Additional Loft Insulation

[cashford]

Hi
I'm new to this forum so I hope I'm posting in the right place. I am after a bit of advice please.

I want to increase the amount of loft insulation I have (current I have about 4" of fibreglass) My plan was to lift the loft flooring boards and place a layer of Double layer Aluminium foil down over the top of the fibre glass before then re-laying the floor boards on top.

Will this work as I understand there must be an air gap under the foil to make it work properly and I guess with the fibreglass there, there won't be. Also will it be OK to place the floorboard on the top of the foil. The boards are not screwed down, just placed there and interlocked together.

Thanks for any guidance.

 

[ajrobb]

Installed correctly with a void on both sides, foil only insulates about the same as 50mm of mineral wool. With the bottom of the foil against the existing mineral wool, you lose half that. When the top of the foil gets dirty, you'll lose the other half.

Also, foil is a vapour barrier and could trap condensation on the cold side of your existing insulation.

There are great deal to be found for mineral wool. A few months ago I got 8m² rolls of 170mm mineral wool for £4.

 

[cashford]

Hi, thanks.for the info and advise. I think I agree that mineral wool would be better, problem is that if I add another 6inches of it to my current 4 I won't be able to put my loft flooring back down as, being a 1900's house it only has 4x2 ceiling joists.

Thanks again, I'll go back to the drawing board.

Charles

 

[freddiemercurystwin]

Just add additional joists perpendicular to your existing, thousands of threads detailing such installations in the Lofts section of the forum.

 

[ajrobb]

Just add additional joists perpendicular to your existing, thousands of threads detailing such installations in the Lofts section of the forum.

Loft insulation should have ventilation on the cold side. Depending on what ceiling paint you have, T&G boards might be more of a vapour barrier than the painted plaster and might cause condensation. Maybe gaps between square edged boards could provide enough ventilation.

 

[cashford]

Hi
Thanks for all the info. We have artexed ceilings painted with standard matt emulsion, so do you think if I was to lay foil over the existing wool and then just leave a gap between all the boards when re laying them over the foil that I may be ok from a condensation point of view and also gain some benefit from the foil from and insulation point of view?

This would certainly be preferable to cross laying new 'joists' to gain either an air gap or the height for more wool.

Thanks again.

Charles

 

[ajrobb]

Hi
Thanks for all the info. We have artexed ceilings painted with standard matt emulsion, so do you think if I was to lay foil over the existing wool and then just leave a gap between all the boards when re laying them over the foil that I may be ok from a condensation point of view and also gain some benefit from the foil from and insulation point of view?

I'm sorry, it sounds like a terrible idea. The foil will be in contact with mineral wool on one side and floorboards on the other. This means it cannot give you any insulation. In addition, it will be a total vapour barrier so will almost certainly cause condensation.

Having 6x2 joists at 400mm centres across the ceiling joists not only allows for an additional 150mm of mineral wool but spreads the floor load across the ceiling joists.

FWIW I have a 1970s terrace house with Artex ceilings under roof trusses. The roof truss joists are strongest where the 'A' beams come down from the apex. My cold water tank was placed on boards across joists across more joists across trusses. In other words, there were two joists running across the trusses at the base of the 'A' beams. There was another layer of joists (parallel to the trusses) running between these joists and the floorboards were on these. Such an arrangement would leave plenty of space for ventilation above additional mineral wool.

 

[cashford]

Hi

Don't be sorry, I really appreciate the advice and will indeed take it. I know very little about these things but am capable of carrying out the work if pointed in the right direction.

Thanks again, more mineral wool it is laid between new joists at 400mm cents

Thnaks.

Charles

 

[ajrobb]

more mineral wool it is laid between new joists at 400mm cents

You might want some noggins between the new joists to keep them vertical. I don't know if you'd need a 'full set' of noggins as it won't be a 'proper' floor. Fixing the new joists to the ceiling joists and fixing the floorboards to the new joists will help stabilise it and stiffen it when you're walking around. The floor will still ultimately only be supported by the ceiling joists so will only be suitable for light loads.

As always with loft insulation, watch out for any electric cables carrying more than lighting current (e.g. electric showers).