Replacing Loft Insulation

26 Feb 2013
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United Kingdom
Hi guys, apologies this is a repeated thread from the Roofing section = posted there by mistake.

@Mods, please remove this thread from the roofing section.

We've just bought a house and found that the loft doesn't have any insulation in it!

I've ordered some rolls of 100mm and 200mm Earthwool, planning to use the 100mm as a base layer between joists and cross lay 200mm over the top.

My question was regarding insulating under the Cold water storage tank, and what the best practice for this is. Our CWST is raised off the floor of the loft by around 1m (see pictures), presumably this was done to increase the cold water pressure to the bathroom taps.

Any advise on how to go about laying the insulation, and how to tackle the water tank would be much appreciated. Many people have said not to lay insulation under the tank, but others say that it should be done. We also have some downlights in a couple of the bedrooms, so would welcome some advice on those too - I've heard to leave at least 75mm of space around each downlight to accomodate for the heat, but if anyone's got any good pointers I would very much appreciate it!

Thank you in advance to all those who reply!


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You'd either use cellotex under the water tank, or garden netting to hang some rockwool insulation under the water tank. If it was on the joists, then you possibly wouldn't bother, but up there, it needs doing. You're going in the right direction for the underlay, but get some fire caps for the downlighters, and that'll keep the insulation away from them, and let them breath. Try and keep the electrical cables away from the insulation, but if you can't move it to the side of the joist, then maybe put a few bits of wood over them to raise the insulation away from it.
Very grateful for your advice Doggit - just to clarify, should I insulate the floor as normal under the water tank (between joists and across the top), and in addition hang some rockwool below the water tank on some netting, up against the wood in the picture? Cheers for your help!
You do the floor (joists) to stop any heat loss from the bedroom, and hang the insulation underneath the CWST to stop it freezing.
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I see your pipes were done by a very old plumber who thinks a few scraps of wool felt will do the trick, until moths eat them.

Due to its thermal mass, the water tank will not freeze until long after all the pipes have burst, allowing the water to run out.

Start by using by-laws grade pipe lagging, paying extra attention to all joints and valves, which will freeze first. The lagging is stiff plastic foam as thick as your arm, much better than economy grade You can cut and mitre it with a breadknife. Tape all bends and elbows to prevent it gaping. Tie luggage labels on all valves to say what they are. Flop the loft insulation over the pipes where possible.

I have a raised water-tank (to improve shower pressure) and it is correct to have a gap in the insulation beneath the tank, but unless you build a little Celotex cabin to box in the space beneath it, very little heat from the house will reach the tank. If there is a landing light on the ceiling below, a little extra heat will leak up.

As regards cables, if they are tight to the ceiling, with the insulation on top, that's fine because the heat can escape through the thin plasterboard. Lighting circuits are not a concern, because the cable (in free air) can carry about three times the maximum design load, and modern LEDs and energy-saving lamps mean that it unlikely ever to reach even the design load. However, immersion heaters, and especially electric showers, are heavy loads near the capacity of their cables, so they must never be buried in insulation. It can be on top or underneath, but not surrounding the cable.
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Thanks for your reply John. I was advised by someone at a local plumbers merchant to make a sort of "skirt" around the frame using a couple of black insulation jackets for the water tank. Would this do the job if I went all the way round and taped the gaps, and where it touches the floor, I can butt the rockwool up against it, making a sort of tunnel underneath the tank down to the floor?
I can see the benefit, but I think Celotex, Kingspan or similar will be easier to build and more durable.

It is a rigid foam insulation board, usually with an aluminium foil skin, that you can cut with a saw, drill, and screw to your timbers. It will not sag or flop.

It is more expensive than the mineral wool, but has about twice the insulating power, so 50mm is as effective as 100mm of mineral wool. I'd use 50mm for the job. 25mm is not much cheaper. 2440x1200mm are cheapest, but it is also sold in half-size and sometimes quarter-size or smaller pieces
expanded polystyrene is cheaper still, but not as good or as durable, and must never be allowed to touch electric cables as it damages the PVC insulation.

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