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-   -   effect of attic flooring between fiberglass? (http://www.diychatroom.com/f103/effect-attic-flooring-between-fiberglass-169811/)

DexterII 01-22-2013 11:48 AM

effect of attic flooring between fiberglass?
 
Going to help a buddy insulate the ceiling of his home addition, and the initial plan is to roll fiberglass on top of and perpendicular to to the joists, working through the joists, then packing between the joists from below. At some point I happened to think it would make laying the top layer of insulation easier, and provide better mechanical access, if we sheathed the tops of the joitsts with say 3/8" plywood or something along that line. Anyway, nothing new or radical, as floored attics are insulated routinely, but my question is, given the option, would the inclusion of this sheathing help, hinder, or have no affect one way or the other in regard to insulation, moisture, etc. Thank you.

AGWhitehouse 01-22-2013 03:09 PM

Quote:

Originally Posted by DexterII (Post 1099443)
Going to help a buddy insulate the ceiling of his home addition, and the initial plan is to roll fiberglass on top of and perpendicular to to the joists, working through the joists, then packing between the joists from below. At some point I happened to think it would make laying the top layer of insulation easier, and provide better mechanical access, if we sheathed the tops of the joitsts with say 3/8" plywood or something along that line. Anyway, nothing new or radical, as floored attics are insulated routinely, but my question is, given the option, would the inclusion of this sheathing help, hinder, or have no affect one way or the other in regard to insulation, moisture, etc. Thank you.

It will help reduce air movement through the batts within the joist. Moisture drive may be affected as plywood can be considered a vapor retarder. To what level of effect it will have depends upon the installation, thickness, and quality of the sheathing.

joecaption 01-22-2013 04:43 PM

3/8 would not be thick enough to safely walk on.

DexterII 01-22-2013 05:11 PM

AG, exactly; I'm sure that it will serve as a vapor barrier to some degree, but I'm just not sure of how much impact it would have. On one hand, of course you wouldn't want it there, but, on the other hand, as I mentioned in my initial post, there have been many instances of homeowner's adding insulation in an attic that was previously floored, and I have not heard or seen any negative results. Not saying they're not there, but just that I am not aware of any.

Joe, yeah, I think you're right; would probably go 1/2"; not that it will ever be used for anything more than mechanical access, but still don't want it to fail.

Anyway, just one of those things that popped into my head, so am going to run it past a buddy of mine when I see him, a quality builder, who keeps pretty up to date on a lot of the details, and am going to ask the inspector about it, so thought that I would see what anyone here might think about it. Not one of those things that I would do on just any build, but something that I don't mind going the extra step on when it's for someone I know might appreciate it.

Thank you.

Gary in WA 01-22-2013 11:02 PM

Plywood is way more permeable than OSB. Air seal the wiring/plumbing holes to stop major air movement; http://www.google.com/url?sa=t&sourc...wWATQw&cad=rja

Will you be building up the joists for the deck or sandwiching it? Is there an interior vapor retarder on the f.g. or plastic vapor barrier next to the ceiling drywall? Will you ADA the drywall ceiling?; http://www.buildingscience.com/docum...wall-approach/

First and foremost; where is your location? CT? Plywood is more vapor open than OSB, the wetter it gets. A face paper is important for your Zone 5, read pp. 3, 5, ----read it all; http://www.buildingscience.com/docum...researchreport

The plywood will be the first condensing surface from room vapor diffusion (small quantities);

Gary

DexterII 01-23-2013 05:26 AM

Thank you, Gary. Yeah, sorry, we're in MI, and I left out some of my initial thoughts, but was figuring to place the sheathing directly on the 2x10 joists, essentially sandwiching faced insulation in the joist bays, face down against the drywall, and then unfaced on top of the plywood. He's got work to do before we get to that point, so have time, and will read through your links, to see how much sense it makes. I definitely appreciate the assistance.

AGWhitehouse 01-23-2013 09:57 AM

The system you're describing really isn't much different than an exterior wall. Up to 1" xps can be placed on the plywood without negative effect (zone 5). Any thicker foam and you'll need to remove the vapor retarder along the interior ceiling face.

DexterII 01-23-2013 11:55 AM

Yeah AG, this is something that I just started thinking about a day or so ago, and that hadn't crossed my mind yet, but you're right in that it is similar to a sheathed wall. That still leaves the issue that you mentioned about thicker insualation on the cold side though, because we'll only get 9-10" between the joists, which means a minimum of something like 9-10" in the attic. Happened to have a flashback last night, and was thinking that we could always use 1x6, like we used to run diagonally for subfloors, but gap it, which seems to me would be a perfect solution, but I think he or his wife would have a fit when they got the bill for that much 1x6 today! Oh well, as I mentioned, it's probably a little way off yet, so will take a look through the information that Gary attached, kick it around in my mind a bit, and not say anything to my buddy until I make up my mind for him! Thank you again.

AGWhitehouse 01-23-2013 02:22 PM

If this is a new install, I would suggest standoffs and place the sheathing completely above the required insulation thickness. Below is a section through the ceiling. I assumed 2x6 existing joists (my house has them) and either a 2x8 standoff or a 2x6 with 2" foam spacer. There are a ton of ways to do this, this is just one example of a method.

I re-read your posts and noticed you have 2x10's existing. Zone's 6 & 7 both require R-49 for ceilings. You can provide a 2x4 to the top of your existing joists and have room for an R-38 & R-11 batt. with the plywood over the top. If you choose to do the rigid polyiso you'll be at an R-54.

Gary in WA 01-24-2013 10:44 PM

Here is a link from Building Science/ Dept. of Energy showing foamboard above the joists (in Chicago) with plywood on top, page 24; http://www.buildingscience.com/docum...ation-packages
The foamboard is on the cold side (protecting the plywood) of the insulated cavity, not the warm side . This is best practices for walls, crawlspace floors, floors over garages and roofs; pp. 27. It is the "in contact" with sheathing that is important, no air should get on the surface to condense because of the temperature difference which drive it.

Gary
PS. Here is the foamboard next to the drywall ceiling, it is for a cooling climate, not heating, pp.83, Fig.3-41; http://books.google.com/books?id=Eq1...page&q&f=false

jklingel 01-25-2013 01:21 AM

why not air seal well and blow in cellulose? faster, easier, and seals way better than fg, esp around obstacles. fg in an attic is not good insulation, imo; not dense enough.

DexterII 01-25-2013 09:50 AM

Well, I'll kick it around a bit, but maybe the best thing to do is go back to my original thought, which was to construct something like a 30" wide catwalk above the insulation, similar to what you described AG, and forego expanding it any more than that. He has a Unico system for air conditioning, which obviously requires access for maintenance from time to time, so I was thinking that we could create a bit more room to work, as well as having easier access for changes in lighting or whatever down the road, but don't want to create potential problems as a result. Jkilingel, yes, I agree with you, and that may be what he does. Personally though, and I guess that I don't have any good reason(s), other than the fact that I am old school, but I still prefer fiberglass, given full and proper attention to details, i.e. ensuring that it is tucked where it needs to be, etc. Thank you again guys; I appreciate the comments and advice.

jklingel 01-25-2013 04:33 PM

Quote:

Originally Posted by DexterII (Post 1101680)
..and I guess that I don't have any good reason(s), other than the fact that I am old school, but I still prefer fiberglass, given full and proper attention to details,....

Yes, it's sometimes hard to let go of habits, but fg is the worst insulation on earth when exposed, even if it is perfectly installed (which it never can be). P, EOS. At the very least, if you are set on fg, put it (or better yet, Roxul) in the bays where it sort of fits, then pump cellulose over it. Cellulose will "crust" a bit, and stop wind washing; fg never will. That way will be a tad cheaper, but way slower. Either way, good luck.


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