Insulation in roof rafters - brand and thickness?

[danmanchester]

http://www.toolstation.com/shop/Celotex+Insulation+Board+/p10635

Evening all,

Need to insulate my loft, already boarded out between joists so needs to be in the rafters. Just looking for recommendations on brands and thicknesses? Currently no insulation in loft (shocking I know).

Thanks in advance

 

[freddiemercurystwin]

Do you have felt under the tiles?

If you wish to insulate at rafter level unless you have either a breathable membrane of no felt at all beneath the tiles you will need to fit continuous ridge vents and ensure your eaves ventilation allows airflow up the gap between the insulation and the underside of the felt which needs to be 50mm by the way.

If you don't ensure the timbers are ventilated, condensation will occur and eventually the timbers will rot

 

[mointainwalker]

This is a terribly inefficient thing to do. You really would be much better off lifting the boards and insulating between joists.

1) This will cost you hundreds of pounds less ( probably , depending on size ) in insulation and other materials as detailed by FMT
2) Shouldn't be too difficult as slight damage to boards not terribly important.

I guess that the heating-cost differential between insulating your rafters and insulating the ceiling will be GBP 100 -200 per year because it's the same as increasing your house size by about 40% and having to heat that.

 

[danmanchester]

Do you have felt under the tiles?

If you wish to insulate at rafter level unless you have either a breathable membrane of no felt at all beneath the tiles you will need to fit continuous ridge vents and ensure your eaves ventilation allows airflow up the gap between the insulation and the underside of the felt which needs to be 50mm by the way.

If you don't ensure the timbers are ventilated, condensation will occur and eventually the timbers will rot

On closer inspection there is a thin layer on insulation currently in the rafters with an air gap then felt before the tiles. I assume I can just add more in the rafters then?

 

[danmanchester]

This is a terribly inefficient thing to do. You really would be much better off lifting the boards and insulating between joists.

1) This will cost you hundreds of pounds less ( probably , depending on size ) in insulation and other materials as detailed by FMT
2) Shouldn't be too difficult as slight damage to boards not terribly important.

I guess that the heating-cost differential between insulating your rafters and insulating the ceiling will be GBP 100 -200 per year because it's the same as increasing your house size by about 40% and having to heat that.

Is this really true? I thought that compressing insulation under the boards would make it less efficient? I'm happy to insulated both in an attempt to keep the heat inside my house!

 

[freddiemercurystwin]

Do you have felt under the tiles?

If you wish to insulate at rafter level unless you have either a breathable membrane of no felt at all beneath the tiles you will need to fit continuous ridge vents and ensure your eaves ventilation allows airflow up the gap between the insulation and the underside of the felt which needs to be 50mm by the way.

If you don't ensure the timbers are ventilated, condensation will occur and eventually the timbers will rot

On closer inspection there is a thin layer on insulation currently in the rafters with an air gap then felt before the tiles. I assume I can just add more in the rafters then?

If the ventilation is as I have intimated it ought to be then go ahead otherwise my post still stands doesn't it?

 

[danmanchester]

Do you have felt under the tiles?

If you wish to insulate at rafter level unless you have either a breathable membrane of no felt at all beneath the tiles you will need to fit continuous ridge vents and ensure your eaves ventilation allows airflow up the gap between the insulation and the underside of the felt which needs to be 50mm by the way.

If you don't ensure the timbers are ventilated, condensation will occur and eventually the timbers will rot

On closer inspection there is a thin layer on insulation currently in the rafters with an air gap then felt before the tiles. I assume I can just add more in the rafters then?

If the ventilation is as I have intimated it ought to be then go ahead otherwise my post still stands doesn't it?

Yes I'd so say. As per my original post is there recommended brand and thickness?

 

[freddiemercurystwin]

Typically you would use either Celotex or Kingspan, around 90mm between the rafters (rafter depth permitting) and 30mm beneath.

You could use polystyrene which is cheaper but it needs to be thicker.

Course you may have a rethink when you see the price of Celotex etc.

(ebay can be a good source for job lots or factory seconds etc)

 

[mointainwalker]

You say

Currently no insulation in loft (shocking I know

which means the space between the joists - 100 -200 mm is empty . If you simply fill this with the corresponding depth of fibre-wool, why do think you have to compress it ?