Insulating bay window upstairs and ceilings

[thediyer]

Hi

This is my first post and a DIY job that is new to me so excuse me for any basic terms that I will use ( still in the learning process).After reading various posts regarding the same project I am undertaking I am well and truly confused.

My 1950's house has bay windows, we are renovating the bedroom as well changing our windows as the old pvc windows are 30 years old and in poor state. The surveyor who came out to measure the windows advised us to insulate the bays.

Considering this, I removed radiators ( help of my dad who's good at plumbing) the plaster and plasterboard to expose the timber frame with no insulation at all, i also did the same for the box above the window which exposed a wooden beam running from my house and all the way across to my neighbours with just roof felt above in the box and the under the window.

The bay window has roof felt and tiles on the outside, I have 2 methods I am considering to use when insulating and was wondering which is the best action to take.

1. Insert 50mm Kingspan / celotex insulation boards as the width from the felt to the timber is only 70mm, use foil insulation tape to seal the boards and the timber together, to create vapour barrier ,however there is gap between floorboards to downstairs window around 2ft I can push it down but this may cause condensation problems and I really can't remove the plasterboard downstairs as we have renovated the room below the downstairs bay window is sat on brickwork this section is fine doesnt not need insulating.

2. To use Knauf 44 earthwool and staple SuperFoil insulation to the to the timber, with this method I could possibly push the wool in to the gap below floorboards as well.

With the above ceiling box I propose to insert the knauf 44 wool and staple superfoil to the timber and plasterboard there will be enough space for ventilation.

I am worried about condensenation forming and ventilation. I wish to get the job spot on so need guidance. I have attached pictures Any help is appreciated dearly.

(Picture of ripped felt is from bay window it shows the tile outside, i also know about the wasps nest in the ceiling )

 

[kp00110]

I did this very similar to my 1930s curved bay window recently but 75mm PIR as the studs were 100mm. Others may know more but as I had new breathable membrane on the roof, the vapour is as not as much as an issue, where as you may trap vapour between your older felt and the PIR is your concern? I would remove the horizontal battens, just use PIR and foil tape on all the joints and across the studs. and then insulate across with insulated plasterboard so you also have insulation across the timber studs? Thats what I planned to do. And put lots of fibreglass insulation above the window bay

 

[JohnD]

You can use the mineral wool in the gaps to prevent draughts, it is easy to stuff in and is a good insulator

You have room for 60mm of mineral wool, which would be adequate, but foam has about twice the insulation power for the same thickness. But it is harder to cut to a precise fit with no gaps. If the wooden frame is irregular, it will be much easier to squeeze mineral wool in.

Insulated plasterboard on the inside will be additional benefit, as @kp00110 says

 

[martygturner]

Condensation will form only when warm moist air hits something cold. So insulate those spaces and fit a moisture control layer, or plastic sheeting as its known.

150mm Fiberglass blanket has a u value of 3.2 and ridge insulation can go as high as 8. So its a question of do you want to pull off all those plasterboard supports....if you do fit rigid and spray foam the joints, fit some plastic sheet then overboard with foam backed plasterboard.

 

[thediyer]

I did this very similar to my 1930s curved bay window recently but 75mm PIR as the studs were 100mm. Others may know more but as I had new breathable membrane on the roof, the vapour is as not as much as an issue, where as you may trap vapour between your older felt and the PIR is your concern? I would remove the horizontal battens, just use PIR and foil tape on all the joints and across the studs. and then insulate across with insulated plasterboard so you also have insulation across the timber studs? Thats what I planned to do. And put lots of fibreglass insulation above the window bay

This is exactly what i want to do, however as the insulation boards come in 50mm or 75mm i will have to use the 50mm option, i am worried about vapour being trapped between the plasterboard and the insulation board as the gap will be 20mm as there is the void is only 70mm this is why i am thinking of using mineral wool instead.

@martygturner I could also possibly stuff mineral wool in the void below the floorboard but again with the not create any vapour issues? as its hard to install any sort of moisture control layer as its the area below floorboards and above downstairs window

 

[kp00110]

I dont think it matters if vapour is trapped between plasterboard and insulation board as it is the warm side so should not condensate?

 

[thediyer]

I dont think it matters if vapour is trapped between plasterboard and insulation board as it is the warm side so should not condensate?

That makes sense, think im over worrying about the whole concept by reading far too many forums lol, The only thing that im unsure of is filling the void between under floorboards, you reckon some wool insulation should do the trick with PIR boards ontop fill the gaps with expanding foam, then seal with insultation foil tape?

 

[JohnD]

You can pack between the floorboard joists with mineral wool. No foam needed.

 

[thediyer]

You can pack between the floorboard joists with mineral wool. No foam needed.

expanding Foam would be to seal any gaps between the pir boards and timber

 

[JohnD]

If you use mineral wool it packs in so there are no gaps.

 

[noseall]

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[JohnD]

That reminds me, a good opportunity to add extra sockets, while you have the wall open.

 

[thediyer]

That reminds me, a good opportunity to add extra sockets, while you have the wall open.

True but I will have my radiator going back there and don't really require any socket there

 

[thediyer]

20160107_085741 (3)

• noseall
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Top job that mate, did you push down the insulation boards down the gap between floorboards

 

[JohnD]

True but I will have my radiator going back there and don't really require any socket there

To the sides , perhaps?