Best way to insulate a roof with no felt?

Question

The roof’s quite old and has no felt underneath. What do we do?

Jay · Answer

Hi 
I would advise not to get the spray foam insulation as this will eventually rot your rafters. 
Insulation boards between the rafters is the best way to, but I would always advise to get felt on your roof first , this will keep the heat in a lot more and keep the cost down 

Hope this helps

Ben · Answer

I would strip back roofing tiles and Battens 
Install 100mm Celotex in between rafters
 install roofing felt 
Put roofing battens back on (if damaged taking of install new )
Reinstate roofing tiles (if any tiles are damaged replace them )

Mihai · Answer

•	Work from the inside (loft/attic).
	•	Fit insulation between rafters (e.g. mineral wool or PIR boards).
	•	Leave a ventilation gap (usually 50 mm) directly under the tiles to prevent condensation.

Tabas · Answer

You could install a warm roof system were a membrane is designed to go over felt roof and then install roof insulation (celotex) and install the rubber roof

Rafal · Answer

If you want to insulate roof, you need to install breathable membrane. Insulation cause lot of condensation,

Thomas · Answer

Pointless trying to insulate the loft space you will loose 50% of your heat because there is no felt

Dimitri · Answer

There is 4 options that you could do to your roof  to insulate it 
1. Re-roof with felt (best long-term fix)
	•	When the time comes to replace the tiles/slates, fit a modern breathable roofing membrane (felt/underlay) beneath.
	•	This gives you proper weather protection, wind resistance, and allows safe insulation.
Best solution for energy efficiency and roof longevity.
 2. Insulate at ceiling level (cheapest & safest short-term)
	•	Lay mineral wool insulation between and over the joists in your loft floor.
	•	This keeps heat in the house while allowing airflow above, so the roof space stays ventilated.
 Does not make the loft itself warm, only the rooms below
3. Insulate the roof slope (if you want a warm loft/room)

If you want the loft space itself warm (e.g. storage, conversion, or habitable room):
	•	Fit rigid insulation boards (like PIR/Kingspan/Celotex) between rafters.
	•	Leave a 50mm ventilation gap between insulation and underside of tiles/slates, because without felt, any rain/dust must still escape.
	•	Add a vapour control layer internally to stop condensation.
This must be done carefully — poor detailing can lead to damp timbers.

⸻
4. Over-roof insulation (if re-roofing)
	•	When re-roofing, you can also fix rigid insulation boards above rafters under new battens/tiles.
	•	This keeps rafters warm and eliminates thermal bridging.
Excellent but usually only done during a full roof replacement.
We can help with this too. So reach out if you need more advice or if you are looking for a quote.

Best way to insulate a roof with no felt?

Follow Us

Cookies

Manage Cookies

Essential cookies

Analytics & Tracking

Advertising