Flat roof insulation

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DaisyDaisy
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Flat roof insulation

Post: #39964 DaisyDaisy
Tue Nov 07, 2006 11:14 am

I have a flat roof over an extension, and the 2 rooms (each about 9ft square - one of them leading open plan onto a room in the original house) under this roof are COLD !!!
The only information I can find on insulating a flat roof says it's best done if the roof is being replaced, but mine isn't.

Can anyone suggest how I can insulate these rooms and not reduce the sale-ability of the house?

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Post: #39976 jondy
Tue Nov 07, 2006 12:38 pm

Have a chat to your builders merchants (try several) There is a product that is fitted, on top of the roof. I think, sort of thermal boards with aluminium foil. I would think that a roof covering is added to the boards after insulation. Again, take advice, but I think injecting foam or insulating material (I have a friend who did this) into the space between the ceiling board and the roof leads to problems. (possibly condensation?)

Let us know what you come up with.

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Post: #39983 Muddypause
Tue Nov 07, 2006 12:55 pm

Adding insulation between the ceiling and the roof capping is usually not a very practical solution.

But you can get what is called a 'warm roof' system. Basically, this means adding a layer of insulation on the outside of the roof. The insulation has to meet certain requirements regarding loading and fire resistance, and stuff like that, so it is specially made for this situation.

There are several manufacturers, making several different types of 'system', so you really need to go to a few builder's merchants and ask them what's available - see if you can get a few leaflets, so the you can see what you are talking about. At its simplest, it is just a sheet of poly-something foam, a few inches thick, with a bit of roofing felt on top, maybe also with something to withstand the odd workman standing on it.

Installing a warm roof is not a great deal more work than having the roofing felt replaced, but I have a feeling it may cost a good deal more. It you are good at practical work, it's the sort of thing you may be able to tackle yourself.

But also consider what the construction of the walls is - many flat roofed extensions were built on the cheap, and are only one brick thick, rather than two brick cavety walls. This makes them very cold, and the only way round it is to line the inside of the external walls with insulation, followed by plasterboard on studwork. This will make all the rooms a few inches smaller, and can be a bit of a task. Also, double glazing is as important in an extension as anywhere else in the house.
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Post: #39989 DaisyDaisy
Tue Nov 07, 2006 1:28 pm

Thanks for all your suggestions so far.
After googling things like "Warm Roof System" & "insulate flat roof" I'm beginning to wonder about waiting until the roof needs repairing - it is at least 20 yrs old and although is not showing signs of its age I guess it will begin to do so before too long - not too soon, I hope :roll:
I've plenty to keep me occupied before then (starting with insulating roof space)

Then I can insulate it (the walls & windows are fine) to my own spec, and add a window into the roof to increase the light into the room at the back (saving electricity eventually).
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Post: #39990 Stonehead
Tue Nov 07, 2006 1:33 pm

Muddypause wrote:Adding insulation between the ceiling and the roof capping is usually not a very practical solution.

But you can get what is called a 'warm roof' system. Basically, this means adding a layer of insulation on the outside of the roof. The insulation has to meet certain requirements regarding loading and fire resistance, and stuff like that, so it is specially made for this situation.

There are several manufacturers, making several different types of 'system', so you really need to go to a few builder's merchants and ask them what's available - see if you can get a few leaflets, so the you can see what you are talking about. At its simplest, it is just a sheet of poly-something foam, a few inches thick, with a bit of roofing felt on top, maybe also with something to withstand the odd workman standing on it.

Installing a warm roof is not a great deal more work than having the roofing felt replaced, but I have a feeling it may cost a good deal more. It you are good at practical work, it's the sort of thing you may be able to tackle yourself.

But also consider what the construction of the walls is - many flat roofed extensions were built on the cheap, and are only one brick thick, rather than two brick cavety walls. This makes them very cold, and the only way round it is to line the inside of the external walls with insulation, followed by plasterboard on studwork. This will make all the rooms a few inches smaller, and can be a bit of a task. Also, double glazing is as important in an extension as anywhere else in the house.


We have two flat-roofed extensions and Muddypause's advice is spot on. We have double glazing and have been draught-proofing, but after that it starts getting expensive, time consuming and labour intensive.

Really, the only way to get these structures properly insulated is to gut and re-roof them!

You can lay insulated roofing over the top, but the advice is that in the long run it's better to take the old roof up and do the job properly.

It's the same with the walls - we could use insulation that's applied like wallpaper or rip the whole lot out and do it properly.

For the moment, we're leaving as is because both options are too expensive - temporary fix or long-term fix.

What's really galling is that the house was burned out by fire a few years back and rebuilt to the standards, including insulation. However, the owner didn't like losing the space so when the house was handed back to him, he ripped all the plasterboard and studding out, ripped out the insulation and then put it back with no insulation - just to save a few inches!!!!! :roll: :angryfire:

Needless to say, our full building survey didn't pick up on this - the neighbours told us after we'd mentioned how cold the house was.
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Post: #40016 baldowrie
Tue Nov 07, 2006 3:19 pm

could you not inject it like cavity wall insulation?

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Post: #40037 paradox
Tue Nov 07, 2006 4:42 pm

Maybe you could paint the flat roof black so it absorbed more heat from the sun during the day.
This would help keep the room at a higher temperature for longer.

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Post: #40041 Stonehead
Tue Nov 07, 2006 4:53 pm

baldowrie wrote:could you not inject it like cavity wall insulation?


No, it's lots of small sections so lots of holes, plus the weight on the plasterboard ceilings. It's also why you can't inject behind plasterboard. You'd have to make dozens of holes.

But you can spray insulation on before you put the plasterboard on. That's very quick and effective.

Funnily enough, once we get a decent snow fall the extensions warm up as there's nothing like two or three feet of snow for insulation. On the other hand, three feet of snow does make the roof creak and groan...
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Post: #40234 baldowrie
Wed Nov 08, 2006 8:19 pm

suppose it depend on how many twangs you put in.

You can buy ready insulation plaster board but I believe it is heavy

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Post: #40243 Boots
Wed Nov 08, 2006 11:09 pm

Will need to call Muddy back in on this one, but if you have a flat roof couldn't you insulate by boxing a slight incline and growing grass or some sort of resistant groundcover? What sort of cross members do you have? Wood or steel?

I think Muddy referred to it as a Green Roof over there. We call it a sod roof (if you want to search it) I know it has terrific cooling properties - does it keep the place warm too Muddy?

He has some pics here someplace, but am sorry, I just woke up and am not real with it yet.
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Post: #40255 Stonehead
Thu Nov 09, 2006 6:52 am

Boots wrote:Will need to call Muddy back in on this one, but if you have a flat roof couldn't you insulate by boxing a slight incline and growing grass or some sort of resistant groundcover? What sort of cross members do you have? Wood or steel?

I think Muddy referred to it as a Green Roof over there. We call it a sod roof (if you want to search it) I know it has terrific cooling properties - does it keep the place warm too Muddy?

He has some pics here someplace, but am sorry, I just woke up and am not real with it yet.


The roof would need a lot of reinforcement - the rafters are wood and while you can, just, walk on the roof it does flex. If I have to go on the roof I lay a ladder from one wall to the other.

Thin sod roofs don't insulate that much and need extra insulation - you need real depth to get the insulation effect.
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Post: #40266 Milims
Thu Nov 09, 2006 10:08 am

Spray in insulation is ok if you know what you are doing, but as stonehead said, lots of holes, also you need to ensure you don't over fill or the inegrity of the ceiling may be compromised and then you might wake up one morning and ceiling may be on the floor! Seen this done and it's not a pretty sight. On the other hand if you need a new ceiling polystyrene or a product called Kingspan are really good insulators but they are'nt very eco friendly. they can easily be cut to size and inserted between the roofing joists then plasterboarded over again. Messy job but if you are reasonably proficient at DIY it's not impossible.
Also with these the insulation values are really high for the section you use. to achieve the same amount of insulation 300mm of rockwool provides you would need about 100-150mm and this also is approved by building regs.

Look into grants available too as if you don't have adequate insulation local authorities can assist. Try your local authority web page for more info or the EAGA partnership if on a low income or classed as "vulnerable" in their classifications

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Post: #40329 Muddypause
Thu Nov 09, 2006 6:05 pm

Boots wrote:I think Muddy referred to it as a Green Roof over there. We call it a sod roof (if you want to search it) I know it has terrific cooling properties - does it keep the place warm too Muddy?


I don't have a great deal of experience of green/living/sod/turf/etc. roofs, but have seen a couple of them, and been involved in some of the structural stuff that supported them. In both cases the roof timbers were massively over spec. compared to a normal roof, and in one case had to support 10 tons of soil (this was twice what the builder had originally figured on using).

I'm not at all certain about their insulation properties. It's tempting to think that that much soil must at least act as a thermal store, but as they must be kept reasonably moist, I also think there would be issues of evaporation, which would be cooling.

He has some pics here someplace, but am sorry, I just woke up and am not real with it yet.


Well, not specifically of the roof, but one of the strawbale builds that I've been involved in can be seen here, complete with green roof. I think it looks marvelous.

I can also confirm Stoney's findings that several inches of snow can be an excellent insulator. I spent several years living in a caravan in the frozen north, and a few inches of snow on the roof in winter made it easier to keep warm inside.
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