Insulation in the Loft - Rafter and Floor?

[tfish]

Hello, I am going to get round to getting planning permission to do a loft conversion (bedroom).. In the mean time I want to start insulation and boarding myself.

I know the recommended depth of Rockwool for the floor is 270mm, My Joists are only 250mm. I am also going to insulating inbetween the rafters (Celotex seems the best solution?), how does doing this effect your floor requirements? do you still need 270mm on the floor or is this reduced?

Also what thickness of Celoxtex do i need inbetween rafters and do i have to put large boards horizontally sitting on the rafters before i board/

Thanks

 

[tfish]

anyone?

 

[Chukka63]

70mm between the rafters and 50mm underneath will be ok.you need a 50mm airgap between the insulation and roofing felt/tiles.the insulation between the joists is for sound reduction so 250mm is fine.

 

[tfish]

cheers.

so underneather the rafters am i right in thinking lay the poly sheets across and fasten with nails, then board direct onto them using longer than usual screws?

 

[mointainwalker]

IMO you are galloping all over the place without any forethought.

1) When do you hope to start your loft-conversion ? If it is very soon, there is little point in putting 250 mm insulation in the joists as it will have to come out and is too thick to use elsewhere.

2) If you convert the loft, you will want heat from underneath so joist insulation will be a hindrance.

3) If loft conversion is some way off, it is pointless to insulate the rafters now ( unless you have some unique reason such as insulation that will otherwise be thrown away ).

It is pointless because exactly how much heat are you hoping to save after putting 250 mm on the joists ?

The amount lost ( from rooms to loft ) should be so small that it will be lost in the frigid loft.

 

[JohnD]

...I know the recommended depth of Rockwool for the floor is 270mm, My Joists are only 250mm...

The incremetal improvement from 250mm to 270mm is insignificant.

However 250mm is an enormous size, are you sure you have it right?

Will you be boarding the loft? What do you intend to use it for? How will you be ventilating it to prevent condensation?

 

[tfish]

the joists might be 200mm not 250.

it will be used as a movie/games room with prospect of a bedroom in years to come.

i wont be removing the joist insulation when i board i think the sound reduction beneifts outway the blockage of heat esp as every room will be have good radiators.

There are two dormer windows in the floor below with the roof slope joining the act main loft, theres vents in the bottom of the dormer rafters i was guessing this will be enuff ventilation esp if i leave a 50mm gap between celotex and felt.

is this ok?

 

[geraldthehamster]

the joists might be 200mm not 250.

it will be used as a movie/games room with prospect of a bedroom in years to come.

i wont be removing the joist insulation when i board i think the sound reduction beneifts outway the blockage of heat esp as every room will be have good radiators.

There are two dormer windows in the floor below with the roof slope joining the act main loft, theres vents in the bottom of the dormer rafters i was guessing this will be enuff ventilation esp if i leave a 50mm gap between celotex and felt.

is this ok?

If you want to turn this into a habitable room, you'll need to submit a building notice to your council's Building Control department. You can ask all these questions of your Building Control officer. It's very difficult to judge what might be appropriate, here, without seeing the details - for example, there's no way to know if your existing floor/ceiling joists will pass muster, without knowing both the size and the span. You're unlikely to need planning permission (as opposed to Building Control).

Cheers
Richard