Insulating loft advice

[Rudie]

Hi,

I have got some kingspan k5 kooltherm from a friend (free ). I know it's for external but am thinking of using it internally? I was thinking to Insulate under rafter with potentialy foil backing on the rafter side and then using foil backed plasterboard on the outside. Insulation is 1200 x 600 x 60mm. Rafters are 4" deep so wanted to keep the airflow clear.

Any advice appreciated.

Thanks.

 

[PrenticeBoyofDerry]

You would be looking for around 50mm for ventilation gap.
To achieve this you could batten out rafters.

 

[Rudie]

Hi mate thanks for the reply. I have enough headroom to insulate under the rafters so none of the 4" depth will be obstructed. I just want to know if I can get away with using this type of insulation?

 

[PrenticeBoyofDerry]

I cannot see why not.

 

[Rudie]

Thanks. Foil the rafter side and board inside with foil backed plasterboard?

 

[Rudie]

my mate said to use tyvec breathing membrane on the rafters instead of foil backed insulation layer before i put on the board insulation. what do you guys think?

 

[Rudie]

Any advice appreciated...thanks.

 

[sh4d0w]

You could always sell (me) the K5 and buy other stuff

 

[Gerrydelasel]

Foil should go on the warm side, which from your description would NOT be the rafter side, correct?

 

[freddiemercurystwin]

Hang on a minute. You can only insulate at rafter level if you have a breathable membrane under the tiles or continuous ridge ventilation. So if you have traditional felt and no ridge vents then you cannot insulate at rafter level.

Thread would have been much better placed in the floors and Loft section.

 

[Rudie]

yeah I plan to use foil backed plasterboard on the inside but thought to use foil pointing out against the rafters to stop it getting too hot in the summer.
Thanks for the offer sh4d0w but I just about managed to get them all in the loft!
Freddy - I wont be insulating to the top, there will be a gap at the ridge so the air can flow from one side of the rafters to the other side it that makes sense.
Just to make sure, im not planning to insulate between the rafters, placing the insulation boards on top (no cutting).
If needed, I may replace the ridge tiles with some vented ones.

 

[freddiemercurystwin]

Really you need ridge vents. If you don't the airflow won't magically flow up one side and down the other, the air will rise up towards the ridge where it will likely condense against the felt, eventualy saturating the rafters/insulation and potentially rotting the rafters.

 

[Rudie]

Ah Get you. So once I get the ridge vents, I wont need any soffit vents or anything else to avoid condensation?

 

[freddiemercurystwin]

No, you need ridge vents and soffit vents.

 

[freddiemercurystwin]

http://blog.celotex.co.uk/technical/pitched-roof-application/