Have laid combi insulation between joists. Then laid old carpet across joists as extra insulation. There is very little sag but have been told the carpet may stop insulation working to max effect as it will stop insulation xepanding to full extent. Is this right or will carpet keep more heat in?
Fibreglass insulation, is not as good as closed cell insulation!
It has a problem, inasmuch-as it is transparent to water vapour and affected by wind circulation.
You may well discover that water vapour will transit the plasterboard ceiling and build up inside the fiberglass making it wet and useless as an insulation.
It is probably best to leave it uncovered to enable the water vapour to escape.
A better solution is to fit an almost water vapour proof plastic membrane under the joists to prevent the water vapour making its way through the plasterboard.
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