Insulation below loft boards

[itsmatt]

We have an old room in the loft (1920s semi) which, for now, is used for a home office. There is no insulation whatsoever between the loft floor and ceiling below which is a gap of about 140mm. For the time being, I can't increase the depth here so need to take an interim step so at least there's some kind of insulation. As of now, I'm planning to put 100mm Rockwool RW3 slabs in which I think should help matters without leading to moisture build-up.

My question is whether I need to leave any space between the insulation and the loft boards or if I can go ahead and add a 30mm layer to more or less fill it top to bottom.

Thanks in advance,
Matt

 

[JohnD]

140mm is quite a decent amount and will save energy. If the loft floor is actually floorboards like planks, there will be slight gaps between them that will give a little ventilation.

Loft condensation is mostly caused by holes in ceilings, especially for downlighters but sometimes for pipes and cables, and especially from steamy bathrooms that do not have effective extractors.

If you have reprehensible downlighters, we can explain how to fit hoods. For minor holes, you can seal with expanding foam (preferably the pink fire grade) but you have to vacuum away the dust and spray the surface with water or it will not stick.

Ordinary loft rolls can be squashed to fit irregular gaps, and will compress a little without pressing on the ceiling, so roll would be easier and cheap. Get the brown grade treated with Ecose that does not shed irritant dust and fibres. Old loft dust can be very unhealthy so hoover up.

The heavier slabs are better for muffling noise.