Loft Boarding in new build house use P4 or P5 boards? help

[howardino]

Hi,

I'm looking to get the loft boarded in a new build house.

I'll be looking to use the LoftZone system, but currently in a dilemma which board to use.
The LoftZone shop are currently selling Supreme Loft Boards which are P5 (which is moisture resistant and load bearing), however, the installer say they only use P4 (not moisture resistant).

Installer are saying that P5 is more for high moisture areas such as kitchens and bathrooms and not for lofts. They state on their website, that water vapour rises in the house and ends up in the loft, if it meets a moisture resistant board, the water droplets will condense and fall back onto joists, causing them to rot over time. This is why they say a P4 board is better.

Is this correct? I would've thought given there is going to be an air gap between the insulation and the board - natural ventilation in the air gap will wick any moisture that rises away.

Would a P4 board be better than a P5. I thought the P5 would be better in case of any spillages or water ingress from the roof, a P4 would just disintegrate causing any stored items to potentially fall through the ceiling. Whereas a P5 board will be stronger and be able to withstand such issues better.

Another thought is perhaps a P4 board will highlight any condensation issues better as it's more susceptible to moisture.

Any help you can provide on which would be the better solution would be greatly appreciated.

 

[Mr diy a lot.]

These days most boards that are supplied are P5 and there is prob not much cost difference however your installer is right so advise you just don’t need moisture resistant boards up in an attic. It’s just an unnecessary over spec. the joists rotting thing with P5 is a lot of scaremongering tosh tho but it’s no more ridiculous than your peice about you needing P5 so your attic floor can cope with roof leaks or spillages! How badly are you expecting your roof to leak!!

P4 boards for an attic are more than adequate.

 

[Deleted member 174758]

The original idea behind P5 boards was that you could lay them in a building where the roof was incomplete and therefore not weathertight, and if it rained they wouldn't get ruined - in fact they will survive 40-odd days of being soaked without damage. That also makes them handy as a sub-floor for rooms like kitchens, bathrooms and hallways which are likely to have elevated moisture levels periodically.

As the previous poster says, though, talk of rotted joist does seem to be scaremongering

 

[howardino]

Thanks guys, that provides me with a lot more reassurance