Loft boarding for storage - Kingspan vs glass fibre

[SimonH2]

Looking to board out the SO's loft. Was planning on 150mm joists to leave plenty of room for insulation - but headroom is already limited and 150 extra on the floor would significantly reduce headroom, as well as making access to the eaves past the purlins "challenging" (especially for someone as corpulent as I am ).

Dad suggested I consider Kingspan or similar and shallower joists. It seems that if I don't mind using seconds (http://www.secondsandco.co.uk) it doesn't seem too expensive. How does it compare insulation wise with glass fibre ? Or put another way, how much would it need to replace (say) two layers of glass fibre restricted by the boards to 150mm thick ?

 

[noseall]

Whilst rigid foam board insulation is probably twice the 'power' of glass fibre per thickness it is not as versatile regards filling gaps etc and requires a bit more effort to fit.

Getting a perfect fit or join is not easy so you need to resort to ex-foam, mastic and foil tape in order to maintain efficiency.

 

[Architexeter]

I should think you want at least 100mm of rigid board insulation between the joists. I've used seconds and co and the stuff I got was as new excess stock.

Why are you worried about headroom - is the loft for storage or as a room?

Have you considered "loft legs"?

Give me some more info so I can advise further.

 

[SimonH2]

Well there's already glass fibre up there, but most of it is squashed down by a load of small boards (the small T&G chipboards sold by the likes of B&Q) so it's only the 4" thick of the existing joists. Headroom wise, going up 6" would just mean more crouching to move around (I can't stand up straight as it is, was darned hard work putting proper lighting in) so I was hoping to avoid making it too much worse.

But, there isn't a huge gap between the purlins and the existing ceiling. There's only 3 brick heights between bottom of purlin and top of the existing insulation (where it's free and not squashed) - which I think is just a single layer of 4" cross laid on top of whatever might have been there before. My guess is this was done "some time ago" - probably when the whole house was "done up" with rewire & central heating (from the test label on the CU, I'd guess in 1986) and before our current standards of "about a foot" of glass fibre came in.

There isn't much gap as the loft doesn't go to the eaves. The ceiling is lifted by something like a foot, so the ceilings follow the rafters down at the front and back of the house. So I'm thinking that if I go and add a 6" joist with board on top, then I won't be able to get through the gap and a large chunk of the loft will become completely inaccessible to me - a problem for access for wiring etc. I'm also assuming rigid foam board will be the best way of insulating the sloping bit of the ceiling - though I haven't ventured down to the side of the loft to see what access is like, and I assume it'll be fiddly getting an L shaped piece past the end of the joists followed by a parallel slab to fill the rest of the gap. It wouldn't surprise me too much if I found a lump of glass fibre stuffed in the top of the slot and nothing further down.

So I though that if I used rigid foam board (on top of the existing glass fibre), I could compromise and use 4" joists and get the same insulation. I suspect I'd still need to cut the edge joists down and leave an access gap - I'll need to work out what gap I can safely and comfortably get through.

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• SimonH2
• 27 Nov 2013
• 1

Untitled

• SimonH2
• 27 Nov 2013
• 1

[/quote]

Oh yes, in terms of span, there's a 2m span from one wall to the brick wall between landing and bedroom, and a 2.8m span from there to the far wall. Existing joists are (I think) 4x2.

 

[freddiemercurystwin]

Have you considered "loft legs"?

Whilst loft legs are effective in making a spacer, they do not spread the load, which is generally high priority when loft storage above ceiling joists is concerned.

 

[Architexeter]

As you are concerned with the height and already considering rigid board insulation I'd probably just remove the existing glass fibre insulation and insert 100mm rigid board insulation (assuming 100mm joists) and then board over. Hopefully the excess glass fibre can be used where you do not need access but be sure to maintain ventilation.

Is your roofing felt breathable?

With the sloping ceilings this is always tough here and you will need to maintain a ventilation path above the insulation here too (50mm for non breathable felt + soffit vents and 25mm for breathable membrane type).

There is little point insulating the flat ceiling to the n'th degree without addressing the sloping ceilings.

Think insulation, think condensation, think ventilation!

 

[SimonH2]

As you are concerned with the height and already considering rigid board insulation I'd probably just remove the existing glass fibre insulation and insert 100mm rigid board insulation (assuming 100mm joists) and then board over.

Hmm, that's certainly a thought. Be fiddly working around all the cables etc, and make access for future cabling awkward (I intend adding wall lights and so in due course). I'll certainly consider it though.

Is your roofing felt breathable?

Yes, it's been re-roofed not too long ago and it's a modern breathable membrane.

With the sloping ceilings this is always tough here and you will need to maintain a ventilation path above the insulation here too (50mm for non breathable felt + soffit vents and 25mm for breathable membrane type).

There is little point insulating the flat ceiling to the n'th degree without addressing the sloping ceilings.

Think insulation, think condensation, think ventilation!

Pretty much my thoughts. I'm not looking forward to doing that bit
Haven't had a good look yet to see if it has vents in the soffits of not.


One slight complication is that I was looking to put a thermal store up there at some point. So one purpose of the cross joisting was to be to make a strong support for that (joist hangers off the gable end wall, across to a brick internal wall.) I'll have another look, I think if I position it carefully I can find a spot that's only the span of the stairwell (ie 1m), so could just put in cross joisting for that bit when I need it.

If only I could lift the whole roof up a couple of feet