Insulating dormer window

[Ratty80]

Hi, I would really appreciate some advice on the best way to insulate our dormer window. We have just moved and the place needs a complete refurb so thought it a good idea to insulate the loft rooms as they have no insulation at all.


I was planning on 50mm of celotex all round but the problem I have just uncovered is the dormer cheeks are only 50mm wide!


Firstly is there any insulation that will be sufficient for this, including leaving an airflow gap or do I need to batten out with 50x50 battens?


The front of the dormer is made in 75x50 stud. Will 50mm celotex be ok leaving just 25mm gap?


Should I use celotex on the roof joists which are 100mm or would rockwool be better and cheaper?


I know 50mm is no way near the 150mm required for current regs but we are keen not to loose room space as the rooms are already very small.


Any advice would be really appreciated

 

[^woody^]

Do what you can. Celotex between and whatever you can fix across the front. No air gaps. Silver tape all joints before plasterboard.

 

[Ratty80]

Thanks Woody. Is that the aluminium foil tape? By no air gaps u mean it's ok to put the celtotex right up to the felt with no ventilation behind for the cheeks of the dormer? I always thought u needed 50mm airgap?

 

[^woody^]

As long as you fill it with insulation and leave no gaps and seal the front with foil tape, then moist or warm air can't get in to condense anywhere. That's why you don't need an air gap.

 

[Ian H]

I'd do 50mm between then 50mm over the lot. The foil tape from Toolstation is good as is the Bostic stuff on e-bay that's a couple of quid cheaper.

 

[garyo]

I'd do 50mm between then 50mm over the lot. The foil tape from Toolstation is good as is the Bostic stuff on e-bay that's a couple of quid cheaper.

Me too. The challenge is how much you can build out without overlapping your window frame too much

 

[bobsuerita]

also if that is the only access to eaves void might be worth topping up the insulation in there too

 

[Adam182]

Just done this recently for a upstairs bay window, I used 60mm celotex, pushed right in leaving no air gap, I then foamed the edges and the taped over them with foil tape, I will be also going over it all with 38mm insulated plasterboard.