Boarding out loft...

[officialsimon]

I am doing a boarding out the loft for basic storage project. The floor/ceiling below joists are 75mm deep by 50mm wide and I am putting the same on top of them to make 150mm deep by 50mm wide (rough equivalent to 6x2) fastened at each end to 8x2 purlins set into the left and right walls.

I intend to put 100mm Kingspan between these members and then board on top. My question is, should the Kingspan be at the top of the 150mm joists, ie the top of the Kingspan touching the underside of the boards I will put on top, or should it be pushed down to the bottom and rest on the bedroom ceiling boards below and then put the cabling from the upstairs rooms' lighting circuit junction box, which is in the loft loose between one pair of floor joists, over the top of the Kingspan?

If the 100mm Kingspan has to go at the top of the 150mm then that would mean putting the cabling underneath that, which I imagine is not good but it would not of course be touching the bottom of the Kingspan as it would rest on the ceiling boards in the 50mm void created.

Anybody help?

 

[mointainwalker]

When adding to joist height, the advice on here is always to add at 90 degrees for stability.

Insulation should be laid on the ceiling BUT with an intervening vapour-barrier . Save yourself a lot of cash and use fibre-wool.

 

[freddiemercurystwin]

with an intervening vapour-barrier

This is optional of course, as the hundreds of thousands of existing houses without such a barrier and no problems in the loft are testament.

 

[mointainwalker]

Hey Fred

Are you taking into account that the Kraft paper on the roll insulation is a vb ( as long as it's fitted against the ceiling ) ?

 

[freddiemercurystwin]

I'm taking into account the hundreds of thousands, no millions of houses across the UK that have mineral wool insulation with no barriers or paper or types of linings to either face of the insulation at floor level in the loft that do not have any problems with condensation.

 

[mointainwalker]

Freddy

You naughty boy, you are avoiding the question - again !

AFAIK all insulation sold in rolls for many years ( before wall-insulation became an issue and supplied in 40- 50 mm specifically for that without v-b ) had a layer of Kraft paper ( vapour-barrier ), so knowingly or unknowingly, people were fitting a v-b.

Have you looked to see if your insulation has the paper ? Would be good to know.

 

[freddiemercurystwin]

A long time ago, it doesn't!

 

[officialsimon]

Re using fibre wool - surely if you do that then the fibre wool is going to spring up and fill the whole 150 deep space formed by the chipboarding above and the ceiling below, which would, would it not, have two implications, 1) that it would be in contact with the electrical cabling even if that is laid underneath on the ceiling panels, which is not allowed is it? And 2) aren't you supposed to have a ventilation gap when boarding over insulation, hence me building the joists up to 150mm (apart from the main reason, strengthening) and then using 100mm insulation?

Am I wrong in any of that thinking?

 

[mointainwalker]

1) Fibre wool will not increase in height, so 100 mm is not going to become 150 mm or 110 mm. It will tend to decrease a little with time because of gravity.

2) Wrong. No gap.