Roof insulation help

[Evs]

Hi guys, I’m after some advice on what insulation to use in my roof. Its quite a ridiculous situation to be in but my local council have no building control offices in, all 4 of them are on the sick, 5 out of the 6 planning officers are also on the sick. I can’t get a building officer to come to site and they have turned all the phones off and we have to send emails which they will reply to when they can.. which as you can imagine is basically never and I can’t get hold of my architect as he’s away for a few weeks. 

 

I’ve got 8” trusses fixed at 400 centres and I have to allow a 50mm air gap and also lay a vapour barrier in the vaulted part of the roof. The U value has to be <0.15. It was recommended to use 100 or 120mm celotex insulation then use 37.5mm insulated plaster boards over that to achieve the u-value. I’d rather not have to use insulated plaster boards unless I have to as I’m trying to keep the cost down, as everyone else can probably relate the cost of materials is through the roof and we’re going over budget, so trying to find the cheapest alternatives where possible. 

 

Here’s the vaulted ceiling. 

 

 

Edited August 1, 2021 by Evs

[Faz]

Blimey - that is a lot of trusses to fill with the Kingspan / celotex - are you going to do it yourself?  The bulk of the cost is going to be the 120mm insulation board - I wouldn't think the cost of the insulated pb really moves the dial that much?

 

It is difficult to see how you get to your u value without it anyway.

[Marvin]

Well I know little, but first suggest that if you do go ahead with the insulating without BC about, make sure you take plenty of photos which clearly show both the work being done and that it's your building.

 

Secondly in my experience I think that you have to have at least 25mm insulation between the joists and the plasterboard to achieve BC requirements.

 

When the joists are insulated in between and the plasterboard fixed directly on to the bottom of the joists to form the ceiling, this can cause cold lines on the ceiling where the timber is which can cause condensation to collect and leave marks. 

 

And as Faz says still need to achieve the U value. So I don't see a way around it.

 

Best of luck

 

Marvin

Edited August 1, 2021 by Marvin
Can't spell

[Evs]

20 minutes ago, Faz said:

Blimey - that is a lot of trusses to fill with the Kingspan / celotex - are you going to do it yourself?  The bulk of the cost is going to be the 120mm insulation board - I wouldn't think the cost of the insulated pb really moves the dial that much?

 

It is difficult to see how you get to your u value without it anyway.

 

Yeah its a lot of space to insulate and yeah I’m going to do it myself too ?. Worked it out, I need 30 sheets of 100 or 120mm insulation boards. 

 

13 minutes ago, Marvin said:

Well I know little, but first suggest that if you do go ahead with the insulating without BC about, make sure you take plenty of photos which clearly show both the work being done and that it's your building.

 

Secondly in my experience I think that you have to have at least 25mm insulation between the joists and the plasterboard to achieve BC requirements.

 

When the joists are insulated in between and the plasterboard fixed directly on to the bottom of the joists to form the ceiling, this can cause cold lines on the ceiling where the timber is which can cause condensation to collect and leave marks. 

 

And as Faz says still need to achieve the U value. So I don't see a way around it.

 

Best of luck

 

Marvin

 

I’ve pictured everything so far, its an absolute shambles I can’t get a building control officer to site. Even other local authorities are refusing to take on my local area’s work so we literally have no one to speak to or to inspect it. 

This is due to the thermal bridging of the trusses? 

 

Thanks for the replies both. 

[Marvin]

10 minutes ago, Evs said:

This is due to the thermal bridging of the trusses? 

As far as I understand - yes.

M

[Faz]

I got away with 25mm insulated pb over 120mm kingspan on mine.

 

You might want to have a look at gapotape - came across it just when I had nearly finished doing the room - will make the process a lot easier and quicker as the trusses will certainly have bowed slightly so none of the rips are true.  Gapogroup ltd - Gapotape, Insulation (gaposhop.com)

[James Newport]

Once the insulation people start sticking PIR to plasterboard, the price goes crazy. You can underboard the rafters with sheets of PIR and then screw up the plasterboard. 

 

All the building control people want is a document from one of the insulation manufacturers that states the U value of your roof - just use the online calculator for the insulation you're going to get, enter all your roof details and away you go. My BCO wasn't interested in coming to see the insulation anyway, he was happy with pictures and the u-value document.

[nod]

Haven’t read the other replies 

So sorry if I’m duplicating advise

If you don’t use insulated plasterboard You will regret it 

Apart from the loss of heat Excess cracking and nail popping can be a nightmare to deal with 

[Roundtuit]

Unless your tight for headroom, you can put celotex between joists and then 25 or 40mm celotex inside, over the joists, fixed with battens, then plasterboard is fixed to the batons.

I had 150mm between joists, 40mm over the joists, 25mm batons and 15mm plasterboard to get to about 0.13 W/m2k I think.

[saveasteading]

Short of headroom, got plenty of insulation as have deep rafters. Don't want to pay silly money for bonded plasterboard. 

Tell me if this is silly. 

The issue is the cold bridge through rafter to plasterboard. 

So how about a thermal break only on the rafter underside? 10mm of marmox or similar would make a huge difference I feel. Can be tacked or stapled on (over airtight membrane) and save a lot of money. That then leaves 10mm gap from plasterboard to insulation but that is another minor insulation gain. 

[PeterW]

Can you not get full fill at 200mm in there if you have a full breather membrane on the outside ..? 

[Mr Punter]

If you use 120mm Celotex between the rafters you will need another 50mm under to get your 0.15 U-value.

 

You can either hold these in place with a batten, which will double as a service void, or you can hold them in place temporarily with screws and big washers fix your plasterboard with long screws.  Make sure you mark the rafter positions very carefully.

[AliG]

Cheapest combination is probably some kind of rockwool between the rafters and PIR below.

 

If you used really cheap rockwool with 0.044 conductivity at 150mm between the rafters, this costs just £2 a square metre. Something like Knauf Earhwool 44. Then 100mm PIR under the rafters at £13ish a square meter then your insulation cost is around £15 a square metre. U-value looks to be between 0.13 and 0.14.

 

It is around £10 more a square metre in materials to use PIR backed plasterboard, but there is less labour involved. However,  you would struggle to get 0.15 with rockwool and PIR backed plasterboard. You would need better insulating rockwool which is much more expensive.

 

To use PIR between the rafters and PIR backed plasterboard under the rafter would come in at about £30 a square metre - 120mm PIR plus 72.5mm plasterboard - I kicked off a couple of pounds for the saving on plasterboard.

 

More insulation below the rafters tends to be more efficient as there is no bridging from the rafters, but you will lose some head height.

 

 

 

 

[Evs]

5 hours ago, Faz said:

I got away with 25mm insulated pb over 120mm kingspan on mine.

 

You might want to have a look at gapotape - came across it just when I had nearly finished doing the room - will make the process a lot easier and quicker as the trusses will certainly have bowed slightly so none of the rips are true.  Gapogroup ltd - Gapotape, Insulation (gaposhop.com)

 

Someone else mentioned this stuff to me but it will add a lot onto the cost of materials tbh. 

 

5 hours ago, James Newport said:

Once the insulation people start sticking PIR to plasterboard, the price goes crazy. You can underboard the rafters with sheets of PIR and then screw up the plasterboard. 

 

All the building control people want is a document from one of the insulation manufacturers that states the U value of your roof - just use the online calculator for the insulation you're going to get, enter all your roof details and away you go. My BCO wasn't interested in coming to see the insulation anyway, he was happy with pictures and the u-value document.

 

Although they’re all off on the sick, for some reason this is usually the thing all the BCO in my athority are obsessed with so I just wan’t clafication more than anything before I start ordering materials. 

 

3 hours ago, Roundtuit said:

Unless your tight for headroom, you can put celotex between joists and then 25 or 40mm celotex inside, over the joists, fixed with battens, then plasterboard is fixed to the batons.

I had 150mm between joists, 40mm over the joists, 25mm batons and 15mm plasterboard to get to about 0.13 W/m2k I think.

 

3 hours ago, saveasteading said:

Short of headroom, got plenty of insulation as have deep rafters. Don't want to pay silly money for bonded plasterboard. 

Tell me if this is silly. 

The issue is the cold bridge through rafter to plasterboard. 

So how about a thermal break only on the rafter underside? 10mm of marmox or similar would make a huge difference I feel. Can be tacked or stapled on (over airtight membrane) and save a lot of money. That then leaves 10mm gap from plasterboard to insulation but that is another minor insulation gain. 

 

1 hour ago, PeterW said:

Can you not get full fill at 200mm in there if you have a full breather membrane on the outside ..? 

 

I have got breathable membrane in the roof actually as its a slate roof. 

 

1 hour ago, Mr Punter said:

If you use 120mm Celotex between the rafters you will need another 50mm under to get your 0.15 U-value.

 

You can either hold these in place with a batten, which will double as a service void, or you can hold them in place temporarily with screws and big washers fix your plasterboard with long screws.  Make sure you mark the rafter positions very carefully.

 

Another 50m?!  Calc I used online said 120m and 37.5mm (25mm insulation on the plaster board) would have achieved it. 

 

31 minutes ago, AliG said:

Cheapest combination is probably some kind of rockwool between the rafters and PIR below.

 

If you used really cheap rockwool with 0.044 conductivity at 150mm between the rafters, this costs just £2 a square metre. Something like Knauf Earhwool 44. Then 100mm PIR under the rafters at £13ish a square meter then your insulation cost is around £15 a square metre. U-value looks to be between 0.13 and 0.14.

 

It is around £10 more a square metre in materials to use PIR backed plasterboard, but there is less labour involved. However,  you would struggle to get 0.15 with rockwool and PIR backed plasterboard. You would need better insulating rockwool which is much more expensive.

 

To use PIR between the rafters and PIR backed plasterboard under the rafter would come in at about £30 a square metre - 120mm PIR plus 72.5mm plasterboard - I kicked off a couple of pounds for the saving on plasterboard.

 

More insulation below the rafters tends to be more efficient as there is no bridging from the rafters, but you will lose some head height.

 

 

 

 

 

Thank you for that reply. 

 

I looked at knauf earthwool but like you say I’d have to really increase the PIR. In the vaulted part of the ceiling I have plenty of room to lose some height but in the bedroom, ensuite and gallery area, not so much. 

 

I also looked at using celotex between the rafters then superquilt ontop, then batten ontop of the superquilt and fix standard plasterboards to the battens. I need to work out what thickness of each I’d need, how much head height I’d lose and obviously the cost of this method. 

[Linto]

Have a look here it's knaufs detail for using rockwool type insulation at the rafter level with PIR it sets out the thicknesses needed for various u values. May not lose as much space as you think.

Plus as you can get it to fit straight in between the rafters it should save on a lot of cutting.

[Faz]

And you need to consider how long this is all going to take - just smacking PIR into those trusses is going to take ages on your own - with the added 'benefit' that it is probably the worst job going on site.

[Faz]

5 minutes ago, Linto said:

Have a look here it's knaufs detail for using rockwool type insulation at the rafter level with PIR it sets out the thicknesses needed for various u values. May not lose as much space as you think.

Plus as you can get it to fit straight in between the rafters it should save on a lot of cutting.

I don't think there will be enough depth to the trusses to allow that and maintain the 25mm breather gap.  A lot easier than smacking 125mm PIR between them mind!

[Thorfun]

14 hours ago, PeterW said:

Can you not get full fill at 200mm in there if you have a full breather membrane on the outside ..? 

 

+1

 

12 hours ago, Faz said:

I don't think there will be enough depth to the trusses to allow that and maintain the 25mm breather gap.  A lot easier than smacking 125mm PIR between them mind!

you don't need the 50mm ventilation gap if you have an air permeable membrane like Proctor Roofshield and can fully fill the rafters. but you do need to batten/counter batten above though. but make sure your BCO is ok with that. ours is.

[Temp]

+1

 

No 50mm gap needed if its a permeable membrane. However you should not fit so much insulation that the membrane is pushed against the underside of the tile battens. Hence the counter batten to raise them off the roof. The reason is that any water blown under the tiles needs to run down the membrane. If you trap the membrane dirt can seal it to the batten causing water to pool above the batten causing it to rot.

 

If the roof is already battened/tiled then you should try to let the membrane drape but don't need the full 50mm ventilation.

 

Edited August 2, 2021 by Temp

[saveasteading]

Lots of good info above.

 

1. Service space between inner board and rafters is something I had not thought of. Definitely got to think this through.

 

2. In another discussion we sort of decided that 50mm ventilation gap above the insulation was for full-sized OSB boards, and that 150mm sarking boards with gaps shouldn't need that, and 25mm should be plenty.

 

3. For our conversion project, the rafters are 18" centres (460mm), so waste in inner boarding matters, ie worse cost with insulated board.

Also waste may be significant between rafters, which are 2". (50mm).

410 mm slices of insulation seems messy and wasteful with mineral wool, esp batts (400 or 445 or 600)  and less wasteful with the bigger sheets of PIR

OR will 400mm batts happen to stretch to 410mm in real life....or 445 squeeze tight to 410.   an experiment is called for.

 

4. My idea of 10mm foam board on inner faces of rafters has been ignored. Obv with the inner insulated solution it is unnecessary. otherwise is it unproven, unnecessary or silly? 

 

5. Will have to check numbers, but my recollection is that the manufacturers' U values are conservative.   That sells more insulation.

I have made a spread sheet, based on previous iterations of building regulations, for the sake of special constructions, an trade-offs. It usually  gets a better number than the spec sheets.

 

6. How horrible out of 10, is it to fit PIR into rafter gaps? I did it into stud walls, maybe 20 years ago, so have forgotten. More a quality than comfort situation I think. (ie tight/slack/gaps.)

 

7. I don't really believe that insulation works pro-rata whatever the thickness. The further it is from the heat source, the less heat there is to escape...I speculate. Not one to argue with the BCO, just for my own decisions if exceeding regs. eg perhaps mineral wool or polystyrene on the cold side, and PIR at the heated side, esp with floors.

[Faz]

As for Q6 - stud PIR 1/10 for horribleness

 

Truss PIR - 10/10 - everyone on the site came up and sympathised!

 

[Evs]

5 hours ago, saveasteading said:

Lots of good info above.

 

1. Service space between inner board and rafters is something I had not thought of. Definitely got to think this through.

 

2. In another discussion we sort of decided that 50mm ventilation gap above the insulation was for full-sized OSB boards, and that 150mm sarking boards with gaps shouldn't need that, and 25mm should be plenty.

 

3. For our conversion project, the rafters are 18" centres (460mm), so waste in inner boarding matters, ie worse cost with insulated board.

Also waste may be significant between rafters, which are 2". (50mm).

410 mm slices of insulation seems messy and wasteful with mineral wool, esp batts (400 or 445 or 600)  and less wasteful with the bigger sheets of PIR

OR will 400mm batts happen to stretch to 410mm in real life....or 445 squeeze tight to 410.   an experiment is called for.

 

4. My idea of 10mm foam board on inner faces of rafters has been ignored. Obv with the inner insulated solution it is unnecessary. otherwise is it unproven, unnecessary or silly? 

 

5. Will have to check numbers, but my recollection is that the manufacturers' U values are conservative.   That sells more insulation.

I have made a spread sheet, based on previous iterations of building regulations, for the sake of special constructions, an trade-offs. It usually  gets a better number than the spec sheets.

 

6. How horrible out of 10, is it to fit PIR into rafter gaps? I did it into stud walls, maybe 20 years ago, so have forgotten. More a quality than comfort situation I think. (ie tight/slack/gaps.)

 

7. I don't really believe that insulation works pro-rata whatever the thickness. The further it is from the heat source, the less heat there is to escape...I speculate. Not one to argue with the BCO, just for my own decisions if exceeding regs. eg perhaps mineral wool or polystyrene on the cold side, and PIR at the heated side, esp with floors.

 

Sorry, I didn’t realise I hadn’t replied to you in my last post. 

 

Nothing is silly, I’m open to all solutions and I appreciate all the help. I haven’t had chance to do any cost comparisons between the methods mentioned above, will get it done one evening this week though and I will reply with my answers. 

 

I need to ring my roofing materials supplier tomorrow to find out exactly what membrane I have.

 

I’ve also got to add a vapor barrier to the vaulted part of the ceiling, surely this a bit of overkill with the amount of insulation I’ll be using in the roof?

 

 

 

[Iceverge]

Vapour membrane is vital for airtightness and to prevent condensation on your rafters causing the rafters to rot. 

 

PIR in the roof is a bad plan for loads of reasons. 

 

Torturous to install. Expensive. Bad decrement delay leading to overheating. Shrinkage of timbers and PIR causes drafts around the insulation making it underperform severely.

 

I have attached probably one of the cheapest options. No PIR. Better still if you can replace the glasswool between reg joists with blown densepack cellulose. Supplier installed.  Excellent for decrement delay, airtightness, toxicity, environmental credentials the list goes on. 

 

[saveasteading]

21 hours ago, Evs said:

Nothing is silly, I’m open to all solutions and I appreciate all the help.

Actually some things are silly. But not as silly suggestions, only if we then do the silly thing having been warned, hence we can help each other.

The suggestions here are really practical, and we might  never know otherwise until too late. 

 

I did used to always say to clients that there were no silly questions, meaning don't be embarrassed to ask. Silly is the wrong word perhaps....non-optimal.

 

Not using PIR in the roof now seems like a good plan. Replacing PIR with rockwool batt changes my U value from 0.12 to 0.15. I think that is acceptable when the practicalities (and real life performance) are factored in. cheaper too.

I favour batt because it pushes in tight, the outer layer can be waterproof , just in case.  Glasswool blanket must be just as horrible to fit as pir surely?

[James Newport]

PIR is 5 out of 10 on the crappy jobs scale - it's just tedious. But rockwool, and rockwool above you in between rafters? Ugh, 8 out of 10, maybe 9. It's itchy, it irritates the eyes, makes you cough. Or you can cover up, wear a mask and eye protection and then drown in sweat.