Spray foam under room in roof floorboards - expensive option?

[readiescards]

To achieve circa 0.1W/m2K for my attic space the architect spec'd 500mm of Rockwool insulation.  Having upgraded to attic trusses instead of roof trusses, the proposal was to have 200mm of Xtratherm and a 25mm Xtratherm insulated plasterboard. However this is quite expensive.

 

Question: is spray foam infilled between first floor ceiling joists likely to be even more expensive?  (The specs suggest the spray foam would not need the insulated plasterboad)

[Onoff]

I had a crack at something like this on an old shed.....with CANS. Did the walls and floor:

 

 

 

The floor:

 

 

One of the STUPIDEST things I've ever done by far.

[readiescards]

Had to admit to expecting to use a professional company but very impressed with your perseverance! 

[PeterW]

Ask @PeterStarck about his insulation - he has a spray foam finish. 

 

I'm not sure where you want to insulate though ...?? Do you mean the floor between the attic and the rooms below as that will then mean the attic space has a lower uValue than the rooms below. 

[AliG]

You can use 220mm Knauf Omnifit Stud which has a conductivity of 0.034 W/mK. With 25mm underneath this would give a U-Value of around 0.14 and be much cheaper than Xtratherm. But the U-value would be 0.13/0.14.

 

With 50mm Xtratherm underneath the U-value falls to 0.11.

 

You need PIR to get to 0.1 whilst keeping the thickness down.

 

Does Xtratherm come in 200mm thick boards, I am not sure and can't find them. Celotex does, originally my architect specced Kingspan and it was much more expensive as it was 2x100mm boards. There was a big labour saving using a single 200mm board.

[readiescards]

Insulated needed in the first floor ceiling void - below the attic 22mm chipboard floor (longer term intention to convert attic space but not at this stage).

 

Xtratherm only do 100mm boards, so yes two lots of 100mm boarding needed.

 

So PIR foam boards beats spray foam professionally installed on price and U value?

 

[readiescards]

Will contact @PeterStarck, thanks

[AliG]

I missed the bit about the attic versus insulating the floor.

 

If funds allow I would insulate the roof as all that insulation will be wasted if you then convert the space above it and you'll have to spend the money all over again insulating the roof. I guess you will still get some benefit if you don't heat the roof space as often as the rest of the house.

 

Particularly if you plan to convert the loft reasonably soon, then insulating the first floor to 0.1 is overkill.

 

It will cost more to insulate the actual roof as the area is larger but that will save you a lot in the long run. Also there will be no VAT assuming it is a new build versus a loft conversion.

 

Also are there light fittings and pipework in the ceiling? This will make it awkward to get a good fit with PIR.

 

Edited November 21, 2016 by AliG

[Ferdinand]

HI @readiescards

 

Obviously you want that insulation on the slope of at all possible.

 

But you don't say what your price is for the Xtratherm. I make it about £25 per sqm of 200mm insulation picking a price off the internet of 36 per 8x4 100mm board.

 

The B&Q celotex deal I pointed out last week comes in at about 21.5 to 22 per sqm, but you would be using 1200x450s at 50mm thick and so more gaps. You would need 100s.

 

Ferdinand

 

[PeterW]

How about putting 100mm rockwool between the first floor and attic which will provide sound and thermal insulation and then putting 150mm of Xtratherm between the rafters of the attic and then tape and batten over. It gives you best of both, and then an attic conversion would then need to just overboard the inside with the additional 50mm of insulated PB.

[Ferdinand]

One which didn't make it through:

 

What about 70mm PIR x3 Or x2 which may shave costs due to 70mm being a sweet spot (due to that thickness just meeting building regs for floors?) Often 70mm celotex is cheaper than 60mm iirc.

 

Ferdinand

Edited November 22, 2016 by Ferdinand

[Temp]

Insulated needed in the first floor ceiling void - below the attic 22mm chipboard floor (longer term intention to convert attic space but not at this stage).

 

Have you considered doing a partial conversion? I think all you need is to put the insulation at rafter level instead of floor level and fit a vapour barrier (plastic sheet). I don't think you would need to plaster board it. Perhaps ask the BCO?

[readiescards]

21 hours ago, PeterW said:

How about putting 100mm rockwool between the first floor and attic which will provide sound and thermal insulation and then putting 150mm of Xtratherm between the rafters of the attic and then tape and batten over. It gives you best of both, and then an attic conversion would then need to just overboard the inside with the additional 50mm of insulated PB.

 

I asked my architect about a similar idea earlier and he felt that would be like having two leaky doors in sequence rather than one well sealed door.  So I'd still be heating the attic space and heat would still be escaping from the attic roof.

 

The downside of the cold attic is that I might end up having to put a hole in the first floor ceiling to keep the water tank warm:

 

[PeterW]

35 minutes ago, readiescards said:

 

I asked my architect about a similar idea earlier and he felt that would be like having two leaky doors in sequence rather than one well sealed door.  So I'd still be heating the attic space and heat would still be escaping from the attic roof.

 

The downside of the cold attic is that I might end up having to put a hole in the first floor ceiling to keep the water tank warm:

 

 

I do question the logic by your architect here ....

 

By just doing the roof make up suggested - and at this point the focus is on thermal efficiency not air tightness - you would end up with a roof value of 0.11. If you then add a 32.5mm insulated plasterboard you are at 0.1 at a future point. 

 

The key is air tightness and doing the partial fill at rafter and sealing either with a VCL or taping the joints will give you that - your other choice to get to a 0.12 would be 400mm of rock wool at ceiling height which scuppers your  idea of converting at a later date. 

 

 

[Ferdinand]

On 23/11/2016 at 07:23, PeterW said:

 

I do question the logic by your architect here ....

 

By just doing the roof make up suggested - and at this point the focus is on thermal efficiency not air tightness - you would end up with a roof value of 0.11. If you then add a 32.5mm insulated plasterboard you are at 0.1 at a future point. 

 

The key is air tightness and doing the partial fill at rafter and sealing either with a VCL or taping the joints will give you that - your other choice to get to a 0.12 would be 400mm of rock wool at ceiling height which scuppers your  idea of converting at a later date. 

 

 

YOu could do 175mm as Wickes, for example, do 175mm Loft Legs at under £1 each.

 

Ferdinand

[readiescards]

Interestingly the spray foam contractor I contacted said the architect proposed 500mm of earth/rock wool is impractical as the rockwool at that depth collapses on itself.  Waiting for his formal quote to work out options for me

[readiescards]

Finally got a quote from the Icynene spray foam contractor £13k for 352m2 (garage and house) a little under £37/m2 - not the cheapest option!  I like the potential significant air tightness improvement over trying to get PIR boards cut and jammed into all the little tight bits,  but still insulated plasterboards are needed to get closer to the 0.1 U value the roof was spec'd as.

 

Architect and BC concerned that the felt won't be allowed to sag under the tile rafters so water will be trapped behind the rafters, the contractor tells me they negate this be a lite first spray and the spray will always expand in path of least resistance ie. downwards.

 

Has any one experience of Icynene (I've yet to hear from @PeterStarck) I'm interested to know if different Icynene contractors charge wildly different rates

[Gone West]

1 hour ago, readiescards said:

Finally got a quote from the Icynene spray foam contractor £13k for 352m2 (garage and house) a little under £37/m2 - not the cheapest option!  I like the potential significant air tightness improvement over trying to get PIR boards cut and jammed into all the little tight bits,  but still insulated plasterboards are needed to get closer to the 0.1 U value the roof was spec'd as.

 

Architect and BC concerned that the felt won't be allowed to sag under the tile rafters so water will be trapped behind the rafters, the contractor tells me they negate this be a lite first spray and the spray will always expand in path of least resistance ie. downwards.

 

Has any one experience of Icynene (I've yet to hear from @PeterStarck) I'm interested to know if different Icynene contractors charge wildly different rates

 

I have external OSB racking on the walls and roof so the Icynene was sprayed onto the OSB and expanded inwards. I've no experience of Icynene being sprayed onto roofing felt. It is a flexible foam so possibly ok. IIRC the quotes I had varied by up to 30%.

[readiescards]

Installed today, pretty pleased with the result. 

[Dudda]

What's the minimum thickness of that foam or as I can still see the rafters what thickness are they?

 

I'd be worried about foam expanding upwards and the felt held tight against the tile battens. Really you should have counter battens outside or good spray foam companies put in trays first which maintains a ventilation gap between the felt and the foam. 

[Gone West]

@readiescards Looks like a neat job. Did you get much overspray?

[readiescards]

Minimal overspray, got some cutting back in the garage but over all not bad - given it is a pretty uncontrollable process!

 

And due to an issue with my roofer (currently inspecting Her Majesty's prison roofs - from the inside) this is what the felt looks like with the foam applied underneath - the window space is bulging just because of the wind  

[Cpd]

Looks grand, however i am intrested that you can still see the inside of the rafters,( how deep are they ?) is this a cold bridge and will it have an effect on potential condensation problems ? Do you need to put extra insulation over the rafter faces to mitigate this ? i am not an expert and i am just intrested as i have a project where i am thinking about insulating the inside of a cold roof with calotex ( slate, breather membrane, 22mm sarking, 50mm air gap, 50mm calotex, fomed and taped, 120mm calotex, foamed and taped to the inside of the rafters face) , i was then going to add a further 25mm of calotex to mitigate the cold bridge. Just asking out of interest and not looking to hijack the this thread. 

[Nickfromwales]

It's quite relative so I'd not worry about hijacking. 

We set a pretty bad example for digressions here

[AliG]

@readiescards can you help me out with some information on your new build up, U-values etc.

 

Was the £37 a square metre for the area of the roof or the area of floor that the roof covers. I am guessing the area of the roof but just wanted to check. Also how thick is the insulation for that price?

 

Currently I am showing around £55 a square metre to supply and fit Celotex XR4200. I think it drops to around £30 if I use Knauf Omnifit Stud, but the U-value would be somewhat lower, similar to that using spray foam I think.

 

What is the R-Value of the product that you have used? I see 0.0345 on the Icynene website.

 

If I used this with 25mm of PIR below I would get a U-value of 0.144, in reality a bit worse after thermal bridges. Are you using much thicker PIR below the trusses or are you ending up with a U-value worse than the 0.1 you were originally targeting?

 

Thanks for your help