Attic loft floor insulation U value

[Camelion69]

I recently changed the floor joists in my new attic from 3 inch to 8 (200mm) and would like to insulate it to current regs standard for insulation. If i search online it states the minimum requirement is 750mm of insulation, which i assume to be earthwool.

As i cannot increase the height of the floor joists to meet this, is there any alternative e.g. PIR board or foam that can be used to provide the same or greater U value, but still fits into the 150mm to 200mm space i have to work with ?

Your advise is much appreciated

Gee

 

[robinbanks]

Where did you get 750mm from? The typical recommendation is 270mm minimum for wool types, 150mm for PIR/PUR board

In your case as you don't quite have 11 inches depth for 270, perhaps consider a hybrid of board and wool. The wool will be easier to fit in and around uneven things leaving it possible to fill the rest of the void with long straight cuts of snugly fitted board, say 100mm of wool and 100mm of board, notionally equivalent to about 300mm of wool

Another advantage of mixing board and wool is that wool is good at expanding to fill gaps that draughts around uneven objects could get down if it were just board alone, and board can help keep draughts off wool. The effectiveness of board is diminished if you accidentally create a void between board and ceiling plasterboards done which cold world air can flow. Wool works great to gap fill and provide a deformable surface for board to mate with

 

[Camelion69]

Apologies for the typo on the measurements Robin, and this sounds like logical advice for me to take onboard and Thank you!

 

[Dereekoo]

What do you mean insulate your attic, putting insulation on the attic floor insulates the room below it, unless you mean you are trying to keep your attic as cold as possible

 

[Camelion69]

Hey Derek, the job at hand is to insulate the attic space as a whole e.g the floor which is what i will do first and followed by the roof space as step 2.

 

[Dereekoo]

Hey Derek, the job at hand is to insulate the attic space as a whole e.g the floor which is what i will do first and followed by the roof space as step 2.

Interesting , what have you got under your attic a garage or heated room

 

[Camelion69]

It's a heated room (bungalow ground floor)

 

[robinbanks]

Hey Derek, the job at hand is to insulate the attic space as a whole e.g the floor which is what i will do first and followed by the roof space as step 2.

Not much point doing step 1 if you're gonna do step 2?

 

[Camelion69]

Are you saying the roof insulated properly would be more than sufficient and heat all the loft space and no need to worry on heat transfer from the room below?

Thanks G

 

[Dereekoo]

Are you saying the roof insulated properly would be more than sufficient and heat all the loft space and no need to worry on heat transfer from the room below?

Thanks G

The insulation does not heat the room it stops the heat in the room migrating out to cold surfaces on the other side so if the room below your attic is warm the heat will not migrate downwards, if anything the warmth from the room below migrate upwards and helps heats the attic hence no insulation between the two rooms is required

 

[Camelion69]

I have to say it's confusing because if you look at the regs for loft floors it indicates a minimum 270mm depth is needed for say earthwool or 150mm for PIR board. Why do the regs look for such thicknesses if it's not a physical requirement in reality ?

 

[Dereekoo]

I have to say it's confusing because if you look at the regs for loft floors it indicates a minimum 270mm depth is needed for say earthwool or 150mm for PIR board. Why do the regs look for such thicknesses if it's not a physical requirement in reality ?

If you just have a open loft/attic then the floor (or really the ceiling insulation of the room below) should have 270mm of loft insulation material in new builds (or equivalent). If you are converting your loft/attic into an habitable space then the roof requires to be insulated with the appropriate materials to achieve the prescribed U vales of the building regs and floor insulation for heat retention is irrelevant

 

[Camelion69]

Ok thanks and clear now. For the floor area, I'll go forth with 150mm PIR board. Some earth wool making sure a slither of gap is available for air flow, with 22mm caber board flooring ontop.

 

[robinbanks]

but you just said that you're going to convert your loft to a habitable room one day..

So if you're going to spend money buying and installing insulation, fit it to the ceiling of the loft not the floor of the loft. This way you only buy it once and you still end up with a continuous warm envelope for your living space

Builders do cold roof because it is easy, quick and cheap. Warm roof is slower, harder to get right and more expensive. In both scenarios the roof insulation joins up with the wall insulation to give a continuous insulated layer for the inhabited space