Author Topic: question about subfloor and vapour barrier  (Read 4584 times)

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Offline rocking23nf

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question about subfloor and vapour barrier
« on: July 22, 2009, 06:21:15 AM »
Hi everyone, my 12x12 addition is underway, I have the concrete piers poured, posts in place, beams and joists built.

since this will sit a foot off the ground, im using 1/4" hardware cloth under my joists to keep rodents out and to hold insulation from just falling out.

My question is, should a vapour barrier go between my joists and plywood? should it go over the plywood? should I just put a vap barrier under my laminant flooring when I get to that step?

Also if I put it between the joists and plywood, how do I use construction adhesive for the plywood floor.

I should add that all joists and plywood on the floor is PT wood.

Offline Redoverfarm

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Re: question about subfloor and vapour barrier
« Reply #1 on: July 22, 2009, 11:36:27 AM »
When I first started reading your post I thought it was a "no brainer" until you said that you were going to glue the ply subfloor down and it changed.  If you are using "faced" insulation you can staple it to the sides of the joist.  The paper facing is the moisture barrier.  But then comes the problem of water infiltration on the subfloor until you get a roof over it. 

I do prefer to put the moisture barrier under the subfloor.  If it is put on top (under flooring) then moisture gets caught keeping the subfloor damp and lead to potential rot.  An Alternative would be to go with sheet styrofoam where as water will not affect it.

Offline rocking23nf

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Re: question about subfloor and vapour barrier
« Reply #2 on: July 22, 2009, 11:40:12 AM »
How about If I just dont use any Glue and screw the floor like mad?


Offline Redoverfarm

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Re: question about subfloor and vapour barrier
« Reply #3 on: July 22, 2009, 11:53:40 AM »
Not real sure. I know that the glued floor will be quieter and less apt to sweak over time.  Maybe DonP will see the thread and give us his expert opinion.

Offline Don_P

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Re: question about subfloor and vapour barrier
« Reply #4 on: July 22, 2009, 05:52:37 PM »
LOL, I'll send that check along directly John.
This end of building is not my strong suit. If I was going to use plastic anywhere it would be on the ground if I knew that it couldn't hold a pool it would keep the ground moisture from seeing the floor joists. I'm in a relatively mild climate, I'd just put the kraft face up, glue and nail the subfloor. For what its worth subfloor glue is a relatively new product, Dad never used it. It does make for a stiffer, quieter floor but isn't a strict necessity.

Offline MountainDon

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Re: question about subfloor and vapour barrier
« Reply #5 on: July 22, 2009, 08:24:03 PM »
Generally the kraft side of the insulation up against the subfloor, just like a wall, paper on the inside. However, climate can affect the choice or material and the location of the vapor barrier.

Plastic on the ground, held in place with sand/gravel is a good idea as long as the ground is graded to prevent pooling underneath.


My personal opinion on insulating the floor before the walls and roof are done, 'dried in' in other words, is that it is a bad idea. If your luck runs like mine you are seriously tempting fate to dump a couple inches of rain in a short time, accompanied by winds. Enough water will get through to cause the beginning of a mold problem. Maybe your lucj runs better. We've built three structures on our mountain property and it has rained during the construction on every one.
Just because something has been done and has not failed, doesnít mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline rocking23nf

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Re: question about subfloor and vapour barrier
« Reply #6 on: July 23, 2009, 08:48:37 AM »
I went to home depot in Canada last night looking for that insulation, I was told its not sold in Canada, only contracters have access to it.

Anyways my problem is that we are only up there every second weekend, and with the addition sitting 8 inchs off the ground, I have to insulate before i put the plywood floor on. Or not insulate the floor at all.

I think im just going to use r28 8.5 inchs think, put a plastic barrior over top, then lay the plywood without glue and put a lot of screws in it. I also put a barrier under the cabin with crushed rock on top.

Offline John Raabe

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Re: question about subfloor and vapour barrier
« Reply #7 on: July 23, 2009, 08:58:35 AM »
First off for the question of a vapor barrier. You already have that with the plywood subfloor. Then, for the insulation you do not want a second vapor barrier but you would like to keep air infiltration from stealing heat from your insulation. If you could use friction fit unfaced batts in the cavity and then Tyvek covered with hardware cloth to keep the critters out you would then have the air barrier. In order not to trap any moisture that might either form in or drain into the cavity I would consider hitting the Tyvek with a box cutter short slash every foot or so before the hardware cloth goes up. That should keep the air in the cavity dry and still.
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Offline NM_Shooter

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Re: question about subfloor and vapour barrier
« Reply #8 on: July 23, 2009, 09:39:53 AM »
Why not faced insulation, installed paper down, then cut slits in the paper and cover with hardware cloth?  Should be less expensive than Tyvek.
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Offline John Raabe

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Re: question about subfloor and vapour barrier
« Reply #9 on: July 23, 2009, 09:53:28 AM »
That would work too. It probably wouldn't be quite as good an air barrier but might be fine in a sheltered location.

In some locations a pier and beam foundation has lots of air movement on the underside.
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Offline rocking23nf

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Re: question about subfloor and vapour barrier
« Reply #10 on: July 23, 2009, 11:32:04 AM »
John im confused, how is a plywood subfloor considered a vapour barrier? are subfloors treated differently then regular walls?  I was always told the rule of thumb is Siding->housewrap->plywood->insulation->vapour barrier->finishing material

So if I understand you right, I would just have Joists with insulation between-->plywood.

I figured I needed a layer of vapur barrier either under the plywood or on top of the plywood.?

Maybe im just misinteripting your post.........

Offline MountainDon

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Re: question about subfloor and vapour barrier
« Reply #11 on: July 23, 2009, 12:01:39 PM »
I'm assuming that it is the weatherproof/waterproof glues plus the close fitting of the sub floor sheathing that causes the vapor barrier effect.


I believe your sequencing of the wall materials/layers is correct in many climates. However it is off the mark when it comes to locations that are hot and humid outside with cooled air inside. John's recent post  http://countryplans.com/smf/index.php?topic=7371.msg94848#msg94848  explains a lot as do a number of articles on the Building Science website.  I believe it is more correct to say that the vapor barrier should be placed on the warm side o the insulation.  

I recall reading there once a statement that was more or less, "People who live in cold northern climates should not design buildings for hot and humid climates."
Just because something has been done and has not failed, doesnít mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline John Raabe

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Re: question about subfloor and vapour barrier
« Reply #12 on: July 23, 2009, 01:35:41 PM »
This from the Energy code information at Oregon: http://www.oregon.gov/ENERGY/CONS/Codes/docs/res15.pdf

Note that just 1/4" of exterior plywood has less than a 1 perm rating (considered to be a good vapor retarder)



The point being you do not need to install a separate vapor barrier if you are in a heating dominate climate where you want the vapor tight side to be on the warm in winter side of the component. The subfloor (plywood or OSB) does just fine by itself.

Yes, my understanding is this comes from the resins used in the manufacture. And, yes #2, this gets more complex in cooling cycles especially in humid hot climates. (rocking23nf - since you spell it "vapour" I think I can assume you are North of Louisiana. :D)
« Last Edit: July 23, 2009, 02:03:15 PM by John Raabe »
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Offline MountainDon

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Re: question about subfloor and vapour barrier
« Reply #13 on: July 23, 2009, 02:36:35 PM »
(rocking23nf - since you spell it "vapour" I think I can assume you are North of Louisiana. :D)

I've adjusted my spelling of certain words since moving way south. (neighbourhood, cheque, armour, manoeuvre, rigour, calibre, endeavour, kilometre,   snorkelling, travelling, colour...  I can't seem to adjust my pronunciation though.

And of course the letter Z is not really Zee; it is really pronounced Zed.
Just because something has been done and has not failed, doesnít mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline NM_Shooter

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Re: question about subfloor and vapour barrier
« Reply #14 on: July 23, 2009, 04:08:52 PM »
Zee?  Zed?

Nope. 

Zulu
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Offline tc-vt

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Re: question about subfloor and vapour barrier
« Reply #15 on: July 29, 2009, 06:32:05 AM »
The other way you could do this is create a crawlspace under the floor and insulate the crawlspace walls instead of insulating under the floor.

http://www.ornl.gov/sci/roofs+walls/insulation/fact%20sheets/crawlspace%20insulation%20technology.pdf

Tom