how to correctly use foil insulation?

[bradycg]

I have a conservatory, and I have had to open the plasterboard walls to access the pipes and cables.
The plasterboard is foil backed, and there is a layer of mineral-wool type insulation (as used for lofts)placed between it and the breeze-blocks of the outside wall.I am considering adding a reflective foil bubble-layer (such as thermawrap) to improve the insulation.

My question is: where should the foil layer be placed?
Should it be btween the plasterboard and the mineral wool, or between the mineral wool and the breeze-block outside wall?

If I wanted to use the same product in a loft, in combination with mineral wool, where is the best place? ie immediately on top of the ceiling baords and underneath the mineral wool, or on top of the mineral wool?

I have been trying to think through the physics, and only succeeded in confusing myself.
Help!!
Thanks

 

[mointainwalker]

Since I believe this will only add a tiny amount of increased insulation value ( much less than claimed ) my opinion would seriously be be not to buy it.

If you absolutely want to, then between outside-wall and mineral wool, but then you should leave 25 mm air-gap each side of foil and that's another problem ( or two ).

In the loft you could use it as a vapour-barrier ( v expensive one) on top of ceiling and under fibre wool, but once again you miss out on those 25 mm air gaps which contribute to their (claimed ) efficiency. In this case you should tape the foil joints too.

 

[ajrobb]

I have a conservatory, and I have had to open the plasterboard walls to access the pipes and cables.
The plasterboard is foil backed, and there is a layer of mineral-wool type insulation (as used for lofts)placed between it and the breeze-blocks of the outside wall.

The foil backed plasterboard was your vapour barrier. Mineral wool has a pretty high conductivity, 0.04, compared to foam boards, 0.024.

I would consider replacing the mineral wool with a foam board product (like Celotex). These are impermeable to vapour so you could use cheaper plain plasterboard.

If I wanted to use the same product in a loft, in combination with mineral wool, where is the best place? e.g. immediately on top of the ceiling boards and underneath the mineral wool or on top of the mineral wool?

If you've already got the recommended (250 mm?) depth of mineral wool in the loft, don't expect to save money by adding bubble foil.

The 'best' place to place these products in a cold climate is in continuous layer 19-25 mm above the ceiling boards and 19-25 mm below the mineral wool with an sealed air gap on each side. (This is not really practical).

DO NOT just put it above the mineral wool as wool needs either a VPL (vapour proof layer) on the warm side or ventilation on the cold side. American product brochures suggest placing it there but you also need a VPL on the warm side and cut slashes through the bubble foil to allow the small amount of remaining water vapour to escape into the loft. However, in a vented loft space, I think it is going to get dirty quite quickly in that position and cease to be effective. (Dirt isn't low emissivity!)

These products are commonly installed under the rafters, where they have plenty of air on both sides and stay clean. This is more to cut air conditioning bills though.

You wouldn't get optimum benefit sandwiching the bubble foil tight between the ceiling boards and the mineral wool as you could loose about 1.3 in R-value by not having the air gaps (based on Proctor Group's "Reflective Technology" brochure).

 

[bradycg]

Thanks to both of you, I am a lot clearer now. Thanks also for the recommendation regarding Celotex or similar.

cheers,
Colin