raising joists/insulation/boarding

11 Nov 2009
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United Kingdom

After lots of searching about what is and isn't recommended for insulating and boarding lofts I'm a bit confused about what I can do.

My plans is to raise my ceiling joists using either 2x2 or 2x4 timber on top of the joists (not at right angles as some people have said), lay as much insulation as needed & then board over the top.

Combined with my existing joists it won't give me the 270mm depth which is recommended but gives me more than what I've got & more than what I can get by using the joists only.

The space is needed for storage only and not for regular use.

Any advice would be appreciated.

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Well what advice do you want ?

You've said what you are going to do, but then don't ask any questions .

Sorry, I think all the mind-readers are on holiday at the moment.
Thanks Chukka.

M/walker......I was aking for any general advice/issues that I may have overlooked. Weight on joist issues, heard people say that the add on timber must reach the wall plates or not to put timber at right angles because extra weight not supported whereas some people say putting at right angles spreads load etc....If I was 100% sure then I wouldn't have bothered asking.

Not really sure why you bothered posting. :confused:
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My take on this is as follows:

If you have an old house with thunderous think roof joists, it's unlikely you're going to overload them. Put your extra wood at right angles to avoid cold spots.

If you have prefabricated rood trusses, just have a think about what you want to store up there. If it's concrete slabs you're probably on to a hiding. If it's the odd box of papers and soft toys you'll be fine. Don't forget you've been wondering around up there stepping from truss to truss they were happy enough to take a 75kg point load. And if there's a cold water tank up there, then that'll be a good few kgs atop a couple of trusses already.

Now if your trusses span full width and rest on a load bearing wall or two, you can put more up there. But if you have that much extra stuff, sell it, give it away, or buy a solid fuel stove and burn some of it. Alternatively move to a bigger house.

By all means put in extra timber to span wall to wall so the walls take the extra load (assuming they were designed to) but you'll have a bugger of a job getting the big bits up there through the hatch and turned 90 degrees unless you take the roof off or the ceiling down.

If in doubt, and maybe you are consulting a bunch of pub experts like us lot, seek professional help. No, not that sort, but try your local building control folk or a proper structural engineer. It might cost you a few quid, but you may sleep better at night knowing you're not about to get a face full of ceiling at any moment.

My comment was because you don't give one single fact that allows anyone to give you advice and you don't ask any specific question.

You don't give joist sizes .

You say you will lay as much insulation as is needed ( needed for what ? )

You don't say if cost is a problem ( PIR boards give better insulation per cm than glass-wool but cost 3-4 times more )

You don't say what weight you want to store.

If I could have answered properly/helped I would have done so but there was no info to work on.

Have either ogf the other two replies helped you ? Are you able to do anything with DIYWell's detailed post about point-loadings and pre-fabricated trusses ?
M/walker, apologies didn't mean to cause offence but was looking for general advice for starters.

My joists are approx 7/8cm high to which I was going to add timber (probably 2x4) which would raise the height to approx 18cm. Lay 150mm space blanket & then board over the top.

The timber won't span the entire joist in one piece, instead I was planning to buy lengths & then cut to suit.

4" timber would be ideal heightwise but would mean buying long screws etc. Not sure how heavy a 3m length is without visiting the big orange shed. This extra expense could be offset by the Homebase buy 1 get 3 free offer which works out at about £3 per roll for 200mm space blanket, meaning that it would get compressed by about 2cm and there would be no 'airspace' under the boards. Is this a problem?

My other option is 2x2 timber giving me about 13cm meaning I could use 100mm blanket & board over.

I've seen various sites that sell kingspan type board (seconds). Is it a case of just cutting and laying between joists or is more prep required (vapour barriers have been mentioned)? Also not sure how the different thickness of boards compare to different thickness of blankets. If I could buy some kind of insulation board that would avoid the need for raising joists (7/8cm thick) or get away with raising no more than 2" then I would look into subject to cost.
sushi you need to give actual joist sizes centres and span
if yours are 3" x2" unless you have a short span and less than 400mm centres you have no real load capacity
cross laying is fairly essential as it means the weight is distributed over lots off joists instead off 1 or 2

Just been up into the loft & measured.

The joists, running the width of the house are approx 6m long. I've seen some of those joining strips nailed into the sides so am assuming that made up of 2 or 3 lengths of timber.

The joists themselves are 56cm apart and are 7cmx3.5cm.

The thinnest insulation I've seen is 100mm so would that mean me having to raise the joists by 3cm to get 100mm in there, then go at right angles raising by at least 100cm to make it worthwhile & then board over?

Loadwise it is general household stuff...pram/car seats etc until needed again, christmas tree etc

If I go down the insulation board route and then floorboard over, can anyone give me any info as to what board is equivalent to say 100 or 200mm blanket?

Also is it a case of cut to size & drop between the gaps or do I need to look at vapour barriers etc? (just seen a post recommending a plastic membrane above the bathroom ceiling)

I am not a builder or SE so will let those better qualified give specific advice on sizing, but with that span, IMO your joists are spaghetti-like and I would forget any ideas of increasing joist height to allow extra insulation,flooring etc until you have received reliable ( professional ? ) advice.

What kind of a house is this and built when anyway ?

If you already have any weighty items like car-seats ( or do you mean child car-seats ? ) up there, I would move them as close to the walls as possible.

Fibre-glass insulation is available in lesser thicknesses than 100 mm, but much less commonly. It may even be more expensive than the 100 mm because there are unlikely to be promotions available.
PIR board is 60 % more effective than glass-fibre, so 100 mm glass-fibre is same as 60 mm PIR.

Boards are available in more thicknesses than g/f so you could find what you need to fill the existing space but it won't be cheap.

If you used PIR and cut it EXACTLY to size between the joists you wouldn't need a vapour barrier as the moisture permeability of PIR is, to all intents and purposes , nil. If however you are not Modelmaker 2009 you might well decide it is easier to lay a membrane over the whole ceiling area - not just b-room - to prevent moisture penetration.

You can buy thin poly sheets from B+Q that they sell as floor/furniture protection whilst you are decorating - cost 5-6 pnds.

If you can't afford/don't want to spend for the full 70 mm depth that you can lay at the moment, then lay 30 mm or whatever because it is the first layer of insulation that will save you most money.

Given current energy costs, I would guess that might pay for itself within one , at most two, winters

It is an early 70's 4 bed detatched house, and if any help it has W shaped roof trusses.

There's nothing up there at the moment because when we moved in 5 years ago, there was just the 2 of us and we bundled all our extra stuff into the wardrobes and stacked boxes in the ensuite (added when extended in the 70's).

Over time we've got rid of a load of stuff but added as well, with the arrival of junior. So it will be his old stuff really, clothes, pram, child car seats etc...although there might be one or 2 car seats up there as well as I've got a project in the garage that I want to start.

May have to go with board for the storage area & use blanket everywhere else.

Is PIR board ok to use above light fittings (not recessed)?

Thanks again.
The general advice is not to lay insulation over electrical cables, although this is, IMO, oversimplified so that people are not asked to distinguish between low-power lighting circuits ( no problem) and power circuits, particularly for things like electric-showers ( high risk ).

As long as you are not talking about anything that has a transformer, which usually gives off significant heat, there should be no problem in insulating above a normal ceiling rose.
is there any walls underneath
or do you have a trussed roof with lots off triangulation on every timber
There's a W shaped truss, I think, every 3rd (without looking) joist. If you can imagine it, the middle bit of the W goes up to the high point in the roof.

A couple of the rooms don't have ceiling roses in the rooms (flush fitting lights) but instead have connector blocks in the loft. I was going to change these for either junction boxes or more substantial connector blocks. Apart from one of the blocks looking a bit small I can't see anything wrong with them (wires tight, don't blow fuses etc).


There are the walls for the 4 bedrooms, the bathroom and the airing cupboard (which has 2 water tanks in the loft above it).

The internal walls are all stud frame/platerboard walls, and the external walls are block/cavity/brick.
The connector blocks should all be in little junction boxes (search for choc box). Don't insulate over recessed downlights.

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