insulating above / below roof boards?

[SamH]

I would be grateful for any advice / feedback regarding roof insulation. The top floor of my C18 house is currently divided into three rooms. I have LBC to convert to one master room with en-suite. The issue is insulation. There currently isn't any. We have exposed beams/purlins supporting wooden boards(laid side by side like floorboards), what seems to be felt over the boards ( I can see this through *****s in the boards) and finally slate tiles on top. My preferred builder wants to remove the tiles and insulate between the boards and tiles rather than on the underside of the boards betwen the pulins - he is the only builder out of five who has suggesterd this approach. I'm keen to avoid taking the roof off mainly because it blows the budget but also because the roof is (by which I mean the visible slates) is in good condition. Local building control won't come out to look until we make a formal application however have stated that as long as we insulate in accordance with manufacturers guidlines they're cool with whatever method we propose.

Regards, Sam H

 

[middi]

Insulating the exterior as " warm roof" like one of your builders has proposed is pretty straight forward as you will end up with an exellent insulation " blanket" and very efficient. It would mean removing tiles and existing battens, install new sealed insulation panel, new counterbattens breather felt above this plus new tile battens .The old tiles will be reused. The roof line will be rised by the thickness of the insulation plus the tickness of the counterbattens. Gable ends and eaves will also have to be attented to by installing new wider boards or whatever is required in order not to deviate too much from the existing detail. this arrangement will leave the interior of the roof undisturbed.

Alternatively you can insulate the inside of the roof, but it would mean loosing some of the details you like and possibly slightly lower head room.

Ask the builders for both cost option then make your decision based on the depth of your pocket.

By having an exterior" warm" roof you can get away with slightly less insulation . Done properly both options will give you what you want.

 

[SamH]

Thanks for your reply. To strengthen his argument for taking the roof off, our builder told us that ,in his opinion, insulating between the purlins and the subsequent lack of ventilation owing to this method will lead to moisture being trapped against the boards and felt above, swiftly causing damp problems and leading to rot. Not quite sure where this moisture is supposed to originate from or whether this is guaranteed to happen.

Also, will the conservation officer need to get involved? We already have lbc to convert the room and insulate it.

Regards - Sam H

 

[Nemesis]

Yes the Conservation Officer will need to be involved if you are proposing taking the roof off.

If you have LBC, then you only have LBC for those works, not anything else.

 

[chuckey]

One problem that your builder has outlined is the condensation that forms under the existing felt and drips down on to the back of the sarking boards. This is because as the air is warm and moist inside the house (from breathing, cooking etc.) it will percolate through gaps in the sarking boards ( your spy hole) and will cool down when it touches the felt where the moisture content will turn into water.
If you can afford the headroom loss, fill the depth of the joists with foam insulation then fit a plastic vapour membrane then re-board the ceiling with some high grade T & G (not big shed stuff).
You don't say when the roof was last worked on ( 1960 - 1990?), but the rot in the sarking boards has started, how long they will look like they do at present is unknown.
Frank

 

[chuckey]

One problem that your builder has outlined is the condensation that forms under the existing felt and drips down on to the back of the sarking boards. This is because as the air is warm and moist inside the house (from breathing, cooking etc.) it will percolate through gaps in the sarking boards ( your spy hole) and will cool down when it touches the felt where the moisture content will turn into water.
If you can afford the headroom loss, fill the depth of the joists with foam insulation then fit a plastic vapour membrane then re-board the ceiling with some high grade T & G (not big shed stuff).
You don't say when the roof was last worked on ( 1960 - 1990?), but the rot in the sarking boards has started, how long they will look like they do at present is unknown.
Frank

 

[jason.p]

It's worth mentioning, regarding cost implication, that however carefully the slates are removed there is bound to be a loss due to breakage, possibly up to 20%, especially if it has been re-roofed relatively recently, and the nails are still in good condition. It would be worth anticipating this and trying to find a source of matching replacements before starting.

 

[SamH]

Thanks for the above posts - all clear and concise advice. I have so far been in touch with the v helpful Thermafleece technical team who have asked me to email a pic of current roof so they can advise on whether their produce will be suitable for insulating between purlins. Anyone have any experience of using thermafleece?

Sam H

 

[Lime]

I'm with the builder who suggests removing the tiles.
Anything else is going to compromise the look of the ceiling and create further problems of condensation.
I would not use Thermafleece in this situation.