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We live in a detached house in the South of England. We would like to install insulation between our rafters to improve the insulation of our house - so it becomes a "warm loft". We are not doing a loft conversion but would like to use the loft for storage. There is already fibreglass insulation on the floor of the loft.

Our reasons for wanting to insulate between the rafters are:

  • Improve the overall energy efficiency of the house, so that the house is warmer in winter and cooler in summer
  • The loft height is limited, so would ideally not want to raise the loft floor to install more fibreglass
  • Protect any stored items from extremes of heat or cold
  • We may choose to later move the hot water tank into the loft, so it would be good to minimise heat losses from that.

We have 100mm rafters, so to maintain a 50mm air gap, we were thinking of installing 50mm of PIR between the rafters. Then to further increase the insulation, attach 25mm battens to the rafters to create another air gap and then staple multifoil insulation to it.

Diagram of roof structure and insulation

My question is around ventilation. We plan to add vents in the soffits - which should help ventilate the gap between the PIR and the roof membrane. And maybe some vents in the roof ridge.

The superquilt multifoil insulation instructs you to ensure that all the joints are properly sealed with aluminium tape - to prevent heat escaping.

Does the main internal loft space also need ventilating? How how do you ventilate without also loosing all the heat?

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    You really, really ought to give that stuff a flame test before you consider it... otherwise you could easily have Grenfell Tower. Houses are increasingly flammable due to use of "new" building materials. Flammable building materials is a big part of why.
    – Harper - Reinstate Monica
    Aug 7, 2022 at 23:19
  • @Harper - You mean the PIR?
    – njh
    Aug 8, 2022 at 15:41
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    yes, PIR means Poly-Iso-Cyanurate. It is a type of plastic. It is derived from petroleum. It is highly exothermic. Sort of like diesel fuel, it resists casual ignition, but once it catches, good luck stopping it. The fire toxicity is pretty heinous... that's what gets people in fires these days.
    – Harper - Reinstate Monica
    Aug 9, 2022 at 23:29
  • I have updated the title of my question because the main thing I am really interesting in is "sealing" the loft with multi-foil insulation, and if the main loft void needs to be ventilated or not.
    – njh
    Aug 11, 2022 at 21:45
  • What's the purpose of the 2nd, 25mm gap? You need the 50mm gap to provide soffit to ridge ventilation, but after that, I think you'd be better served by uninterrupted insulation.
    – FreeMan
    Aug 12, 2022 at 14:22

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Assuming Harper's concern is not an issue... You probably need both soffit vents and a ridge vent. Although I don't know if ridge-vents are possible with a tile roof(?).

The main loft space ventilation requirement seems to be a function that it is just being used for storage--in which case I don't think ventilation of the main area is a must.

I would not move the water heater into this space--primarily due to plumbing leak considerations. A LOT of damage is possible if/when the tank leaks. (Usually occurs when no one is home. Murphy's H2O attic plumbing law).

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    Water heaters can be put into catch pans with a drain to contain & control leaking water.
    – FreeMan
    Aug 12, 2022 at 14:21
  • @FreeMan There's a lot not covered by a drain pan. In my case the thermal expansion tank developed a pinhole leak, spraying water out parallel to the floor.
    – Armand
    Sep 11, 2022 at 3:57
  • fair point, @Armand. Sorry to hear that!!
    – FreeMan
    Sep 12, 2022 at 12:25

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