DIY Doctor UK DIY Forums

Help, I'm confused - still struggling with an issue and getting conflicting advice:

I want to put thermal bubble wrap at the rafter level of the loft. At present it is a cold loft (with joist level insulation) but after severe flood damage, I want to make it a warmer loft to prevent pipe freeze.

I have boards covering the joist insulation over most of the loft and have been thinking to bring the bubble insulation on the rafters right down to pin it to the loft boards (adding new boards where necessary).

The intention is during colder spells to leave the loft door open and kind of heat the space- making it a warm loft.

However I have been told by friends that I must leave a gap for ventilation to avoid condensation between the bubble wrap and the joist insulation. If I do leave a gap I'll have trouble keeping it warm. (the usual 50mm gap between sarking and bubble wrap would be there of course)

If I don't insulate with bubble wrap and leave the loft door open, won't I get condensation anyway?

Condensation occurs when warm moist air hits cold air, i.e, above the insulation line.

Therefore the aluminium bubble wrap would need to achieve a thermal insulation value similar to the glassfibre insulation between the ceiling joists.

Effectively you are rendering the glass fibre as obsolete and making the thermal envelope at rafter level. If you do this then it should not matter whether you leave the loft hatch open.

However, you need to ensure that the insulation at rafter level is of a decent standard, if not then you will still get condensation in the roof space because it will be colder.

I hope that this makes sense, but it is not easy to explain.

Good luck,

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