Plastic VB for unfaced insulation?

[RaysnCayne]

Hey all,

Background:

I'm building a 28x28 detached with an 8/12 pitch roof and storage trusses. Full-vented soffitt and vented gables (with draw fan on one end) and no ridge vents. I live in middle VA and it gets fairly hot in the summer. But I plan to have an decent a/c unit cranking in the real hot days so it should be somewhat coolish. I'll use a couple radiant kero space heaters in the winter.

After pricing faced vs. unfaced R-30 insulation and discovering the faced R-30 will cost me exactly twice as much as unfaced, I've decided I gots to go with the unfaced.

So now, my question is, can I staple a sheet of plastic to the bottoms of my trusses, nail the 1/2 OSB (for my ceiling) to that and drop in the unfaced insulation on top of that plastic and not have to worry about any sweating/moisture issues?

 

[ForceFed70]

Yes you can. Use 6mil polyethalyne vapor barrier. This is standard construction practice in Canada (we rarely use faced insulation). Make sure to tape the seams with Tuck Tape.

 

[RaysnCayne]

Then I got to thinking, what if I simply nailed up the OSB and then laid in the plastic from up top? (As in lay one continuous sheet down in between the trusses (against the OSB) and then up and over the truss, and so on.) Does that improve the insulation "sealing" or anything (by sealing it off the vertical sides of the trusss)? Just wondering if it would help at all or just be a wash.

 

[ForceFed70]

You'll get a better seal if you install the plastic 1st. Plus, I'm pretty sure it would be faster/easier to install the plastic this way.

Took me 3hrs to hang and tape the ceiling in my 32x40 shop. 4 rows of plastic.

 

[rsa]

So now, my question is, can I staple a sheet of plastic to the bottoms of my trusses, nail the 1/2 OSB (for my ceiling) to that and drop in the unfaced insulation on top of that plastic and not have to worry about any sweating/moisture issues?

Don't use any vapor barrier or vapor retardant, but do make the ceiling plane airtight. From: Lstiburek’s Rules for Venting Roofs and Building Science Digest-102B: Understanding Attic Ventilation:

...nowhere in this discussion did the term “vapor barrier” come up. If you really want to have a vapor barrier in the ceiling, limit it to climate zone 6 or higher, but that’s really not important compared to the airtightness of that ceiling plane.

The building code calls for a vapor retarder in climate zone 6 or higher. It’s okay to put one in, but if you don’t, take a Valium and relax. You don’t want to go through a lot of brain damage in a renovated house to try and add a vapor barrier underneath insulation in an attic. What you really want to do is make that ceiling plane airtight, make it airtight, declare victory and be done. Don’t mess around with permeability’s and calculations and whatever.

To recap, airtightness on the ceiling; washing the underside of the roof deck; unbalanced ventilation should be in favor of the lower vents because you don’t want to depressurize the attic; no to powered attic ventilation or the whirligigs; and you don’t want to squeeze the insulation at the perimeter so it’s less than the R-value of the wall.

No interior attic assembly side vapor control is required or recommended in climate zones other than Climate Zones 6 or higher for vented attic assemblies.

Airtight drywall ceilings can use the "airtight drywall approach", but how are you going to make the OSB ceiling plane airtight? I haven't spent much time researching sealing an OSB ceiling, but Peel-and-Stick on the seams comes to mind. See this discussion. You've also got ceiling penetrations and the ceiling perimeter to seal.

 

[aloxdaddy99]

I would second the 6mil plastic and tape.

 

[green.bubbly]

FYI, HomeDepot has unfaced R30 for less than 32 cents per square foot!



http://www.homedepot.com/webapp/wcs...sp=BldgMat-_-Frag1-_-NLP_Insulation-_-ShopNow

 

[HoosierBuddy]

Sorta OT...but when I priced insulation for my garage project, a contractor quoted me a lower price to install the insulation on my job (material included) than what it would have cost for me to buy it at Lowes and install it myself.

Worth checking out!

Phil

 

[RaysnCayne]

Interesting video...

So I'm hitting one of his rules with my fully-perforrated soffit on both sides. And I'm sort of following up on his point with some pretty big gable vents.

I'm using OSB, #1) b/c it's $3.50/board cheaper than drywall. #2) I've never taped or mudded in my life and I hear it's not easy. And I don't want to try it for my first time "upside down".

Why was the good doctor against VBs? He never said. In my case, I'd think it'd help make my ceiling air tight and thus be a good thing, right? The other article you linked said there's some downsides to caulk, but didn't say what they were. I don't see the harm in using silicone caulk to seal all my OSB seams (and fixture pentrations) and then cover the whole thing with a thick layer of paint.

FYI, HomeDepot has unfaced R30 for less than 32 cents per square foot!

Nothing new there. That's the same deal all the box stores have going for the last 2-3 months. Lowes is selling Johns Manville, same price, same stuff. I can only guess the insulation companies goofed and made waay too much of that stuff and now the box stores are trying to unload it for them.

And at that price, HooiserBuddy, I'll have about $220 + a day of my time in the insulation. Not sayin' that's terribly cheap, but I can't imagine any contractor in my neck of the woods even bothering to fire up their dually F-350 for that little - let alone less. Either you had a whale of a job, the contractor was a good friend, or they're just more desperate for work out there.

 

[rsa]

IWhy was the good doctor against VBs? He never said. In my case, I'd think it'd help make my ceiling air tight and thus be a good thing, right? The other article you linked said there's some downsides to caulk, but didn't say what they were. I don't see the harm in using silicone caulk to seal all my OSB seams (and fixture pentrations) and then cover the whole thing with a thick layer of paint.

I'm certainly not a professional in this field, but I've been doing a lot of reading lately planning my dream garage.

Lstiburek says, "With vented attic assemblies moisture that diffuses into the attic space from the conditioned space is vented to the exterior by attic ventilation." I'm sure it goes beyond that (condensing surface), but I'm just quoting the good Doctor.

About vapor barriers making your ceiling air tight: small holes in vapor barriers don't materially reduce their effectiveness. Small holes in air barriers do. Reference: Air Barrier or Vapor Barrier?.

I believe any objections to caulk are related to longevity and that the material you're caulking is going to be moving around. What I've seen recommended for air sealing is non-hardening caulk. Painter's caulk and Tremco Acoustical Sealant are examples.

Thanks for reading the links.

 

[RaysnCayne]

Wow, that one made my face hurt a bit. I read it twice and it's still going over my head. A barrier's a barrier in my simple mind. I'm not going to use drywall so maybe I don't have any sort of "air barrier" in the mix here and I'm just talking

I'm not nearly as worried about moisture going from the garage area up through my ceiling and condensing in the attic nearly as much as I'm worried about the moist air (you're in NC, you know how steamy our mid-atlantic summers can get) flowing through the soffits and settling down in/under my insulation and making the top side of the OSB moist and then moldy. That's the main reason I was thinking I should install a sheet of plastic. (Although, now that I type that, having the plastic catch the water and hold it against the bottom of insulation doesn't seem like a good idea either. Argh!!)

But please bottom line it for me, or see if I'm understanding it correctly, given the materials I'm using, do you think Listiburek would say ditch the plastic, get the OSB installed as tightly as possible and then seal it as best as you can?

As for non-hardening caulk, that's what silicone is. It's designed to squish and expand and not harden over time. So I'd think I'd be safe with that.

 

[rsa]

do you think Listiburek would say ditch the plastic, get the OSB installed as tightly as possible and then seal it as best as you can?

Ditch the plastic and seal it as best as you can? Yes. Install it as tightly as possible? I don't know. OSB in exterior installations and sub floors is generally installed with a gap between panels. I don't know if it's necessary when OSB is used in place of drywall. Others here have actual experience using OSB on interior walls (and ceilings?). Yes or no, I assume that you'd want to let the OSB condition on-site for a few days before installing. Even if it's not required, it can't hurt.

Remember my disclaimer. I too am struggling to get it right. I've just been reading longer than you have. Good luck.

Stewart

 

[RHD 4 LIFE]

Try this!
http://www.ontariotile.com/vapour_barrier.html