Need advice on underlayment/vapor barrier

6 years ago

Our house (a single level ranch type) sits above a dirt crawl space. Eventually - within a year or so - the crawl space will have a vapor barrier on top of it and will be sealed close to the top of the foundation with closed-cell foam insulation. In the meantime... We are down to subfloor in the living areas. Very soon we'll be installing a plywood underlayment - to create a smoother, more even and hopefully more level base. Very soon after that we'll be installing T&G bamboo flooring (by nailing it, we believe).

Do we install a vapor above the plywood but under the bamboo flooring?

Thank you so very much for any insight you can provide.

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Comments (4)

User
6 years ago
The manufacturer of your flooring will provide installation/warranty information.
SJ McCarthy
6 years ago
Getting bamboo into a home will all depend on how well you deal with the venting and the cross-venting of the crawl space. The moisture barrier needs to be under the home. This isn't just for the flooring's warranty...it is for everything.

Where you live and whether or not you have a heated crawl space or simply a vented crawl space will also determine how this works with moisture barriers.

Traditionally, you CANNOT put down a vapour BARRIER over top of wood (plywood, OSB, particle board, etc....doesn't matter it is all wood). The best you can do is a vapour retarder. And the type of vapour retarder will require a certain 'perm' rating. And that rating will be dictated by your home's environment.

And JUST FOR FUN....what is your humidity CONTROL like in your home? Can you control the humidity? If not, you will want to have a hard look at bamboo. Bamboo is trick. Right after that, it is tricky. And then it gets tricky. And if you haven't figured it out at that point, it gets SUPER tricky.

Bamboo is a very difficult product to keep happy. The HIGH end stuff ($12/sf or higher) is easier to live with (but tough to pay for)....but it is still tricky. If your product comes in around $4/sf then things could get ugly...right it got tricky.
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Laura B
Original Author
6 years ago
Thank you, SJ. Yes, the vapor barrier will be under the entire house. It will not be vented; the closed-cell insulation will seal the crawl space (we've received conflicting advice about this, too, whatta surprise, right!?). Crawl space is unheated.

Humidity control is not a problem, we have mini-splits in all areas with humidity control options built in the functionality. My understanding is that the fluctuations in humidity levels are the real problem - is this also your position, or are you concerned about consistent high levels of humidity, as well?
SJ McCarthy
6 years ago
Both...fluctuations can be a problem EVEN IF the humidity stays inside the 'happy zone'. And you can have issues with high humidity if it is TOO HIGH for the floor's happy zone.

Do not let humidity swing more than 10% - total. That is to say, if you install the floor at 40% humidity, it should NEVER get any higher than 50%....Ideally you would allow the home to swing between 35% - 45% if you installed at 40%...see where I'm going with this?

And mini-splits MUST be able to deal with humidity on it's own...adjusting all by themselves every 30minutes or so. If you have to do it manually, you will be running back and forth between every room multiple times per day. Every day. For the rest of this floor's life.

Did I mention that bamboo is tricky? Again, how much is your budget for purchase of the bamboo?