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giannit 01-21-2010 09:14 AM

Roxul without vapour barrier?
I am continuing my research on crawl space insulation. My crawl space is very dry, has a concrete floor and is ventilated. The floor in the family room above is an old hardwood floor covered by berber carpet with underlay. Also the 2 outside walls of this crawl space are above ground due to the slope of my property.

1) If I insulate the ceiling of this crawl space using Roxul, do I need to put vapour barrier up considering how dry this area has been. My thought is that the various layers of flooring above are a significant vapour barrier in themselves.
2) The other concern I have is that, if I do put up vapour barrier covering all the joists and subfloor, do I actually run the risk of trapping moisture and creating a mold problem.

Thanks for your comments.

giannit 01-23-2010 12:09 PM

I am replying to my own post to report my solution as follows:

1) Took out a floor register to see what is on the floor in the family room above the crawl space. I found berber carpet (which is essentially woven plastic), underlay and hardwood flooring. From the crawl space I can also see (in between the 6 inch floor boards) that there is a layer of tar paper under the hardwood flooring. I am taking the leap of faith that the various layers are a functional vapour barrier.

2) In the ceiling of the crawl space (ie on the bottom of the 6 inch floor boards) I taped 6 inch square pieces of clear vapour barrier in several places. I also put a piece on the the concrete floor. The idea is that any moisture from above or below would be trapped and condense. After 24 hours they a were all bone dry.

I think I am good.

ccarlisle 01-24-2010 08:41 AM

What parts of the US homeowners call a crawl space is a bit different from what we call it up here; there it seems a 'crawl space' is vented but in actual fact has an opening to the outisde, usually covered by a grill or some other air-permeable covering. So it is open to the elements...

Up here, what we call a crawl space is in fact a miniature basement, surrounded by concrete (even the floors) and is inside the building envelope, as opposed to outside the envelope. In our case, we don't need as much insulation and vapour management as other people do.

It that your case? when you say "dry", it's a bit relative. Put a humidistat down there, leave it for 2 hours and report back on the relative humidity at what temperature reading you get. Then you'll know if it's dry or not.

Then do the same thing in your basement, and report that. IMO there shouldn't be much difference at all between the two readings.

Chemist1961 01-24-2010 12:04 PM

Roxul is more resistant to moisture according to what I have read...and seldom available in the US

My guess is the "tar paper" you see is actually meant to resit squeaking under your hardwood. Vapour barrier should go between the cold and warm so against the insulated outer wall of your foundation to prevent damp air from penetrating.

If you had a floor cantalevered over outside air sapce you would insulate the floor and use vapor barrier there

giannit 01-25-2010 02:43 PM

Ok I am finished with this project. Roxul is awesome. Fibreglass would have made this project very awkward because I was putting batts in a ceiling space. They would have probably needed bracing whereas the Roxul just stays where you place it.

I can also report that I took temp readings at various points on the family room floor, above the crawl space, before and after the insulation was installed. In each location, the floor was 2-3 degrees F warmer after the insulation was added. The room is also noticebly more comfortable according to my wife (the living thermometer)

Wildie 01-25-2010 04:29 PM

As somebody else has noted, in Canada we don't usually insulate the floors.
We make the crawl space part of the conditioned area. The walls are insulated and any opening to the exterior are closed and kept closed!
When I insulated my crawl space, I used Roxul also. The Roxul was held in place with pins glued to the concrete walls.
If you have placed Roxul up between the floor joists, there should be no vapour barrier below this, as moisture from the house will condense here and encourage mold to grow.
If you have already installed the vapour barrier on the bottom of the insulation, you should perforate it and allow the moisture to move down and away!

Gary in WA 01-25-2010 06:12 PM

A good read:

Be safe, Gary

giannit 01-26-2010 11:25 AM

This is for Wildie.

I am not familiar with the method you describe for holding up Roxul against a wall. I assume this means that no studs are required. Can you tell me where you found the "pins" which you mentioned?

Wildie 01-26-2010 04:39 PM


Originally Posted by giannit (Post 389404)
This is for Wildie.

I am not familiar with the method you describe for holding up Roxul against a wall. I assume this means that no studs are required. Can you tell me where you found the "pins" which you mentioned?

I bought the pins at a construction supply company whose name is Total Construct. Materials.
These pins are commonly used in commercial building applications!
They come in various lengths and have a perforated metal base. A dab of construction adhesive is used to glue the pin base to the concrete.
After the adhesive is set, the insulation is pressed onto the pin, and then the vapour barrier is pressed on, over top!
The pins come with a square spring keeper used to keep everything in place.
In my case, I bent the sharp end of the pins back in towards the wall to avoid any chance of injury!

P.S. No studs are required! The bats are butted against its adjacent mate.

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