Insulating bay window roof

[Feltwell]

My flat-roofed bay window will be getting some work in the next couple of months, as although the roof doesn't leak it slopes the wrong way in places and drips water over it's front edge, rotting the window frame below and making the dwarf wall at the base damp. So, the nasty felt will be coming off, with a plan to replace it with lovely lead, as it most probably was originally.

The roof decking will probably want replacing as I sort out the slope of the roof, so it's an ideal time to add some insulation. All work will have to be done from above - the ceiling won't be touched.

The question is how to do this and avoid condensation problems. The options appear to be:-

- Cold deck roof. Put Kingspan/Celotex or similar between the roof joists, leaving a 2" gap above that (below the roof deck), and try to ventilate that gap as best as I can to the outside without using nasty modern vents. Problem is I can't put a continuous vapour barrier underneath the insulation as the ceiling is not being disturbed. I would have also thought you'd get some cold bridging with the joists.

- Warm deck roof. Put a new deck of ply on, lay Kingspan/Celotex on top of that with a vapour barrier underneath. Problem is, according to all the info I've found normally the felt then goes directly on the insulation. I'll need to have another deck of ply to lay the new lead on, and join that to the bottom deck somehow without creating a cold bridge. Will increase the roof height and so will necessitate a larger wrap of lead over the front edge, not necessarily a great problem, but it will severely limit the thickness of insulation I can use - although apparently I can put about a third between the joists, under the bottom deck, so long as two-thirds goes above.

http://blog.kingspaninsulation.co.uk/ventilation-for-flat-roofs/

Warm roof seems the way to go. Any thoughts or tips anyone?

Edit - this is quite informative https://www.youtube.com/watch?v=HoLHb7ptGfI

 

[malcolm]

I've got one of these to figure out too. My thought is a bay window roof is a tiny percentage of the overall wall and roof area and insulation for the sake of avoiding heat loss could well be environmentally unfriendly. Either you throw away 80% of a sheet of celotex, or you have a lorry travel half the way across the country with environmentally friendly materials. Left over celotex is probably OK.

You get the warm air inside that holds lots of the water that your wife breathes out while she is complaining at you, but you have cold air outside in the winter can't hold the water so it condenses within the fabric of the building. Positioning the dew point where the water condenses is important - don't let it be inside your wood. I prefer internal insulation, then a ventilated air gap, then the covering.

External insulation (warm roof) is good in principal. The vapour will condense in the insulation not in the wood frame. It works well in breathable buildings where the condensation can evaporate. I am not convinced it will work under lead as I can't see where additional water will go the next time your wife complains at you apart from drip onto your timber. Others might have better ideas.

What are you doing for the drainage on top of the lead? The bay I am worried about currently falls towards the house. Seems a reasonable system with lead coverage but wants a system for the water to drain.

 

[plasticpigeon]

Even I'm not that fussy! I would bung some insulation between the joists, re board and put the lead on. Where would there be any condensation? I wouldn't think there will be much on the inside of the lead, as the air around it (inside and between lead and wood) would all be cold, and the ceiling might be lime but I can't see much water vapour escaping from the room to the space between ceiling and leadwork! And if you did get a bit of condensation, it will evaporate off again!

 

[Feltwell]

Malcolm, the lead contractor I'm using has advised that warm roof and lead is a no-no. He could put in a system to provide vented cold roof but for a bay he reckons not worth it - just put some insulation in - he's a stickler otherwise, very thorough, so I'll go with his advice. The bay will drain into a trough along the join with the house wall - not an ideal design but it's as it was originally and I don't really want guttering round the front edge of the bay. The trough will be lead lined in rather hefty lead - code 8 ! - and drains into a hopper at one end.

Anyone used loose fill Vermiculite insulation? I'm thinking that will be the easiest to put in, best to get in the inevitable nooks & crannies, and being loose fill there must be some element of breathability. Plus for some long forgotten reason I've got 2 big sacks of it underneath the garage that my Dad gave me ages ago!

The only downside I can see online is lots of warnings about asbestos contamination from an American mine, but that closed in 1990, and the bags I've got aren't that old.

 

[malcolm]

Sounds like you have a sensible lead worker. I insulated my upstairs bay last week with 4 inches of loft insulation on top of the bay ceiling. The roof is constructed on top of small rafters sitting on a batten at one end to give it a slope then boards on top of that. The gaps between the rafters allows ventilation to the outside.

I am considering a properly drained rear sloping bay downstairs. I think mine became rear sloping due to overloading of timbers but it works OK even with a big puddle sitting on top of it. Properly installed lead is very effective for a very long time.

 

[Feltwell]

Properly installed lead is very effective for a very long time.

Yep - it's the properly installed bit that's key. I've really seen the difference in quotes between 2 different lead workers - the chap I'm going with has made some very valid points about how the lead should be installed on the porch I'm also having done, points that I wouldn't have thought of - the "obvious" way of doing it would end up with premature failure. The chap I didn't go with made none of these points and would have just done the work in the obvious way.

I'm hoping that by the time this need doing again, it will be someone else's problem as I'll be worm food!