Roof insulation at both ceiling and rafters

gmb

Can anyone advise me whether the spec for the roof insulation on my house build makes sense before I speak to the engineer - (I've google'd through boards.ie but can't find an answer to this specific issue)

The house is a 1.5 storey and there are no plans for an attic room

The engineer's spec states:-

Attic to be insulated with 300 mm blown rockwool insulation on the flat ceiling area. Fit 1 sheet of 100 mm Kingspan Kooltherm K7 between each sloping ceiling rafter and 50 mm thick K17 Kooltherm insulated plasterboard below the rafters.
The insulation should stop short of the eaves to allow for adequate through ventilation of the attic space. The contractor to fit insulation around all tanks and piping in the attic

My question is whether it makes sense to insulate both the sloping roof and the ceiling. If the roof space is ventilated and insulated at the ceiling level surely that makes the insulation on the rafters redundant as there will be little temperature difference between the roofspace and outside.

Would it make better sense to insulate at the rafter level using the system specified, seal the roofspace and omit the ceiling insulation so that the roofspace would be kept warm warm.

Any comments would be appreciated

Mellor

gmb wrote: »

My question is whether it makes sense to insulate both the sloping roof and the ceiling. If the roof space is ventilated and insulated at the ceiling level surely that makes the insulation on the rafters redundant as there will be little temperature difference between the roofspace and outside.

You thinking here is spot on, it makes no sense to insulate both.
If the attic will not be a room, them insulating the ceiling is best

sinnerboy

agree with mellor that you understand the situation correctly , but i have a differnent approach to offer

300 quilt insulation will bury your ceiling joists . Ok , its not a room but your water tank is there - so you will have access to it and will want to store things there - and will risk crushing the quilt .

oversize your roof structure and use 225 x 50 for rafters and joists .
you will need less of them ( 600 spacings vs 400 spacings ) and this will allow a greater % of insulation in the construction , which will improve overall heat loss

to the flat ceiling - 2 x 100 layers of kingspan/xtratherm or quinntherm
you can install flooring over now - from wallplate to wallplate

to the rafters - same as above but use 50 kingspan/xtratherm or quinntherm over the rafters too . then breather membrane , counter battens and batten and tiles . your vent path is between the underside of tiles and top side of breather membrane.

install a rad , on its own circuit , to maintan 10 degrees temp in the attic .

don't use recessed lighting anywhere

get hold of a good air tightness tester now to

- advise on specification
- do a pre test visit after first fix m+e but before plastering
- test visit near completion

This will cost more . spend now on well built house or pay later with fuel costs set to at least double in 10 years

sydthebeat

+ 1 on SB....

it makes more sense to carry the same construction over and around the roof, than trying to create and junction two different constructions.... seening as the dwelling is going to be 1 1/2 storey....

gmb

Thanks for the replies, the engineer concurred with your advice and I now have a solution I'm happy with...:cool:

sinnerboy

did you check out pm i sent you re air tightness ?

muffler

Nice to see a happy ending

championc

sinnerboy wrote: »

install a rad , on its own circuit , to maintan 10 degrees temp in the attic .

Two quick questions please

1. What is the idea behind having attic at 10 degrees ?

2. My attic has Moy Fibreglass insulation. It is worth replacing it with the newer materials of today. I was considering the foil backed 4inch or so type polystyrene panels


C