Any views on my loft boarding scheme?

[PaulUszak]

Does anyone have any comments (good or bad) for this scheme to board out a part of my loft:-

I'm going with 38 x 89 CLS studs running at right angles over the top of the existing trusses. Topped with 320 x 1220 chipboard sheets. This will be the central third of the loft. The sheets will be screwed at 150 centres to the studding and Gorilla glued together at the tongue & groove joints. I don't think that I need to screw the studs to the rafters as they will be held in place by the floor sheets, and along the edges with existing timbers. It will effectively float over the rafters. The insulation will remain as is at 100mm beneath the boarded third. There will be an 89mm gap for ventilation beneath the boarding.

The outer two thirds, one each side of the central portion will remain un-boarded, but the insulation will be topped up with another 170mm of insulation.

Not the ideal amount of insulation, but I believe two thirds of the loft at the current recommended depth will be an acceptable compromise.

 

[jabuzzard]

My recommendation is forget the CLS and get some 100mm Kingspan/Cellotex or similar. Just cut the Kingspan into strips so you can get it through the loft hatch lay over the joists and cover with the chipboard. A few (and I mean a few) 150mm screws can hold the chipboard in place.

My top tip for the hatch is to build up the rim with wood, apply the chipboard so that it overhands and then use a flush cut bit on a router to trim it down flush before finishing off with some sandpaper. I also use some 4mm plywood to line the loft hatch hole so things don't catch going in and out before doing the flush cut with the router.

This method is much faster, gets you the full depth of insulation required and adds less weight to the joists than all that CLS. If you are worried about the kingspan being supported on thin joists, just cut some 6mm plywood strips and screw/nail over the top of the joists to act as load spreaders. Yes it costs a bit more but having done both in the past I would never again do the wood battens method.

Personally I also think strips of 15mm chipboard are cheaper, again add less weight to the joists and work just fine laid on top of the Kingspan/Cellotex. The sheds will cut a sheet into three 400mm wide strips, and unless you have a really small loft hatch with hardly any height in the loft you can usually maneuver these in through the hatch. Just lay them in opposite directions to the Kingspan/Cellotex strips.

 

[PaulUszak]

I would never again do the wood battens method

Any reasons for this?

Just cut the Kingspan into strips so you can get it through the loft hatch lay over the joists and cover with the chipboard

It's funny how these questions begin to circulate. I'd be concerned about condensation on the underside of the Cellotex as it would be touching the top of the existing insulation. Especially with the metallic coating. There would be no ventilation gap as some people recommend. The missus would be cross if the ceiling collapsed.

 

[jabuzzard]

I would never again do the wood battens method

Any reasons for this?

Because it is not much cheaper, its more work, adds more weight to the loft joists that are generally not sized to carry flooring, and you loose more height in the loft if you want to achieve the same amount of insulation.

Just cut the Kingspan into strips so you can get it through the loft hatch lay over the joists and cover with the chipboard

It's funny how these questions begin to circulate. I'd be concerned about condensation on the underside of the Cellotex as it would be touching the top of the existing insulation. Especially with the metallic coating. There would be no ventilation gap as some people recommend. The missus would be cross if the ceiling collapsed.

Some people talk utter nonsense, and have a very poor grasp of thermodynamics. You don't put ventilation gaps between layers of insulation. The condensation is going to form on the cold surface of the insulation layer which is the top of the insulated layer not somewhere in the middle. If you are worried, and it's actually a good idea to seal all penetrations in the ceiling, which will also have the effect of further reducing heat loss. For extra thoroughness use an intumescent sealant to maintain/improve the fire rating of the ceiling.

It's five years since I did my sisters loft with this method, and it has not fallen down yet, and there is no evidence of condensation either. I had to lift a bit of it to adjust some electrics last summer. If you are really paranoid I would replace the insulation between the joists with something thinner, but it would be a total waste of time and money.

 

[PaulUszak]

Frankly I don't know what to think with this. I actually bought a single box of Loftlegs to see what they're like. No ventilation gap with those according to their spiel. I also read about Loftzone, whose entire marketing message revolves around ventilated insulation. I can only assume that since there is no evidence as clear as "don't drink and drive" , it doesn't matter how you board it. Ventilation of the insulation must be irrelevant as there would be widespread guidance as to the correct approach.

Seems to me that chipboard laid directly on top of Celotex is an identical insulation /barrier situation as chipboard directly on top of mineral insulation. Funny that, as there are quite a lot of lofts/ attics in the UK. Surely some one would have noticed if one system works better than the other.

Maybe I should install that swimming pool in the spare bedroom instead. The guidance seems more explicit

 

[jabuzzard]

In terms of performance they are I guess both the same. For lots of lofts the loss of height might not be important. I can stand up in mine no problem. To be honest I had done several with the "CLS" method before I hit on the idea of just using 100mm Celotex. My sister wanted her new house insulated and boarded for storage and it has limited height, which made me sit down and think about solutions as loft legs would have been an unacceptable reduction in the headroom. Thinking of solutions for this is when I hit on using Celotex. I have since revisited my mothers loft that has even less headroom, by replacing the fibreglass between the joists with 100mm Celotex and putting 50mm Celotex over the top giving 300mm fibreglass equivalent which is a big improvement on what she had before.

My guess is people go the fibreglass route because that is what is normally done, then boarding out is considered as secondary and there needs to be a way to stop it being squashed so all sorts of schemes from timber to various schemes involving legs etc. Like I said I have done several of these with various methods in the past to raise the boards over the insulation and stop it being squashed. However going forward I would always go the Celotex route now I have hit on the idea.

As regards condensation, seal the penetrations in the ceiling and forget about it, assuming your loft is adequately ventilated. It might well be that you need to address this as well as insulating your loft will create a cold space and condensation can form on the underside of any bituminous felt under tiles with inadequate ventilation.

 

[PaulUszak]

adds more weight to the loft joists

I've just run some numbers regarding weights. It seems that the weight of additional CLS joists is only 15% of the total additional weight. The bulk (85%) of the new flooring weight will be due to the chipboard sheets. Total additional weight being approximately 15 kg/m2. That's lighter than I initially assumed.

No one can accuse me of over thinking my projects! Oh wait...