I am about to raise the ceiling of two bedrooms in the our recently purchased cottage. The new plaster board ceiling will follow the slopping roof line roughly half way up before leveling off across the roof. I had planned to insulate behind the plaster board with kingspan or another type of sheet insulation.
Can anybody tell me what gap I need to leave between the roof lining and the insulation? The lining looks to be a form of polythene sheeting.
Can anybody also give advice on roof ventilation? The cottage has very thick stone walls and the wall reneder meets near enough flush with the bottom tiles so at present there is no soffit area. If I added a form of ventilation at eave level both sides would this be adequate?
Any other advice on this subject much appreciated.
You need to maintain 50mm clear airgap between the top of the insualtion and the underside of the sarking felt / polythene. If you are renewing the ceilings, consider underlining the rafters with 25 - 40mm insulation too, as this will vastly improve the thermal effiency and will avoid any possible cold bridging.
Structurally, if you are raising the ceiling, you may be removing a structural cross tie from the property at low level (ie the ceiling joists). You may wish to run this past a Chartered Surveyor or Engineer to make sure you are not creating a long term structural defect that will be expensive to sort. Sounds dramatic, but it can ccertainly happen.
If your are struggling for obtaining eaves ventilation, you will need to look at thru slate ventilation. Ventilation of the roof is very very important to reduce condensation, which can lead to dry rot, beetle infestation etc over the longer term.
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