Insulate loft floor or ceiling

[gna03633]

Sorry another loft insulation question ;-)

I've got 100mm of PRI insulation between the loft floor joists (the bedroom ceilings) overboarded with chipboard boards. I've aquired some sheets of

100mm PRI and was trying to decide the best way of using it:

- lay on top of loft boards. I would obviously have to move my stuff lay insulation and then cover it with something to stop it getting damaged. I'd have to muck around with the loft hatch.

- insulate over the ceiling rafters. I would cross batten then screw PRI onto there. This would have the advantage of not having to move stuff around much, would make the loft a little warmer/cooler and cleaner. I'd need more insulation for this one though.

What do the group think, is my insulation best on the floor or the ceiling.

cheers

[fred]

In article , gna03633 writes

Sell it, you already have enough.

I assume you mean PIR foam insulation.

[Tim Watts]

I prefer the insulation on the underside of the roof as it makes the loft drier for storage.

However, watch that you have ventilation above the insulation or you risk rafter rot...

[gna03633]

Cheers Fred, yes I meant PIR I've got a work acronym in my head ;-)

So there is little return on adding an additional 100mm either on the floor or the ceiling?

[js.b1]

Some, but 100mm PIR is expensive.

One benefit of PIR insulation is that it is totally waterproof, which avoids the small risk with conventional glass fibre loft insulation of it getting very slightly damp. I recall 1-2% moisture content reduces performance considerably, and when you go from a warm roof to a cold roof you can get condensation if ventilation is not up to modern standards. For some roofs it is, but some roofs are also a bit leaky re no eaves protector & felt having failed near the guttering, wind driven rain etc. It comes down to environmental conditions in some cases.

PIR is the best stuff possible for a loft, waterproof, foil covered, and very very high insulation value. A typical 45m^2 of 100mm PIR is probably =A3700? I would much prefer PIR over glass fibre, but not re cost. The glass fibre in foil bags seems a bit better - the solar reflection seems a little better if south facing.

[fred]

In article , gna03633 writes

Although you will have a little cold bridging caused by the rafters I'd say that 100mm was plenty and certainly enough to meet the latest strict building regs limits. Adding more would depend on the use you plan to put the loft to but if it's just to store a few items that don't mind the cold then I'd leave it as it is. If you do decide to add more to the inside of the sloping roof surface you need to be careful to allow ventilation of the space above the insulation to avoid any trapped moisture leading to rot. It's not hard but you would probably need extra vents at the soffits and at the apex of the roof so requires some thought, effort and a little extra expense.

[Ghostrecon]

if you go to

can probably get close for half that price....

[george [dicegeorge]]

get an infra red thermometer gun thingy from maplius or elsewhere and ind where the cold spaots are

[g]

[js.b1]

Ah yes, Seconds. I will probably need them in the future.

[RobertL]

is it? I thought that you always needed to put something just below the rafters to bring the insulation up to buidling regs - TLX battened on for example.

Robert

[george [dicegeorge]]

is there foam sealing between the insulation and the joists, or is there lots of very thin very long air leaks?

i'm off to buy another 12 cans of pink dryfix foam tomorrow for my inslation sealing!

[g]

[fred]

I may be out of touch but 100mm would be enough for me. I wouldn't use any mutlifoil product, snakeoil IMO, every one.