Storage loft, kingspan+chipboard over joists not wool+stilts

[Wassy]

Hi everyone,

I have just bought my first house, built in 1979, it has a cold system attic, currently with about 150mm glass mineral wool style insulation laid between 100mm high joists (about 35mm wide). I'd like to increase the insulation but also add some much needed storage space. I've read up a lot on this subject and am well familiar with the various systems available. But before I launch into my preferred option I'd like to get opinions from fellow diy'ers or tradesmen on whether it sounds like a good idea or not.

The loft joists are fairly standard and therefore not designed to carry heavy load, so I'd like to keep the weight down. With that in mind I thought I would avoid adding cross battens, extra insulation roll and then chipboarding over the top as that seems like quite a lot of additional weight with all the cross joists/battens.

I could use loft stilts/ loft legs, extra insulation roll and then chipboard, but this seems like quite a lot of effort as by my calculations, for the area I want to board between the trusses would require 110 stilts in total.

I like the idea of using 100mm kingspan, laid directly cross over the joists (yes I know this would compress the existing 150mm wool roll which is a bad thing), giving a total of 100mm wool roll (after compression) and 100mm kingspan, which has an insulating equivalent of 200mm of insulation roll or more, bringing me up to 300mm. I would then lay chipboard directly on the kingspan and screw through both into the joists. Building savvy friends have said the kingspan is pretty good under compression and shouldn't damage too much just from having light storage boxes placed on top (nothing too heavy) and occasional foot traffic. The weight shoukd also be spread out from the chipboard on top right?

Seconds and co are selling 100mm kingspan at the moment at a price that would make the whole job only around £100 more than the loft roll and stilts option, and in my mind it should be a lot easier to do and I get to keep about 100mm more headroom in the loft by using kingspan.

I don't think I should need to worry too much about trapped condensation as the kingspan is not foil backed, so hopefully any rising vapour will permeate as before and it will only be laid in the middle of the loft, with more wool roll laid either side towards the eaves, so air should be able to circulate under the kingspan from the sides.

I know the kingspan will crush slightly on the ends where the weight above is held by the edges overlapping the joists, has anyone tried this method and has anyone found this to be a problem?

What about down lights to the rooms below, would I need to put metal diffusers round them before the kingspan goes over the top, leave a hole in the kingspan above them or box them in somehow?

I'm interested to hear peoples thoughts in general about this solution and especially if anyone has done this themselves.

Thanks[/u]

 

[anybloodyid]

What you suggest may work but I wouldn't chance it as the kingspan is not sat on a flat surface but straddled across joist and I suspect it will fail at these points.
You don't really need more headroom if it's only for storage of light weight items. I would go for the stilts and definately don't cover the lights.

 

[freddiemercurystwin]

There are literally thousands of threads on the forum in the Flooring section about this topic, have a search.

 

[Wassy]

@anybloodyid, thanks for the advice. I think I might end up down that route.

@freddymercurystwin, yes I have done a search, and read about 10 or more different loft threads, however, as everyone loft design, joist size/weight/centres are different, I thought I should ask about my specific situation. Plus there weren't many relevant threads about putting kingspan down over the joists and placing boards on top

Would 35x150mm cross battens be ok on top of 35x100mm joists with ~550mm centres do you think?

 

[anybloodyid]

Would 35x150mm cross battens be ok on top of 35x100mm joists with ~550mm centres do you think?

I think not. the weight of the cross bearers alone would be too much.
Loft timbers are only designed to hold the plaster ceiling. One of the reasons cracks appear when too much weight is placed on them from above.

 

[JohnD]

Look for the Knauf Space board guide which I think is similar to what you want, although this particular product may be hard to find, similar insulating slabs are common.

http://www.space-insulation.com/products/space-board-eco/

http://www.space-insulation.com/wp-content/uploads/2012/12/Space_Board-Installation.pdf

http://www.space-insulation.com/wp-content/uploads/2012/12/Space_Board-Product_Datasheet.pdf

http://www.space-insulation.com/wp-content/uploads/2012/12/Space-Board-Leaflet.pdf

The insulating "power" of the foam slabs is about twice that of the same thicknesss of mineral wool.

Obviously you can't expect to walk on the foam, you need chipboard or ply on top.

Stilts are IMO the worst option because they have little sideways stability, and put the load onto spots rather than spread. We often find shills for the manufacturer recommending them.

 

[Wassy]

Thanks for the info, I have looked at these before. They seem a bit scarce though these days and it looks like they aren't selling them anymore.

@anybloodyid. I appreciate the words of warning, and I think it probably is best to air on the side of caution, but people put 2x4 (granted not as big as what I was suggesting) cross battens in their loft all the time for storage. This seems to be the most common method for adding a bit more insulation and loft boards. Is this really going to be too much weight? It seems to happen a lot and I haven't seen many posts in the 'DIY disasters' section claiming that their ceiling is falling in.

I'm now thinking adding 2x4 cross-battens with 75mm or 100mm kingspan between the battens, then putting boards on top might be the way to go, as it would be slightly less weight than 35x150mm.

 

[Wassy]

@John D - P.s., yes that's also what I've heard about the stilts, which is why i'd like to go fo something sturdier than I don't have to worry about when I walk on it or move stuff around in the loft.

It'll be a bite more expensive, but do you think the cross battens, kingspan and chipboard will be light enough not to cause bowing on the 35x100mm joists?

Thanks

 

[JohnD]

I think you could use Kingspan or Celotex in the same way as the space boards, and dispense with the battens.

I battened and boarded the central part of my loft years ago, but if I bulk it up in future, I'll do that (especially the low edges where I don't usually stand)

 

[Chri5]

What sort of overall space are we talking about?

Post our loft conversion we ended up with a inner gable area and eves section for storage. About 140 sq ft 12x5 strip and 9x9 area in the gable.

I laid 100mm kingspan and floated 18mm TG over, glued the TG and built access to the lighting via mini trap doors.

It's only a storage zone, movement is limited to every month or so and has been fine for years.


Unless the existing wool compresses comfortably to 100mm you do run a risk of pushing the board or lathe (you don't mention which) which could bow or cause cracks. So either replace that wool with a thiner depth or (nasty job) skin back existing wool.

The lights you have generate heat and must have air around them, they can't have insulation or ks over them. So fabricate or buy covers, and maybe consider trap door access to them.

 

[Wassy]

Hi Chri5,

Thanks for the great description of the method you used.

I plan to board the area between the trusses (approx 5.5m x 2.5m), making cut-outs in the T&G to fit around the trusses so that I have space to push the boxes back slightly into the area between the trusses, to leave the centre walkway clear. Apart from this, the area from the trusses to the eaves will be left clear and have more glass wool laid on top. The ceiling is plasterboard and I could potentially take up the 150mm wool roll and use that to top-up the eaves, buying in some more 100mm roll to go in the section that will be boarded.

I have changed my mind now and decided to add cross-battens instead, to make a sturdier platform as I might use it as a hobby space once a week or so. I've bought 75mm (non-foil faced) kingspan from seconds and co to lay between the battens. I can get 38x88mm studwork from wickes for £1.50/m to use at battens which should spread the weight of the T&G better and shouldn't add too much weight.

I have bought Aurora Sola Universal downlights which are fully fire rated and come with their own loft caps and can be covered with insulation as long as it's not packed in tightly at the sides. I have also bought 1mm twin and earth cable rated to 15A, which when covered with insulation, after calculating the downgrading (~50%) is still 7.5A which is more than the MCB on the lighting circuit.

I'm not sure whether to leave little trap door sections in the T&G (and have a removable square section cut into the kingspan), or whether to just leave enough loose T&E cable to be able to pull the downlights all the way out from below?

I think that covers everything I am planning to do so far.