Flat roof and insulation

[GMM]

Hello folks - Any ideas on this one?

I have a single storey area at the front of the house (not really an extension as it was part of the original) with a flat roof on. Half of the roof covers the downstairs bog and hallway, while the other half makes a carport. What I'd really like to do is insulate the roof over the first part. Now, the flat roof needs attention which might be re-felting or might be re-boarding as well. My roofer tells me that the state of the boarding can only be assessed properly once it's stripped. He's a good guy, who I've used before, so I'm inclined to believe him. If it needs re-boarding, I'll be able to get access to the void and put some thing (Kingspan or whatever) in - job done. If, on the other hand, it can be fixed just by re-felting, I won't have this luxury, so will need a cunning plan for the insulation. Of course, I could just ask him to re-board it anyway, need it or not, but this will obviously cost quite a bit (dunno how much yet - he's coming to quote at the weekend).

So...>Does anyone have a cunning pan for insulating a flat roof void without taking off the boards or taking the ceiling down? I suppose I'm half-wondering whether something like cavity wall insulation could be put in via the ceiling, or insulation boards could be slid in from the side. I'm half aware that it's possible to have insulation on top of the boards, but I don't think that would be a viable option here, for a number of reasons.

Any (sensible) suggestions welcomed!

[Tim S]

GMM coughed up some electrons that declared:

Hi,

I think it may be difficult to get any loose fill insulation in evenly - if there's a slope (there must be some slope), any loose stuff is likely to collect down the lower end blocking off the ventilation and leaving cold spots at the top.

Pushing wool from one end might work if you can not stretch it too much.

But, personally, having looked at my flat roofs from below, after the felt comes off, it's likely that lifting the board (which will either be ply/some sheet material or planks - mine are planks) should be a comparatively trivial job - unless there's something about your roof that makes this difficult.

I'd consider asking for the boards to be lifted - or get the roofer to leave a week between de-felting and refelting so you can DIY it.

I can't see it costing a massive amount extra in labour to take the boards off and drop some Kingspan in. But please ensure that you get adequate ventilation installed too - you need air in and air out, flowing over the top of the kingspan, otherwise you risk condensation and rot. You don't need a howling gale, but you do need free air movement. Depending on the rest of the roof, you might need mushroom vents at the top which the roofer should be able to integrate into the refelting.

HTH

Tim

[GMM]

Good points Tim, although I hope my ceiling doesn't slope too much (!)

My assumption would that, since the boards have been there for a few decades, taking them off would mean replacing them as they'll probably be wrecked by the process. Not too sure I'd want to leave it without felt for a week, unless we get some more reliable weather forecasting, but the blokey who will do it is very happy to work interactively, if he can.

The point about ventilation is undoubtedly a good one, but it's got me thinking (makes a nice change!). Since the airspace in the void is effectively sub-divided by the joists, wouldn't each space need its own ventilation? In which case, that sounds like a lot of vents on a

5 x 6m roof, which I've never seen on any other structure. (Doesn't mean it'd be wrong, just a bit unusual). Likewise, wouldn't the space above the insulation be pretty much equivalent, in vapour terms, to the uninsulated space that's there now? If ventilation were a problem, wouldn't it have failed long ago?

Cheers

[Archie]

Good points Tim, although I hope my ceiling doesn't slope too much (!)

My assumption would that, since the boards have been there for a few decades, taking them off would mean replacing them as they'll probably be wrecked by the process. Not too sure I'd want to leave it without felt for a week, unless we get some more reliable weather forecasting, but the blokey who will do it is very happy to work interactively, if he can.

The point about ventilation is undoubtedly a good one, but it's got me thinking (makes a nice change!). Since the airspace in the void is effectively sub-divided by the joists, wouldn't each space need its own ventilation? In which case, that sounds like a lot of vents on a

5 x 6m roof, which I've never seen on any other structure. (Doesn't mean it'd be wrong, just a bit unusual). Likewise, wouldn't the space above the insulation be pretty much equivalent, in vapour terms, to the uninsulated space that's there now? If ventilation were a problem, wouldn't it have failed long ago?

Cheers

Look at installing the insulation on the outside and converting to a "warm roof" construction. I am considering this approach when the felt on my utility room/garage is due for renewal.

Archie

[Tim S]

Archie coughed up some electrons that declared:

Ooops - please watch the quoting - had trouble seeing the reply...

Anyway - good point. Lateral thinking never was my strong point; this is an excellent idea.

To the OP - this would mean your roof goes up a few inches - is that a problem?

Cheers

Tim

[Archie]

/snip

Wierd. My Outlook Express is set to indent original text with >.

Archie

[John Rumm]

Install insulation under the ceiling and add a false ceiling under that. Then just skim and paint.

[Dave Plowman (News)]

My fairly recent flat roof was constructed with high tech insulation between plasterboard and roof joists precisely to retain good air flow.

[Dave Plowman (News)]

Looks fine here on this compliant browser. Dunno what Tim's on about.

[Archie]

My previous reply to GMM indented.

Archie

[Tim S]

Dave Plowman (News) coughed up some electrons that declared:

For some reason I didn't see any quoting?

[Andrew Gabriel]

I redid a lean-to roof which basically just consisted of 4x2 joists. I bought roofing battens and fixed these at 90 degrees across the bottoms of the joists. Then cellotex onto the underside of the battens, and plasterboard onto the underside of that.

The roofing battens provided something like a 20mm airflow gap under all the joists (can't recall exactly how thick they were), combined with air vents at either end to provide a cross airflow. That was

8-9 years ago, and it's been fine since (and warm).

[Archie]

I my case that won't work due to the height of the door from kitchen into utility room. I presume that squeezing Celotex into the space between the joists would cause ventilation problems and result in condentation.

I also make use of the space between the joists in my garage to store stuff like long lengths of wood and copper pipes.

Archie

[John Rumm]

me neither...

(its not in the message source either - so its not an artefact of the reader)

[GMM]

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I wish I could John - Unfortunately, the ceiling is so low already that almost anything would take it down to door height, so it'll have to be something on top rather than below.

[GMM]

=A0 London SW

That sounds good, but I can't picture anything other than a complete new construction to achieve it: To keep the ceiling at its current (too low as it is in my opinion) level, I would have to re-construct the whole roof above it, or am I mis-understanding the layout, Dave?

[GMM]

Again, with a restricted height under the existing ceiling, this solution wouldn't really work without dismantling the whole thing, unless I'm picturing it wrongly.

I'm pretty much coming to the conclusion that in the absence of a warm roof, as suggested by Archie (and I'm not sure I can do that in this location, for a number of reasons, but will talk to my roofer), I might have to just leave the whole thing as it is (apart from getting new felt to stop the leaks) - I was thinking "stuff some Kingspan in there while the lid's off" but if that's going to become "completely re-construct the whole area", it might be better to put up with it being a bit cold for a few months of the year !

I must admit, I'm a little puzzled as to why ventilation should be more of a problem once the roof is insulated than it is at present when it's cold. In a hallway / downstairs bog, I had rather thought that the humidity would be pretty low anyway as it's not really inhabited for long (apart from evaporation from the toilet of course). Isn't there some way to paint on a vapour-proof coating (gloss paint?) to the ceiling, or shove plastic or foil onto the top face of the plasterboard to cut vapour ingress into the void?

[Dave Plowman (News)]

It was a new build so not a problem.

[Clot]

Same problem here using OE

[Archie]

My original post to DMM was not indented. I have no idea why as my OE is set to the default of indenting with the >character. It will seen by everyone like this for ever.

Archie