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As part of a local council grant, I am getting my attic floor insulation (2 inch thick fibreglass) replaced with 4 inch thick insulation. It will lie between the joists, on top of the plasterboard which forms the ceiling of the rooms below.

I then plan on flooring the attic - not as a living space, but for storage.

As the joists are 2x4's, this means there will be no air space between the insulation and the flooring. From various articles I am unsure as to whether this is a problem, so my question is a 2 part one:

Do I need an air space between insulation and flooring in the attic? And if so, would adding joists on top of, and at 90 degrees to the existing ones and placing flooring on them be a practical solution?

I have read this question and I don't think it covers off my lack of knowledge in this area.

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3 Answers 3

up vote 10 down vote accepted

4 inch thick insulation is thicker than a 2x4 (only 3.5" thick), which means you'll be compressing the insulation and decreasing the R value. Instead, I'd suggest laying the new insulation on top of the old (no need to throw out the old unless it's turned moldy), without a vapor barrier on the new (you only want one vapor barrier, against the ceiling), and going perpendicular (to the old to reduce the air gaps).

With 2x4 joists, I'd suggest that they are not load bearing and I'd recommend against putting any further load on top of them.

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BMitch's suggestion would trap air in the spaces over the old insulation and under the new. It also covers the existing 2x4s, eliminating thermal bridging. Both of these factors will further increase the insulation value. –  bib Jul 18 '12 at 16:03
    
+1 for everything in this answer! –  auujay Jul 18 '12 at 16:03

It's very important not to compress the insulation to allow 'lofting' Install raised storage stilts. You can the fix 18mm ply to the stilt and have insulation up to 300mm. An air gap of 25mm is often a good idea to allow the insulation to breathe.

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You do not need any air space between the insulation and your flooring because your floor will let air and water vapor through it (unless it is plastic, which you should not do because it would be a vapor barrier). The insulation value of fiberglass is approximately constant per inch at any reasonable density; compressing a batt of insulation will reduce its overall R-value. There is no reason to remove your old insulation, unless it does not contain a vapor barrier (in which case, you can put it over the new insulation). If you are in a non-tropical environment, you want a vapor barrier (such as kraft paper (the facing of fiberglass batts)) applied against your plasterboard. You do not want multiple vapor barriers to exist in your ceiling.

In order to get the suggested amount of attic insulation, you will need insulation more than 3.5" thick. You'll have to build up your joists so that you can have thicker insulation beneath the attic's floor.

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