How do I insulate under a flat roof?

[Dave Rove]

I've got a "chalet-style" semi-detached house with ~30% flat-roof:

How do I insulate under the flat roof?

From inside the loft, the space under the flat-roof can be accessed. The

5"x2" joists continue under the flat roof and a sheet of polystyrene foam runs across their top. Strips of 2"x1" wood are on top of the foam, aligned with the joists, to space the wooden panelling of the flat-roof an inch above that:

I broke the foam back a few inches and poked it out above the joist, and sure enough, the flat roof appears to be held up by the foam. I don't know if this is a common form of construction:

This leaves 5" of space between the ceiling plasterboard and the foam that I could fill with some insulation. A web search suggests that foam- blocks are commonly used to insulate under flat-roofs, but those would be difficult to retrofit because the rafters block an inch and a half off the width of the access to the space between the joists (see pic 2 above).

Or I could use 4" thick mineral-wool if I could figure out how to poke it under there. Or some other material? Any suggestions? Do I need to leave a gap above the insulation material to allow moisture to escape?

[Tim Watts]

On Tuesday 21 January 2014 16:17 Dave Rove wrote in uk.d-i-y:

I've dealt with 6 flat roofs on my property - 4" rafters on 4 "bay" type outcrops and 2 dormers with 5" rafters.

The 4 bays I did with the ceiling off.

The 2 dormer flat roofs I had done from the outside when it was re- roofed as the decking needed replacing too.

In both cases, I had to ensure there was a 2" or better air gap over the top of the celotex and vents under the gutterboards to allow it to breath. Failure to vent means a real risk of condensation and rot.

I'm really not sure the best way to do yours without access from the ceiling or the top - but if I were to stuff some sort of wool in, I would be inclined to use sheeps wool. It can absorb and thus buffer moisture so it may mitigate the risk to a degree. But I'm not really able to say it would work for sure.

However, if you decide to stuff something in there, sheeps wool would probably be the lowest risk option - especially if you could monitor it, perhaps next winter and see if timbers are getting any condensation build up.

Have you got any vents on the outside?

[Bob Minchin]

My roof is similar with 4 flat roof dormers. I used celotex boards cut to fit as tightly as possible between the joists. The foil coating will help provide the essential vapour barrier on the warm side of the insulation. with 5" joists, 100mm of celotex will leave you a ventilation gap. If you are taking the rest of the loft insulation up to the 250/300mm that is becoming recommended, make sure there is a gap through to the ventilation gap.

I solved the problem of the rafters getting in the way by each space between joists being filled by two narrower pieces side by side.

Say the space between joists is 14". Cut a 7" wide strip and fit in the gap on the side where the rafter joins the joist. then slide in a second

7" strip. The whole area can be filled in this way.

[Nightjar]

If it is old enough not to have insulation put in when it was built, the flat roof may be nearing the end of its useful life. I would consider replacing the roof and putting insulation in when that is being done.

Colin Bignell

[Tim Watts]

On Tuesday 21 January 2014 17:13 Nightjar wrote in uk.d-i-y:

That would get the best result - vents could be easily added at the same time.

[GMM]

Not quite the same situation but I had a leak in a flat roof over two small rooms in my last place that damaged the ceiling in one part. Then I could access the space over the other room. I used plastic covered loft insulation and taped the end to a strip of thin (6mm) mdf and slid this along the gap between the joists. These days I would probably feed in PIR insulation, TBH.

[harryagain]

Blown in fibres? Some hire shops have the equipment.

[RobertL]

I have a roof like this, approved by building control. It is called (IIRC) a "warm roof". The insulation and vapour barrier are on top of the rafter s. The rafters are at room temperature and room humidity. The big advanta ge of a warm roof is that there is no need for ventilation or semipermeable coverings.

However, you must not insulate between the rafters in a warm roof or you wi ll get condensation as the upper part of them will be cold but humid.

Robert

Robert

[Dave Rove]

No, there are no external vents that I can see, but thanks for suggesting the sheeps wool -- I might well use that.

e.g.: I could buy 4 packs of 3 rolls of 75mm thick 37cm wide sheeps wool from this company, roofingsuperstore.co.uk for £186:

I presume that I can't claim a government grant if I do it myself?

However, one annoying thing is that the rolls are 37 cm wide, and the spacing between the joists in my house is about 41 cm, so I'd have a gap of 2 cm either side of the insulation. Would leaving a gap that big undermine the advantage of having thick insulation?

[Dave Rove]

Thanks. I'll consider that because I assume that getting a tight fit is important. However, it does seem that it would be a lot of work because the gap between my joists seems to vary between 40cm and 42cm so I would have to measure and custom fit each piece. I hope it's easy to cut.

[Dave Rove]

I'm not planning to renew the flat roof because it was replaced 12 years ago, and I didn't think of the insulation at the time. It's still in good order because I've given it a layer of bitumen paint every few years.

[Tim Watts]

On Thursday 23 January 2014 16:16 Dave Rove wrote in uk.d-i-y:

You could probably stretch the sheet as it comes of the roll that much.

[Dave Rove]

If that's so then I will have to leave the flat roof as it is, and just replace the insulation in the loft. The layer of polystyrene foam under the flat roof is just 1 cm thick, though, so I don't know how effective it is. Also, the cold air from the loft space exchanges with the warm air from between the rafters of the flat roof, so heat from the house would be lost via that route.

I wonder if putting, say, 2" thick Celotex boards under the flat-roof, leaving a 3" space above them would be OK to carry away the moisture? Or would that still be against regulation for this type of roof?

[liquidroofcoatings]

These days, level tops have the 'warm top' sort. Encasing can either be put beneath the climate confirmation layer, which is known as a 'warm top deck development', or above the waterproof film, whic h is called an 'upset warm deck'.The main purpose of putting the encasing o n top of the waterproof film is the point at which you can't take it off, a s when, for instance, it is bitumen.

[Jim K]

Q. Who the F besides you uses these "phrases"?

A. ah- in this case google tells me all I think I need:-

No results found for "upset warm deck". No results found for "warm top deck development"

Jim K

[Andrew May]

You missed the bit about it being American.