Fiberglass Batts - "polarity"

I am adding another 3 1/2 inch insulation layer under the floor of my utility room, which is cantilevered out from the house, with a 4 foot breezeway underneath. It is an unheated area that I want to reduce the temperature swings since it has three exterior walls. Between the floor joists there is currently 3 1/2 inch R11 with foil facing on the "up" (subfloor) side. I have enough craft-backed R13 3 1/2 inch to do the job, but am uncertain whether to orient the craft backing in the up or down orientation against the existing batts, or do I have to remove the backing and use some sort of wire to hold the insulation in place?
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If you already have craft-backed fiberglass insulation that you're installing over old batts, you want to remove the craft paper from either the old or the new. If the craft paper is against the opposite surface, remove the craft paper from the new. If the craft paper is facing you, remove the craft paper from the old. Actually, you don't need to remove it, perforating it (so that vapor can go through it) is just as effective. There's a little roller thingy with pointy spikes on it that can be used to do that, sorry that I don't remember the name of it, it's been many years. Anyhow, the point is that you don't want two vapor barriers there, because vapor will build up between them and condense in the insulation.
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Eric Lee Green mailto: snipped-for-privacy@badtux.org


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