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Under floor insulation questions

(13 Posts)
CheeseBadger Fri 28-Nov-14 14:41:22

I've been tasked with insulating beneath the floor of the nursery this weekend, and have some stupid questions. Here goes:

1) Kingspan boards or stuff in a load of rockwool?

2) Do I need to leave any space around the joist ends, or can I insulate up the the walls? I dimly remember something about leaving space for air circulation to prevent damp, but can't remember.

3) How important is it to keep the insulation away from power cables? This is a room above the kitchen, so there are recessed spotlights and power cables. Cables are massively over rated for the current carried as we've changed to LED bulbs.

Background: House is a 120 year old end terrace. Room to be insulated under is the back bedroom, in the gable above the kitchen. Kitchen ceiling is original lath and plaster. Reason for insulating is partly heat retention but mostly sound reduction. I want to be able to put the radio on in the kitchen while PFB sleeps above.

Thanks in advance!

CheeseBadger Fri 28-Nov-14 17:04:49


PigletJohn Fri 28-Nov-14 17:36:14

for sound reduction, dense mineral wool batts are what you want. Lightweight loft insulation has little mass to muffle sound. You can get them from a builders merchant or Wickes. When you pick up the (rectangular) pack it will feel quite heavy. Look for the dark brown stuff, yellow fibreglass sheds more dust and fibres. I would use a dustmask and gloves anyway. AFAIK the dense wool is not sold in rolls.

Stuff it tightly into the gap, especially round the edges of the room. It will cut out draughts under the skirting, and the black dust stains. As it is compressible you do not have to cut it to a precise fit. A breadknife will cut it but may be blunt afterwards.

If it flops down, stretch garden netting tightly between the joists to retain it, and fasten the net with garden staples tapped in with a small hammer.

PigletJohn Fri 28-Nov-14 17:39:12

you do not need to allow for air circulation, because there is not damp earth beneath.

Electric cables can be under or below the insulation, but not surrounded by it. Cables for immersion heaters and electric showers are heavily loaded and must have a gap all round. Lighting cables are not heavily loaded.

CheeseBadger Fri 28-Nov-14 17:44:29

Thanks PJ. Is it worth stopping short of the external walls, which are of solid construction, to prevent moisture wicking into the insulation? Or am I being paranoid about damp yet again?

And the thing about needing to leave room around the joist ends for some air to circulate, I assume that's only for suspended timber ground floors is it?

Knew you'd sort me out...

CheeseBadger Fri 28-Nov-14 17:46:49

Ooops. Cross post. Just the thing about stopping short of the solid walls outstanding for me to get wrong now... [Where'sTheEmbarrassedFace]

PigletJohn Fri 28-Nov-14 18:13:53

you can stuff the wool right to the ends. If the actual wall is wet due to cracks or gaps in the brickwork or render, or a leaking pipe or gutter, get it fixed.

mineral wool is not water absorbent (the batts are intended for building into new walls during construction, as cavity insulation) but they are permeable to air.

PigletJohn Fri 28-Nov-14 18:18:51

you mention recessed spotlights <boak>

You don't officially need smoke-proof or intumescent hoods, though you can buy them if you want, but the insulation should not flop onto the lamps. You can bodge up little boxes to go over them from plasterboard (it is very easy to cut but there is a knack) glued together with plaster. Plasterboard is fireproof.

PigletJohn Fri 28-Nov-14 18:21:03

(if there is noise transmission, it will be getting through those great holes cut in the ceiling. L&P ceilings are usually thick and heavy, and good for blocking noise as well as fire. Some people, including me, disapprove of holes in ceilings)

PigletJohn Fri 28-Nov-14 18:23:43

PigletJohn Fri 28-Nov-14 18:28:02

CheeseBadger Fri 28-Nov-14 21:15:06

Thanks again John. I was going to bodge enclosures together to surround the cursed lights to prevent contact with the insulation. I have quite a lot of planed square edge boards in the cellar. Will look for 6 blinking hoods to add to the bill as well instead now. Not that LEDs dissipate that much heat, and I was always skeptical about the fireproofing properties of dry lath and horse hair impregnated plaster.

Fully agree with the boak. But apparently it's the done thing if you're a buy to let bastard without a plan to cover the mortgage payments [BoughtARepossessionLongStoryFace]

The lath and plaster doesn't transmit too much noise. Just thought doing the job was easier than arguing about whether it needed to be done. Might have a conversation with the commissioning engineer (Frau Badger) in the morning before work starts.

Inspired by another thread, do you have a preferred charity if (when) your advice saves us mucking up and wasting loads of money?

PigletJohn Fri 28-Nov-14 21:23:22

I suppose you could try Riding for the Disabled Assoc.

The fire hoods are much more to prevent smoke and flame from a downstairs fire rising through the hole and suffocating the occupants of the downstairs room, rather than for a faulty lamp.

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