 3.3" o.c. OR 3" cc + 2.5" o.c.?

If dual-density foam, 2" cc + 3.5" o.c. is the best of what you put up there, but a flash of 1" cc + 4.5" oc would be even better.

If open-cell, put up CertainTeed MemBrain under the gypsum, not poly.  Next best would be a half-perm "vapor barrier" latex.

You can definitely get rock wool batts at a least one of the box store chains nation-wide.   The air retardency of R15 batts is pretty good for both fiberglass & rock wool, but rock wool is more protective in a fire, and doesn't have the floating micro-paricle indoor air pollution issues that fiberglass does.

Thanks Dana1.

Finally got something figured out within our budget. 1" cc + cavity fill oc in walls. full cc fill in 5" energy heel around perimeter of house. R50 cellulose in attic. R50 oc in vault ceiling.
Since we were not able to afford the skim coat of cc in attic sheetrock, I am not wondering if I need vapor barrier in the ceiling? I googled it, but like with anything I can find promoters of it and those who don't recommend it. I looked at building science, but not sure I found a definite answer. I saw a lot about a sealed gypsum ceiling. So if I do a good job of foaming all penetrations through top plates, cover the cans and foam around that, will that be good enough? Or should I go ahead an use vapor retarding primer/paint in the ceilings?

Another question I have is what to do for noise walls. Contractor proposed just using batts, but we did that in the basement and doesn't seem to reduce much for the "noise". Any other options for noise wall insulation?

Thanks.

http://www.roxul.com/residential/products/roxul+safe%27n%27sound%E2%84%A2

Lowes is a distributor.

Posted By Boontucky-girl on 06 Dec 2012 10:33 PM
Thanks Dana1.

Finally got something figured out within our budget. 1" cc + cavity fill oc in walls. full cc fill in 5" energy heel around perimeter of house. R50 cellulose in attic. R50 oc in vault ceiling.
Since we were not able to afford the skim coat of cc in attic sheetrock, I am not wondering if I need vapor barrier in the ceiling? I googled it, but like with anything I can find promoters of it and those who don't recommend it. I looked at building science, but not sure I found a definite answer. I saw a lot about a sealed gypsum ceiling. So if I do a good job of foaming all penetrations through top plates, cover the cans and foam around that, will that be good enough? Or should I go ahead an use vapor retarding primer/paint in the ceilings?

Another question I have is what to do for noise walls. Contractor proposed just using batts, but we did that in the basement and doesn't seem to reduce much for the "noise". Any other options for noise wall insulation?

Thanks.

At R50 thicknesses most  half-pound oc foam is running about 1.5-2 perms, and with standard latex primer & paint on an air-tight gypsum ceiling you'll have about 1-perm between the conditioned space and the roof deck, which should be fine for the vault ceiling.

In the vented attic part just the 2-3perm latex ceiling will be enough, since the drying capacity on the cold-in-winter side of the cellulose is excellent.  As long as the ceiling is air tight the moisture can dry faster through the cellulose into a vented attic than it can pass through the painted gypsum.

Noise abatement can't be achieved with insulation alone- it's primarily about air-tightness and mechanical decoupling.  Unfaced batts and staggered-stud  studwalls does a pretty good job for most residential apps.  But without similarly blocking ceiling & floor joists there are still huge acoustic bypasses to even the best sound-insulated walls.  Do a web search on "sound abatement" and start reading.

Thanks! Unfortunately drywall is coming on wednesday, so not much time to read up on sound abatement. Contractor normally does just batts, so thinking of skipping it since it sounds like that won't amount to much "sound abatement". Plus there is return air vent in one wall, so that won't work at all.

So of course now that it's crunch time, we're over budget and a lot of things, so no OSB on the ceiling, electrical is all in, and now contractor wants to put 1/2" drywall in ceiling. I keep telling him that R50 cellulose will make that sag, but he said it should be enough.

I want to go at least 5/8" drywall. I re-read several pages back on this thread about the whole discussion loading my trusses, so I think at r-50 cellulose and 5/8" gypsum I'm safe for dead loading . Anything I can do to help support that cellulose load when I have no option to OSB or strap?

Thanks
BG

You could go with a double layer of 1/2" high strength sag resistant ceiling board on the ceiling.

http://www.usg.com/sheetrock-sag-re...board.html

Another option would be to buy a pallet or two of reclaimed 1-3/4" polyiso roofing insulation from Insulation Depot or similar and rip the sheets down to ~22" wide on a tablesaw with abrasive blade.  Lay these sheets between the trusses on top of the ceiling board.  Then run a bead of canned spray foam along each side and at all ends.  This thickness of polyiso will add R10 to your insulation and its rigidity will help support the load of the cellulose above.  Plus the spray foam will help with air sealing.  Now you only need to add the standard R38 to R40 of cellulose above to get your R50.  The downside is that it is a lot of DIY work.

Anything I can do to help support that cellulose load when I have no option to OSB or strap?

Stabilized cellulose.

Thanks for the quick replies! I finally got a hold of the spec for the cellulose brand, and at R-50 it says 1.5lb per sq ft of weight. The 5/8" spec for the brand of drywall says maximum of 2.2lb per sq ft of insulation on 24" o.c. trusses if a textured ceiling is applied. So it appears that 5/8" should be enough. I sure hope so because drywall ceiling is going up today!

Insulation is all done, and here where we ended up:

1" R board on exterior, 1/2" OSB, 1" cc skim coat, 4.5" OC fill shaved flush. I got the "mini" energy heels all around the exterior of the house foamed in full with CC foam to max. insulation at this spot. I foamed all penetrations through the double top plates of all interior walls into the attic, caulked bottom plate of ext. wall to subfloor, caulked every butt joint between double or more studs, and caulked around the whole double top plate butt joint. Foamed around all windows with foams, and stuffed fiberglass for the rest of the cavity since I couldn't afford to fill with foam completely. This weekend we'll be foaming in the attic around every light box, wire, and can lights poking through the ceiling. Finally, we'll get R-50 ALL Borate cellulose blown in where we have the height for it. My worst spots will be the first 13" in from exterior walls since at best I'll have R30 cellulose there between sheetrock and wind baffle.

If we have issues with ice damming, I will have to crawl in the attic and fill this 13" space with sheets of blue foam foamed together. Because of lack of time and funds, I can't do that now. But we've given it our best effort and did the best we could with the funds and time and capabilities that we have available.

I really want to thank everyone for all the great help, discussion, ideas, tips, tricks... everything. I learn so much from the help that can be found on this forum!

BG

I predict that the baffles will be effective in preventing ice dams from being a problem.

I predict the R50 will prevent ice dams from being a problem! :-)

But in fact keeping the attic ventilation rates reasonably high would mitigate ice damming even the whole attic were R30, not just the thin-spots.

Boontucky-Girl
congratulations! I know you've been at this a long time and know that it will pay off. Your home will be far better performing than a similar house without all the detail work you've done. And I know that it is attention to these details that makes the difference. We frequently discuss here whether the Rboard, for example, should be 1" or 2" in every aspect of the envelope, but however you determine that, you're aware of the issues and the basic fact that houses cannot be slapped together as has been too common in residential construction. People like yourself are the leaders in the new understanding of how houses work; you'll show your friends and neighbors and it will expand from there. Thanks!
Bob