Insulating an unfelted roof space

[IM]

We recently moved into a 1930s house, where the previous owner has wrapped the entire area of the loft in plastic sheeting.

Apart from being rather odd, I'm worried about condensation.

I had thought it was a DIY attempt at insulating/weather-proofing in response to missing roof tiles.

However, having had a roofer in to check the general condition, he thinks that it was put up to stop stored items in the loft being covered in dust from the lime mortar securing the tiles (the roof is un-felted).

I want to remove the plastic sheeting, and insulate the loft (it is currently 'boarded' with old doors etc. and there doesn't appear to be much underneath), but still have it useable for storage.

I've not measured, but I'm assuming the joists won't be deep enough for me to lay rolled insulation to a sufficient depth/U-value and still lay boards over the top.

Seeing as I'm going to have to put some kind of barrier (something like a breathable membrane) up to prevent the mortar covering everything in dust, should I add additional insulation between the rafters?

If so, what is the best option? I'm presuming I need to take care to keep a gap between the insulation material and the loose mortar.

 

[IM]

Anyone?

 

[freddiemercurystwin]

Do you want the stuff in your loft to be kept warm? ie do you wish to insulate at floor level or roof level? The standard practice is to lay one layer of insulation between your existing joists, then counter batten another set of battens perpendicular to your ceiling joists on the top of your ceiling joists and lay another layer of insulation between those, thereby ending up with 2 layers of crosslaid insulation. Then board over the top of that to avoid compressing it too much.

 

[IM]

Thanks for the reply.

There's nothing going to be up there that needs to stay warm - the usual Christmas decorations, suitcases etc.

I guess with this double layering approach I'd lose head-height, but for all the time I'm up there it doesn't really matter.

I'd still need to add a barrier to stop the dust from the mortar - so would you suggest just using a sheet of membrane and not bothering with additional insulation (assuming I add it at joist level)?