Boarding a loft over very thick insulation - Problems?

[Parrotwoman]

We've just had our loft insulated with really thick insulating material (fibre glass kind) and the chap who measured up said that, as it will be well above the joists, I will have to raise the height of the joists with some 2x2 if I want to board it out, otherwise the insulating material will 'sweat'. Does anyone know if this is true? I have read on the forum about the threat to the ceiling beneath, but the fitter said the 'sweating' was a load of old tosh. Anyone out there know the truth please?

 

[JohnD]

If you are going to board it, you will have to put extra timbers uo, otherwise you will squash the insulation and instead of having it 200mm thick it will only be 100mm thick, so it will only be half as good (ish).

Fibreglass is not airtight so warm moist air can rise through it and will condense on the first cold surface it finds. this will be your loft flooring so an airspace is recommended for ventilation. It will be worse above light fittings, or where cables or pipes mean there are holes in the ceiling. Bathrooms also generate a lot of steam.

You can if you like cross-batten your loft by running the extra timbers at right angles to the existing. thsi will help spread the load of your weight while you are clambering about up there. if you screw them down tightly, and the boards on top, they will be much more rigid, but not strong enough for a habitable room. This also enables you to give space to pipes and cables.

Condensation in a loft if often caused by excess moisture in the house. Two bad common causes are: no extractor fan in bathroom, and draping wet washing about the house.

 

[Parrotwoman]

Many thanks for the quick and comprehensive reply. It's much easier to do things when supplied with the correct information and the reasons why.
Once again many thanks.