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Old 11-16-2009, 08:24 PM   #1
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Floor replacement


Next Spring I will be replacing the rotting main beam, joists and floor of my 50 year old seasonal cottage. It has a perimeter block foundation on granite, with a shallow crawl space. I can improve the crawl space ventilation but will never entirely eliminate occasional rainwater intrusion into this area, with resulting periods of high humidity. The depth of the crawl space precludes working from below, so all work will be performed by cutting out the existing floor and structure, supporting interior walls, and building a new floor structure from above.

My plan is to use preservative-treated two by tens for beam construction and joists, with a three quarter inch preservative-treated plywood subfloor. I will use ACQ-rated deck screws throughout.

Any comments/advice on what I have described so far will be appreciated, but my main concern at present is this next step: I am contemplating placing a vapour barrier over the subfloor and then installing an underlay of three quarter inch regular (untreated) plywood or OSB prior to installing some type of flooring.

I am a bit uneasy about the vapour barrier but without it I know from previous experience that there will be times that a considerable amount of moisture will migrate up through the floor. (This would really limit the type of flooring I could use.) So essentially what I am trying to do is confine the occasional high humidity to the crawl space and the preservative-treated wood used below the vapour barrier.

Can anyone comment on this?

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Old 11-16-2009, 09:02 PM   #2
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Floor replacement


Your crawl space water has to be handled first. And you have a ventilation problem if the joist bottoms are within minimum 18" of the dirt (which should be covered with 6mil. poly).

Rather than all the treated wood, I would rigid insulate and air seal: http://www.buildingscience.com/docum...n-crawlspaces/

This on vapor barriers: http://www.energysavers.gov/your_hom.../mytopic=11810
Be safe, Gary

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Old 11-16-2009, 10:06 PM   #3
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Floor replacement


I have recently completed a project almost identical to what you describe and will offer what I did for you to consider. First let me say that GBR's reply, especially the first link to building science of the crawl space, is right on. I wish I had read it before completing my project. Just to emphasize GBR's comment, you really need to get control of external moisture intrusion to give your project the best chance to succeed.

You didn't say whether your crawlspace is insulated and in my case it was not. Mine is a 50-year old seasonal cabin in northern Minnesota with a poorly ventilated, 18" uninsulated crawlspace built on a perimeter stone foundation using untreated wood. I took the old floor and joists out right to the sand and replaced the joists with pressure treated materials; used 6-mil poly over the sand and sealed at the foundation; PT 3/4" plywood on joists; 30# roofing felt over PT plywood (15# would have been fine but 30# was available so I used it); then 3/4" untreated OSB on top layer. In my case I used 8d hot-dipped galvanized nails for attaching all flooring materials.

Good luck with your project. It sounds like you have thought it through very well.
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Old 11-17-2009, 08:46 PM   #4
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Floor replacement


Thanks for the replies. My cottage is on an island in Northern Ontario, and is built on uneven bedrock. Rainwater unfortunately follows a channel which flows underneath, so when it rains it gets wet in the crawlspace, but being bare rock it dries up fairly quickly. I am attempting to divert this flow but this is a slow process involving a hammer drill and a sledge hammer.

The crawl space is not insulated, and I have no need of floor insulation for warmth, as I am never there in winter months. I am contemplating the use of rigid insulation with a moisture barrier, as described in the link GBR provided. One caution with any such approach is that any water that did get in to the joist area (e.g. plumbing leak) would be there for a long time and could cause a lot of damage before it was detected. I'm also wondering if this approach would have any unforseen issues when applied to a structure which is not heated in winter, when temperatures sometimes get down to -40 C.

I'm thinking the safest bet would be the plan I originally described but without the vapour barrier between the subfloor and the underlay. Flooring choices would be very limited but in a building which is not heated over winter most flooring products are not recommended anyway. Does anyone know of a wood flooring and paint/stain combination that would breathe and not trap moisture?
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