Jump to content

Another Dormer question

Night Owl

Recommended Posts

Interested to know how forum members would set about insulating the dormer of my 3 bed late 1950's bungalow?  Timber construction as in the picture below with a mineral felt roof covering.  I've only got approx. 100mm deep joists on the roof part and 50mm on the cheeks.  The rest of the room sits in the roof space (not insulated at present but have probably sheeps wool installed between joists) and is of old 'concrete' block construction.


My thoughts were to use sheeps wool throughout but not sure if vapour barriers / air gaps are needed?  Also planning to add batons to 'external' walls of dormer room and add sheeps wool there.  I like sheeps wool due to it's natural properties and breathability.


How would you set about this job?





Link to comment
Share on other sites

I agree that insulating externally would be ideal. At the least you should be able add warm roof external insulation above the flat roof. If insulating internally / between the timbers, add additional timbers to the existing external insulation to make space for thicker insulation.


I don't have time to reply on air barrier & vapour control - plenty of threads on here discuss them anyway - but you do need an air barrier, don't need a vapour barrier, and may want a vapour control membrane.


I wouldn't use wool insulation; if you want a natural product there's wood fibre insulation board, or preferably hemp (which has better vapour control properties) - for example https://hemspan.com/product/bio-wall-natural-fibre-hemp-insulation/.

Link to comment
Share on other sites

I like wood-fibre a lot, and have used it a lot, so it gets my vote. Unless you use the flexi stuff it is heavy, though - 140 - 180kg/m3. You could use an (expensive!) 'Intelligent' VCL like Pro Clima's 'Intello'. Used a lot of that, too. I am not plugging for Pro Clima. I imagine other manuf's make intelligent membranes but I do not know any - yet. Yes - did a quick search. There appear to be others. Search 'intelligent VCL'.

Link to comment
Share on other sites

Thanks for the replies.  I hadn't considered external insulation but agree it would be the best solution.  Unfortunately it will probably be a bit outside my budget and bryond my skillset to make a decent job of it.


Will definately be looking to make use of the pro clima intello membrane.  Hadn't heard of it before but sounds like a very good idea.  One of the reasons sheep wool interested me apart from being a natural product was it's fire retardent properties.  Hemp sounds a great material but a bit lacking in this department.  There are now companies claiming to sell sheep wool that doesn't have an issue with moth infestation.  See link -




No idea if this 'ionic' system works to solve the problem with moths.


Would fibre sounds interesting too, gain I wasn't aware of the product.  So many products and ways to insulate a property these days makes it really confusing.  Utimately, I guess any insulation I install will be better than none as is the current situation - providing I don't introduce a condensation / damp problem.  That is the key.  Looks like I've got more research to do but appreciate listening to what others have found to work for them.

Link to comment
Share on other sites

  • 1 month later...

Hello again.  Have now had the window replaced on the Dormer so ready to start insulating in that area.  Unfortunately trickle vents were added in a location that reduces the thickness of ceiling I can have so made the decision to remove them (Left one already done).  Will seal them up with foam or silicon.  I intend to install whole house ventilation later so hopefully won't be an issue.




Still confused as to the best method for insulating the dormer roof and cheeks.  Currently the roof has two layers of green mineral felt torched onto the wooden boards.  The side cheeks have a covering of lead onto the wooden boards.  As mentioned in the original post, I have 100mm in the roof joists and 50mm in the cheek batons.  The maximum I could probably goto now on the ceiling is 170mm including thickness of plasterboard & skim.  The maximum on the cheeks is 80mm including plasterboard & skim.


Still keen to use sheep wool if I can and would look at putting 150mm in the ceiling (leaving a 20mm air gap) and 50-60mm in the cheeks leaving a 20-30mm air gap.  Although they say wool doesn't need a vapour barrier I would look to install an Intello membrane over it to help reduce moisture reacing the timber.


I've also considered using phenolic insulating boards as it is supposed to have the best insulating properties where space is limited.


My questions are -


Do I really need to be leaving an air gap with either method if I install the Intello membrane?


Could a combination of both materials be used i.e wool between joists/batons, the cover with phenolic plasterboard?


Cost not really an issue for this part of my refurb. 

Link to comment
Share on other sites

I'd be going rigid board as it gives more insulation for a given thickness. Another thought is the foil type insulation such as Actis. Personally, I'd send your picture which illustrates exactly the situation, to the company's technical department and let them suggest a solution.

  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...