Advice on Rafter Insulation

6 Apr 2006
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United Kingdom

I want to add extra insulation in my loft. Currently I have good insulation between the joists, but I want to insulate the rafters as well. The loft is used for storage.

My house is from the 1900s. There is currently no membrane or anything behind the slates. I can see chinks of light through the slates, although the roof appears to be waterproof.

Do I need to put some kind of membrane up before I insulate?

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Although that sounds like a simple question, there are many considerations and most have already been covered in this forum.
Try using the search button facility, and look for rafter, lofts and insulation and also look in the condensation sticky, at trop of forum
You will be an expert after reading it up, but you can then come back with any specific problem.
Good luck.
Although that sounds like a simple question, there are many considerations and most have already been covered in this forum.
No it is fairly simple, assuming you are not trying to carry out a conversion and getting building control involved. Insulate between the rafters with about 90mm of either Celotex or Kingspan, if you wish to plasterboard beneath the rafters add an additional layer of around 30mm of Cel or King and then foil backed plasterboard. Depending on your rafter depths leave say a 10mm gap between the tiles and the insul. No need to worry about a membrane beneath the tiles as because you have no membrane there will be plenty of ventilation. It may be wise to ensure there are no leaks in the slates before you insulate as once the insulation's in you will not see a leak until the rafters/boards show visible signs/the leak appears elsewhere.
Thanks for that. The rafters a pretty deep so there will be loads of distance between the slates for ventilation

There appears to be lots of different types of insulation from Kingspan etc. Do I just need normal polystyrene insulation - or something that is specialised for roofing - I see that some have a felt-covered side etc.?

Also - do I need to go all the way to the top beam and the bottom (where rafter meet joists) and make sure everything is sealed, or should I leave gaps at the top and the bottom to maximise ventilation behind the boards?
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You can avoid Celotex or Kingspan but they are generally the most efficient brands hence their recommendation. You can use many others but will need thicker layers to achieve the same heat loss reduction. If I correctly get your gist, fit the insulation so that the underneath is flush with rafters and then fit the boards to the rafters.
Thanks again,

I was actually asking if the best method is to insulate all the way to the top of the roof and seal the boards right up against the top beam - or should I leave a small gap to let air enter?

Same thing at floor level - do I put the insulation boards all the way down to the eaves, or do I leave a gap?

I'm not putting plasterboard up, just insulation board between the rafters.
Install the insulation tight between the rafters, seal any gaps with expanding foam, you can tape the joints if you wish. Extend the floor insulation as far as it needs to to meet the roof insulation. For clarity there is no membrane therefore there will be plenty of ventilation immediately beneath the tiles there fore you do not need to leave any gap between the tiles and the insulation although it may be prudent to leave say a 10mm gap.

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