Loft Insulation and Boarding

[AkiraDIY]

The previous owner of my flat rewired the place and the budget spark they used pulled up all the insulation and piled it in a corner. I had a surveyor out to get it insulated under GBIS but he says that since it was insulated properly before, they cannot do it under the scheme despite it being ruined by the spark. So I'm looking at DIY-ing it and boarding some (roughly 50%) of the floor-space for storage while I'm at it. I'd like to run my plan by some more experienced DIY-ers/tradespeople on here before I pull the trigger.

I live in an old 1900s tenement, my ceiling joists are 2x6 beams. I'm planning to put down around 300mm insulation (Knauf glass mineral wool or whatever is most cost effective when I come to buy it) and use 8" loft legs/stilts (considering these at the moment). That would leave a little over 2" of air gap between the insulation and boards. Some of the joists are twisted but a bit of shimming should sort that. It's just for storage so doesn't matter if the boards aren't perfectly flat.

For reasons unknown to me, the joists have been drilled and the wiring run through (completely unsupported/unclipped), instead of going over the top. All lighting and plug sockets, no down lights, no electric shower but my oven says it's 4kW (I think that's ok but best to check). The surveyor told me insulating over the top of wiring isn't a problem. But to be safe, the 150mm (~6") insulation that goes between the joists, I plan to leave a small air gap around the wiring. The 2nd layer of 150mm insulation I'll lay perpendicular to the joists and cut slits to slide over the loft legs.

I know I need to leave an air gap at the roof eaves for ventilation and my bathroom has a false ceiling that looks sealed. There's also mechanical ventilation through the bathroom window and we always leave the door open fan on for a little while after a shower.

Have I missed anything or does that sound like a decent plan?

 

[muh3]

Hi, the wires should not be covered with the insulation, you could put them in conduit, you can probably buy the one you can twiddle around the cable. Second issue you have is that the 2x6 is probably only to support the ceiling. What is the biggest span for the 2x6 you have?

 

[AkiraDIY]

thanks for the reply. From what I read, shower cables and downlights are the main concern but I'll pick up some corrugated conduit when I get the insulation too. Doesn't hurt to be safe and won't take that long to fit. Just a side thought though, if the cables are supposed to be clipped/secured to the joists and the regs require 270mm insulation minimum, aren't the cables always going to end up under insulation if going through the loft?

The biggest 2x6 span in my flat is around 4m but the area I'm planning on boarding out is around the unused cold water tank for the building (500-800L at a guess). I assume the joists around there are going to be the strongest. The majority of the span's in that area are around 2m as there is a wall in the centre but I would need to double check.

My roof is similar to the picture, with a collar tie between the purlins and minus the centre column (water tank is left of centre and the joists are parallel to the purlins). I would be planning to board ~50% of the area between the purlins only.

 

[muh3]

Hi, I'm not an electrician but I assume if cables are run through a loft with insulation they will be run in conduit. You can alternatively run bigger cable or downgrade the breaker.

Assuming you joists are about 400 centers you should be fine with a span of 2-3m but I would avoid the 4m span.
Also I'm not a fan of the legs I would rather counter batten with something like 38mm x 195mm but this will add quite a bit of weight and I know this command is something nobody likes on a forum but you should consult a structural engineer.
You also need to bear in mind that you need an air gap above the insulation, you should not board straight over.
A 2 inch gap is normally sufficient. So in your case if you stick with legs you need 8inch legs and not just 6inch.
You can get downlights which can be run in insulation, might worth replacing otherwise you can buy or build huts to keep the insulation off.

I hope that helps.

 

[AkiraDIY]

that's perfect thanks. I forgot to check the centers last time I was up but 400 sounds about right.

Access to the roof is via a skylight in the loft and annoyingly, the loft hatch is in the stairwell facing the front of the building and the skylight is at the back of the building. So contractors need to walk around the stairwell in the loft, over my flat. The walkway is just planks of wood nailed between two joists. Need to figure out how to insulate that and keep the walkway as I asssume i cannot just block off access or risk contractors messing up the insulation again.

There's been many a portly builder up there and there doesn't seem to be any cracks forming, apart from the usual age related cornicing ones. The 4m spans don't feel bouncy either when I walk on them, so the 2-3m spans should be good enough for storage. Heavy stuff can go inside the old cold water tank, light stuff can go on the boarded out area.

If I was attempting to make the space habitable, then I'd be getting a structural engineer but I'm unlikely to get planning permission for that.

Thanks again for the help, got enough confidence to get this sorted I think. I'll add it to the ever growing todo list!

 

[muh3]

You could just insulate under the walkway and next to it, alternative would be to lift the boards put some extra timbers on top of the existing joist, sister that with some off cuts from you flooring or just some ply and put back.

I agree I think you be fine. post some picture of your project and let us know how you get on.

 

[AkiraDIY]

I need to scope out the walkway properly, see how fixed the planks are or do they come off easy. I think the mains electricity enters my flat around the walkway area so might be best to just leave that half meter walkway alone.

I'll get some pictures when I get round to doing it, need to clean the loft out first. Get rid of the old crusty insulation and rubbish left behind by various lazy builders over the years.

 

[AkiraDIY]

You could just insulate under the walkway and next to it, alternative would be to lift the boards put some extra timbers on top of the existing joist, sister that with some off cuts from you flooring or just some ply and put back.

I agree I think you be fine. post some picture of your project and let us know how you get on.

I've been thinking of using timber instead of loft legs to do the walkway for better stability since I don't think contractors will be particularly light footed when walking on it. I was reading some posts about using timber and people say weight is a possible concern. Got me wondering why though?

My ceiling joists are 6x2s, which are the same as floor joists no? If floor joists can take the weight of people, children running around, furniture, etc; some additional timber and boards shouldn't be that much of a problem no?