very thin insulation

[david1701]

Hi all,

I'm looking to insulate my Mums flat for her as it costs a bomb to run, but its a top floor flat in a flat roofed building with plastered laff boards then an air gap then more laff boards with the actual roof on (the last part I think but am not 1000%). To minimise the mess made I'd like to leave the ceiling up there as it makes hella mess pulling down laff boards.

I've looked into filling it with foam but there are problems with condensation with cold roof buildings if there is insufficient air gap above the insulation so I think that kinda rules it out.

So I'm thinking to lay insulation up and plasterboard over it. I've looked at the Celotex thin stuff which comes in 25mm but is fairly shitty as an insulator (R of 0.118) VS foil blankets at 38mm which are way more efficient (R of 2.218) giving me a total depth of about 2 inches with a thin sheet of plasterboard.

Does anyone know a thinner way to get decent heat savings or do we have to live with a 2" loss (ball parked costs of £12/msq already so if we could limit that a bit too....)

Generally open to any opinions

 

[Globalti]

How would you get the insulation into the gap? I'm not familiar with Celotex, is it the same stuff as Kingspan?

If it's an old lath and plaster ceiling it's probably cracked and sagging. I would pull it down, put in some decent insulation then get a plasterer to board and skim, having first got a sparky in to wire for some decent lighting.

Oh, and go and ask on DIYnot.com, you'll get some proper replies from experienced people!

 

[david1701]

I've run the numbers for pulling it down and filling it with this rigid insulation, less efficient than a thin foil blanket. An equivelant thickness foil/fiber blanket between the beams is twice as efficient as the foil if you can fit it 200mm deep, if the beams are less than 150mm its no better (as I need a 50mm air gap, so I think the next job is to measure the beam depth then decide. Cost is the same too.....

 

[Archie_tect]

Loss of floor to ceiling height is only a problem if it's less than 2.4m.

The multilayer foil insulation sheets need an airgap of at least 25mm both sides to be effective, so batten exg ceiling, fit foil [no airgaps and tape joints] then batten again before 12.5mm duplex [foil backed] plasterboarding [and 5mm skim for best airtight finish] so at least 70mm thickness o/a below exg ceiling which may be sagging as Globalti suggests [easy to check with batten]. Jury's out with thin multi-foil insulation.

Celotex RR is the best rigid board insulant you can get. Could just line underside of exg ceiling with composite 50 Celotox RR/12.5mm plasterboard + skim.

Don't puncture new ceiling with inset downlighters or you'll have problems with heat loss through units and possible fire/ condensation shorting out fittings. [New lighting- use LED fittings].

 

[Tim Bennet.]

I'd check out the figures for the foil wrap. I think they've included its effectiveness against radiant heat loss in their calculations which wouldn't apply when it's used behind another layer (plasterboard, etc).

In the same way that the silver 'space blankets' are only effective against radiant heat loss and as a cold wet body doesn't radiate any heat, they're only as effective as any other impermeable material of the same thickness - ie not much! They have to be one of the greatest triumphs of marketing over reality ever! I've heard the foil insulation is the same.

 

[colly]

It's a long shot but what about blowing polystyrene beads into the gaps above the ceiling?

I did that years ago when some built in wardrobes against an outside wall kept getting condensation and mould on the inside.

It pretty much cured the problem straight away.

 

[Archie_tect]

Good thought Colly but don't fill an open roof void with blown fill if you don't know whether the roof needs to maintain ventilation, ie a 'cold' roof.

 

[WychwoodTrev]

Or you could lay closed cell insulation on top of the flat roof warm roof and cover with pebbles



or



get roof refelted and get 100mm insulation fitted


Yes I am a flat roofer

 

[dellzeqq]

first of all - check the lease. Who does the roof belong to? It may be that the freeholder owns the roof, and could be persuaded to improve it, the cost being shared with the other leaseholders. Look at it this way - if the freeholder had to repair the damp proof course, your mum would get part of the bill.......

second - do a cost benefit analysis. Putting a further 25mm of insulation in might reduce the bill, but by how much? Your utility company should help you here, if you provide them with the details and the numbers. While you're at it, ask what grants are available - the utility companies have a responsibility to reduce the bills for elderly people (I'm assuming your mum is getting on). They fitted secondary glazing to my parents place for free.....try this site http://www.moneysavi.../grant-grabbing

third - check to see how the roof void is ventilated. Do nothing at all unless you are sure that you can maintain adequate cross-ventilation

fourth - wychwoodtrev's overlay idea is a good one (note the key words 'closed cell'), but just consider, briefly, the weight of the pebbles. If I were you I'd be pm-ing wychwoodtrev and asking for advice, and also going to Kingspan for advice, because they are one of the very few insulation companies that will offer advice and back it up with an e-mail (are you listening, Celotex, and are you wondering why I never specified your stuff?). Calculate the weight of the pebbles, which may vary according to location and the height of the building, take it to a structural engineer with details of the roof structure. This sounds like overkill, but I've laid pebbles over insulation, and it took two tonnes..........

fifth - having established how you want to go forward, ask wychwoodtrev how much you should be spending

sixth - Archie Tect's politeness has got the better of him. Multi-layer foil? I'd rather drink a bucket of cold snot.

 

[the snail]

There's also this stuff:
http://en.wikipedia.org/wiki/Aerogel
expensive and not widely available though.

 

[Archie_tect]

Archie Tect's politeness has got the better of him. Multi-layer foil? I'd rather drink a bucket of cold snot.

You've got one handy?

 

[david1701]

first of all - check the lease. Who does the roof belong to? It may be that the freeholder owns the roof, and could be persuaded to improve it, the cost being shared with the other leaseholders. Look at it this way - if the freeholder had to repair the damp proof course, your mum would get part of the bill.......

second - do a cost benefit analysis. Putting a further 25mm of insulation in might reduce the bill, but by how much? Your utility company should help you here, if you provide them with the details and the numbers. While you're at it, ask what grants are available - the utility companies have a responsibility to reduce the bills for elderly people (I'm assuming your mum is getting on). They fitted secondary glazing to my parents place for free.....try this site http://www.moneysavi.../grant-grabbing

third - check to see how the roof void is ventilated. Do nothing at all unless you are sure that you can maintain adequate cross-ventilation

fourth - wychwoodtrev's overlay idea is a good one (note the key words 'closed cell'), but just consider, briefly, the weight of the pebbles. If I were you I'd be pm-ing wychwoodtrev and asking for advice, and also going to Kingspan for advice, because they are one of the very few insulation companies that will offer advice and back it up with an e-mail (are you listening, Celotex, and are you wondering why I never specified your stuff?). Calculate the weight of the pebbles, which may vary according to location and the height of the building, take it to a structural engineer with details of the roof structure. This sounds like overkill, but I've laid pebbles over insulation, and it took two tonnes..........

fifth - having established how you want to go forward, ask wychwoodtrev how much you should be spending

sixth - Archie Tect's politeness has got the better of him. Multi-layer foil? I'd rather drink a bucket of cold snot.

we are not messing with the roof, deffo not messing with the roof, its an old hotel turned into 16 flats all of which have a share in the freehold and the roof was redone a few years ago so way too complex to mess with it.


I'm stuck on tinfoil being the most attractive but apparently crap option vs a ridiculous amount of work/mess to pull down and replace all the ceilings :s

 

[david1701]

btw I have read all of the replies its just that Dells was most quoteable. Bleurgh this is complex

oh and working out a decent electric central heating system is also pretty complex too, still stuck on wet vs dry atm though if we can get economy 7 it would make wet a much better option...

 

[dellzeqq]

we are not messing with the roof, deffo not messing with the roof, its an old hotel turned into 16 flats all of which have a share in the freehold and the roof was redone a few years ago so way too complex to mess with it.


I'm stuck on tinfoil being the most attractive but apparently crap option vs a ridiculous amount of work/mess to pull down and replace all the ceilings :s

I don't think you are. If the wind uplift is not so great (and the need for pebbles similarly not so great) then wychwoodtrev's idea will be relatively inexpensive and will be quick. And, again, look to the leases - even if the freehold is held in common there may be an indication on how costs to the envelope (sorry) of the building should be shared.

I think, by the way, that the closed cell insulation has to be laid on to fancy little plastic battens fixed down with double sided tape - but, again, ask the expert above!

 

[slowmotion]

If you put the insulation on top of the roof, just make sure that absolutely no UV from sunlight can reach the foam. If it does, you can kiss goodbye to the foam in short order.