Improving loft insulation with kingspan in rafters

[adaml]

Hi,

Is it OK to stick Kingspan or similar in the rafters and between hangers to improve insulation in loft?

It's an 1890s terrace, 4 massive perlins going from gable end to party wall and the rafters aren't that deep. Hangers coming off the lower perlins to the ceiling joists. It's then got hardboard nailed onto rafters and hangers in various states of disrepair all warped and broken

Can't really add insulation under the floorboards, it's used for light storage and would be major hassle, plus it's only ceiling joists so not much depth to add insulation and then relay floorboards.

Thanks!

 

[Mechanical Mike]

I think the right way to do it is to make the rafters a bit deeper ( by adding wood) so the insulation has a gap between it and the underside of your roof tiles etc. This is airflow to remove any moisture that gets through.50mm is enough.

 

[adaml]

Ok. Attach some 4x2's on rafters to make a deeper space and make sure theres a good air gap between the kingspan and the roof?

So dont need to be changing ridge tiles for summat that allows airflow?

 

[Mechanical Mike]

No, the main thing is to create a draught free insulated space in your loft, with the tile space well ventilated as it is now. That way it drys out in the wind etc. But you don't lose as much heat out of your loft. If you don't seal it off you won't get the benefit you are after. You can use expanding foam and the wide sticky foil tape to seal gaps etc.

 

[cdbe]

Could you put celotex on the floor and board over it (probably 12mm ply over at least 100mm celotex would work well), otherwise you are heating a huge loft space before you get to the insulation.

 

[Mechanical Mike]

I think in the original post that was discounted by the poster but is probably the most conventional method. Other options, less mucky?, would be to lower ceiling in the room below and fit insulation there instead. Some of those old terraces have very tall rooms anyway.

 

[TicTac]

Ok. Attach some 4x2's on rafters to make a deeper space and make sure theres a good air gap between the kingspan and the roof?

You need a 2 inch airflow up and over the insulation, (and it's got to be coming from the sofits) so with 75mm rafters, you could use 3x2's. You haven't mentioned if there is felt under the tiles though.

Can't really add insulation under the floorboards, it's used for light storage and would be major hassle, plus it's only ceiling joists so not much depth to add insulation and then relay floorboards.

But this statement suggest you want to insulate the loft, in which case, you need to lift the floorboards, increase the depth of the joists, drop in you insulation of choice, and then relay the floorboards.

Your method will do nothing to retain heat in the bedrooms, but will cause problems in the loft, as you're not protecting the loft from excess condensation. Cdbe's suggestion of 100mm of kingspan on the loft floor, and then ply or even P5 chipboard on top of it might just be the best intermediate solution, but would still give a heat loss from the bedroom into the space under the floorboards, so not that efficient.

 

[geraldthehamster]

If your house was built in the 1890s, and if it still has its original roof, then it probably has no roofing felt. If this is so (can you see the underside of the tiles/slates?) then you don't really need an air gap between the Celotex and the tiles.

If the roof has been replaced and you have the old style impervious bituminous roofing felt, you need a 50mm air gap, as well as vents at the soffits and the ridge. In that case you can add more Celotex across the loft side of the rafters.

If you have a modern breathable felt, in theory you don't need an air gap but it's good practice to have one.

In either case, you don't need to batten it.

But as has been suggested, you would be better off insulating the loft floor. That way you're not wasting all the heat that will rise from the house into the loft, and the loft remains properly ventilated, for the avoidance of condensation and damp.