Insulating loft

[Adrian Simpson]

I've been following various threads here over the months about insulating lofts, but none of them quite seem to answer my question.

I've already got Rockwool type material laid between the ceiling joists, which gives a nominal four inches, this is now in the process of being boarded over for a combination of storage and a hobby area. However, partly to increase the overall insulation, and partly to reduce the extremes of temperature in the loft, I want to insulate the underside of the roof. My idea is to fasten some form of sheet material to the underside to the roof joists (4x2s), with a flat section at the top (overall I've got about 8ft of headroom in the main area), thus leaving an air gap above it.

Now the question is, what would the panel recommend that I use ?. A Kingspan type material looks good (but there appear to be several grades), as it weighs very little (so little extra weight on the timbers), and the foil finish should reflect some of the light, but are there better alternatives ?

TIA

Adrian

[Weatherlawyer]

Since the roof space needs a lot of venting, you will need to have the eaves open all the time. Therefore insulating the roof is going to be a waste of time.

A huge amount of moisture rises from the living quarters into the void. Sealing it will cause too much condensation to build up in the attic.

[Timothy Murphy]

I have a slightly different query - I'd like to improve the insulation in a roof-space where there is no flooring, just joists, and I am wondering if there is a simple way of blowing insulating material into the space? (There is some fibre-glass insulation between the joists now, but it rather thin.)

[Stuart Noble]

50mm celotex between the joists should leave you the required 50mm gap between it and the roof tiles. Easy to cut with a handsaw, but a bit pricey

[colinstone]

Many years ago - 14 I recall - I did my loft. I used 2 inch 8 ft x 4 ft polystyrene panels which I cut with a hot wire cutter - car battery and a bit of wire - into strips that fitted snugly between 4 x 2 joists, leaving a 2 inch air gap above to the roofing felt. Below I used the blue membrane to seal followed by plasterboard. It has been fine. No doubt if I was doing it again there are better materials on the market such as the kingspan.

[Stuart Noble]

Sure, if you don't mind it continuing to blow about every time there's a breeze.

Can't get much easier than rolling out some extra fibreglass, or get someone in for £200-£300.

[Andrew Gabriel]

In the recent gales we had, all the gardens around me were left with large wet clumps of fibreglass insulation dotted around. No idea whose house it came from.

[Andrew Gabriel]

Not unless you've got lots of holes through the ceiling, badly fitting loft hatch, etc, in which case repair them. Air/Moisture transfer through plasterboard itself is insignificant.

The attic won't be cold anymore.

You will need to ventilate on the cold side of the new insulation.

[Andrew Gabriel]

Since this isn't needed for any Building Control regulatory reasons, you could pick up seconds from ebay or the like. A friend did this a couple of years ago, and got it much cheaper. Might want to check it's fire resistant though.

If you don't mind losing an inch or so, 25mm sheets fixed under the rafters would give continuous runs. Could combine that with 25 or

50mm between rafters as Stuart says.

BTW, I found the bread knife was best for cutting it!

[Weatherlawyer]

Your health hazard is a major fire risk too.

[FKruger]

Thats only correct if the plasterboard is moisture resisting ie with aluminium foil on the reverse side.

[Stuart Noble]

I didn't want to jeopardise the lunchtime sandwiches:-) An old handsaw worked well for me

[Andrew Gabriel]

No, it's true regardless. The amount of air which will permeate a sheet of plasterboard is completely insignificant compared with any tiny amount of loft ventilation. Moisture gets into lofts in air through cracks, oversized pipework/cable holes, badly fitting loft hatches, recessed downlighter holes, etc.

[Adrian Simpson]

The roof space is a "L" shape, with vented soffits on the two longs sides (the short sides come down to a flat roof), with a gable at one end, and the party wall at the other, so there is currently some ventilation from the gable (a gap big enough to get your hand through). The section running across to the party wall has plenty of height, hence the idea of placing the insulation on the underside of the timber, rather than in the gap (makes it easier to do as well)

The existing ceiling is the original 1939 plaster board, so how moisture proof that is I don't know. There are very few holes in it, apart from where the cables for the light fittings go through, the only others are the ducting from the bathroom, and the hatch itself, which is a fairly good fit (although some draught strip to help it seal would be no bad thing).

I don't think insulating the roof is entirely a waste of time though. Bearing in mind the main reason for doing it to allow it to be used for a hobby room, in the summer it gets very hot up there, I would hope the insulation will do something to help regulate that, and in the winter, I think with the aid of a small fan heater, it will be moderately bearable.

Comments about the fire risk (particularly with polystyrene) elsewhere in the thread are noted.

Adrian

[Dave]

I was told that using an electric carving knife was the quickest and easiest way.

Dave

[Stuart Noble]

Anything with teeth cuts it really quickly, and it's easy to shave a few mm off too. Put another way, it's not like trying to cut foam rubber

[Ian White]

An old hardpoint saw with the set ground off the sides of the teeth works pretty well. It cuts straight and fast, but with a very narrow kerf more like a knife.

[r.bartlett]

Anyone use this?

a tad expensive but I wonder if it would be suitable to use between the joists as I can then board the loft isntead of either squishing the -now- glassfibre or getting some Kingspan

Thoughts?

Cheers

Richard

[Andy Hall]

This is a really good idea. Not.

All of the application illustrations show examples where controlling the thickness of the sprayed gunk doesn't matter.

You would have real fun cutting it back in order to board over the top.

It's probably a job better left to the professionals - apparently they are going to regulate its use.....