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I have leftover insulation from doing my basement, little bits and pieces.

In between the joists on my basement ceiling it is insulated with vapour barrier already from when the house was built.

Will condensation form if I put all the extra pieces against the vapour barrier on the inside of the wall but do not put a second vapour barrier? Or will it be fine and help my house stay better insulated? Or is it a complete waste of time.

It just seems like such a shame for all these smaller pieces to go to waste.

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A picture would help, showing where you intend to add insulation. –  gregmac Feb 6 '11 at 18:11
    
If the basement if finished and conditioned space, then the vapor barrier in the ceiling is serving no purpose. (I'm not a fan of vapor barriers even on walls of basements, but that's another issue...) –  DA01 Oct 19 '11 at 20:37

4 Answers 4

Rule of thumb in Canada is that insulation can be placed inside the vapour barrier, on the warm side, but it must be 1/3rd or less of the R/RSI value of the cold side insulation. So to extend Evil Greebo's example:

Warm side | Insulation | Vapor/ Moisture barrier | Insulation Insulation | Cold side

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Insulation is always installed as such:

Warm side | Vapor/ Moisture barrier | Insulation | Cold side

You ONLY want a moisture barrier between the warm side and the insulation, because warm air holds onto more moisture than cold air, and when warm air meets cold (air, wall, whatever), the air has to let go of that moisture as it cools, forming condensation. The moisture barrier prevents that moist air from getting into the insulation where cold and warm meet.

If you put insulation on the wrong side of your barrier, the moisture will form in the insulation, and as Shirlock said, if you create TWO layers of barrier, you create a potential moisture trap.

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+1 for your text-based diagram. –  aphoria Oct 19 '11 at 19:58

Little pieces of insulation are fairly useless because there are so many joints for air to move through.

If your existing vapor barrier is plastic sheeting, I would not place insulation in front of it (warm side of the room). Moisture will migrate through the insulation and likely condense against the existing vapor barrier, thus trapping water in the wall.

If your existing moisture barrier is the kraft paper face on the original insulation, then probably no harm will result, since the paper-faced insulation allows mositure to migrate more quickly.

Note: All mositure barriers are permeable, even concrete. It is just a matter of how quickly the moisture can move through the barrier.

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Concrete is not a moisture barrier. Concrete is anything but. A moisture barrier, like 6 mil plastic, is most definitely not permeable (if it's installed properly). I do agree that paper faced insulation isn't a good moisture barrier (it's more like 'mold candy' IMO) –  The Evil Greebo Oct 19 '11 at 18:07

Do not sandwich your vapor barrier between layers of insulation, and do not add a second vapor barrier creating a multilayer thing. This will only trap moisture. Not exactly sure where you intended to use the leftover insulation.

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Some good information on vapor barriers: buildingscience.com/documents/digests/… –  gregmac Feb 6 '11 at 18:12

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