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Question DetailsAsked on 2/7/2017

Under laminate flooring, should there be a barier if appling over pine boards ?

This is a heated room. the pine board flooring is mounted on joists with insulation applied underneath. The insulation was applied years ago and the paper was applied to the bottom side not near the flooring. Below this is about 3 feet of air and then dirt. if a barrier is applied between the laminate flooring and the pine flooring, will there be trapped moisture?

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2 Answers


Hmmmm - heated room over unheated (presumably) crawlspace. As you appear to know, the vapor barrier would normally be on the room side in an area with winters, or the crawlspace side in an always warm climate with high humidity prevailing. The former is which is what they actually did, and while kraft paper faced insulation is not a great vapor barrier (asphaltic kraft paper is much better, and of course visqueen much better yet) and of course there are gaps all along the sides of each batt and none over the joists so its effectiveness is limited, for a dry crawlspace situation in areas where it gets cold that would be the right way to do it - short of isntead having put down vapor barrier over or under the subfloor sheathing, that is, which would be better because less air gaps.

If a wet or constantly high humidity crawlspace, while putting the vapor barrier directly over the ground is best to protect from ground moisture, vapor barrier on the underside of the insulation would be second best, though not very close because in warm weather it would work right, but in cold weather might act to trap household moisture in the subfloor and insulation.

Not knowing which climatic situation you are in, can't really say which would have been best in your area.

Now - to your situation with the faced insulation already facing up under the floor. The laminate flooring will retart vapor transmission fairly well (because of the platic surface wear coating and laminate plastic layer), so to avoid the issue of possibly trapping moisture between two vapor barriers I would treat the subfloor and flooring as part of the conditioned space and use no intentional vapor-retarding layer.

Otherwise - I would go for a breathing padding - open cell or open matrix material, and synthetic plastic material only - not organic thread based to minimize possibility of any condensation mildewing. Then the laminate over that. Given that it is a heated room and you have insulation underneath (presumably 5-1/2 to 9-1/2" worth if between floor joists as I assume) the floor should hopefully never get down to the dew point and condense what moisture is in there. Of course, using an inorganic laminate (plastic rather than wood-based) as opposed to an engineered wood would also reduce both the moisture transmission through it and the risk of damage from moisture condensation, but unless you are in an area where it gets pretty cold in winter (say below 10 degrees or so for extended periods, which is hopefully unlikely if you have crawlspace construction) I would not get too worried even with normal laminates or engineered wood, provided it sits on an inorganic breathable padding.

Of course, if the joist bays are not filled full depth with insulation, you could consider filling them the rest of the way with unfaced fiberglass insulation if in a cold area or if in a very high air conditioning use area - though for the latter it might not pay off, because with crawlspaces commonly being relatively cool the floor normally does not contribute a major part of your air conditioning load.

Of course this is a different issue than household air getting trapped in the subfloor, but if the crawlspace is damp or wet, then permanently draining the water level down (if high) would obviously be a good idea to protect your subfloor framing from excessive moisture too, and vapor barrier the crawlspace to reduce the moisture coming from there if it is bare dirt and damp. You need to seal the vapor barrier to the foundation for it to be truly effective - some caulk (with a caulk or adhesive that does not dissolve the vapor barrier) it to the foundation - usually with firring strips nailed over it to hold it on (leave slack so any movement under there doesnot tear it free) - others skimp and make it a bit less effective and wrap the vapor barrier up the foundation wall a ways and tape it (which will not hold long-term) then spread an inch or two berm of sand along the edges to hold the vapor barrier to the foundation wall.

Answered 3 years ago by LCD



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