Under floor insulation

[JM]

I have a 1930s house with the ground floor consisting of wooden floorboards on timber joists. In turn, the joists are supported by brick pillars or by the main walls. Under the floor there is a 2 foot high crawl space ventilated by airbricks. There is access via several crawl hatches. The underlying soil is dry to the touch.

I'm looking to insulate under the floor without removing the floorboards.

I've read about various possible approaches using rockwool / loft insulation roll between the joists suspended by net / nylon cord / battens, or alternatively using rigid panels of Kingspan / Celotex / Floormate cut to fit between the joists.

Are either of these approaches sensible and practical, or are the any alternatives? Or alternative products to use?

If I use rigid panels, I have concerns about creating a possible future damp problem if I block up the end of this cavity/channel and air is effectively trapped in a cavity between the underside of the floorboard, the joists, and the rigid panel. If I don't block the ends of this cavity, is it just a waste of time? Or should I push the rigid panel next to the underside of the floor board?

Presumably I should not have insulation in contact or close proximity to electric cables for risk of leaching chemicals from the cable insulation?

What are the possible options and suggestions?

[Dave Fawthrop]

|I've read about various possible approaches using rockwool / loft insulation |roll between the joists suspended by net / nylon cord / battens, or |alternatively using rigid panels of Kingspan / Celotex / Floormate cut to |fit between the joists.

Beware loose rolls of insulation which will fall on your head and itch terribly. There is glass fiber roll on a polythene outer, which should be better.

Polypropylene cord on clout nails will hold it up OK.

[Andy]

I insulated with rockwool held up by wire strung zigzag fashion between nails I hammered into the joists. As mentioned, although this works you will need long gloves, a boiler suit, goggles, a facemask and a scarf to protect you from the rockwool as it sheds fibres directly onto you: not nice.

Also, in retrospect I would have used some form of netting to hold the rockwool in place, not wires, since netting would hold it in place more positively. The cheapest way to do this IMO would be to use garden netting, or maybe garden fleece or membrane.

A scan of my floor with an IR thermometer showed ( referenced to a spot I didn't do because I ran out of rockwool ) a 1 degree Fahrenheit difference, occasionally 1 degree Centigrade, so a worthwhile result.

I would say that the silvery bagged rockwool I have seen is somewhat more expensive than the loose roll version so if economics is all, rockwool on a roll plus netting may be the cheapest option.

Andy.

[Phil L]

Use polystyrene, it's available in various thicknesses, 100mm should be ample. it's clean and cheap. it's easy to work with. A slight downside is that it gives off toxic fumes if it catches fire, but when was last time you had a fire under your floor? Celotex et al will set you back more than you could ever save and fibreglass/rockwool are horrible for working with.

[The Natural Philosopher]

Both work.

But kingspan/celotex is probably the easiest and most effective, and the most expensive.

The latter. Push it up. However thats all on the 'warm' side of the insulation so it should not be a big problem.

Not an issue with celotex. Its very inert

[Christian McArdle]

It should be fine. To avoid condensation, each void should be considered either part of the outside (i.e. outside the insulation envelope and well ventilated) or part of the inside (i.e. inside the insulation envelope and with at least some ventilation to the rest of the house).

You want to avoid void areas sandwiched with insulation both towards the outside and inside, as you can get interstitial condensation unless the insulation is "short circuited" with ventilation.

The space between the rigid insulation board and the floorboards should count as part of the inside.

Some ventilation to the cavity would be good particular if you have an impermeable floor covering. However, it should be ventilated to the house space, not the underfloor space. Gappy floorboards are ideal for this. Otherwise, you could channel air through gaps between the skirting and the floor.

Christian.

[JM]

Thanks for these responses.

What if I were to use Knauf space blanket? It's on offer at B&Q right now.

[Harris]

wrap" (great description!). They are apparently as good as the thicker mineral wool, but i would imagine a lot less messy? Don't you just staple them to the joists, so no messing around with nets and wires?

[JM]

This sound like the same stuff. A silver space blanket enclosing fibre glass / mineral wool on a roll. Must less messy to deal with.