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PIR above rafters


williamlincs
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@Temp Many thanks. Looking at a refurb of one of our properties - can i put further PIR 50mm insulation on ceiling, with then the roof void above open, or will i get issues with moisture in the void? Ill have 100mm PIR outside rafters, and i dont want to cut and fill spaces between rafters with PIR, if i cant help it. Ceilings are high and so i thought i could put insulation under ceiling joists, counterbatten and then plasterboard.

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13 minutes ago, williamlincs said:

can i put further PIR 50mm insulation on ceiling, with then the roof void above open, or will i get issues with moisture in the void?

 

Bad idea. Rotten roof territory. 

 

15 minutes ago, williamlincs said:

dont want to cut and fill spaces between rafters with PIR, if i cant help it

 

I seem to spend half my time on here telling people that this is a terrible plan. Out of interest where did you get the idea that it might be the way to go? It seems to be everywhere. 

 

 

What is your current roof build up, what level of performance are you hoping to achieve?  

 

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As above, not normally good practice to have insulation at different levels in a ‘warm’ roof. Check with insulation manufacturer and ask for condensation calculation. With a correctly detailed vcl it may work. Also check MIs and BBA certificate, 38mm min thick counter battens normally required

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54 minutes ago, williamlincs said:

@Temp Many thanks. Looking at a refurb of one of our properties - can i put further PIR 50mm insulation on ceiling, with then the roof void above open, or will i get issues with moisture in the void? Ill have 100mm PIR outside rafters, and i dont want to cut and fill spaces between rafters with PIR, if i cant help it. Ceilings are high and so i thought i could put insulation under ceiling joists, counterbatten and then plasterboard.

 

there are only two ways to insulate a roof, warm and cold.

 

cold roof is easiest as it needs no vapour barrier as moisture rising through it and dew forming inside it is vented as its open to outside air. 

 

warm roof on the other hand has to be sealed so moisture rising up from the house cannot get into it as once it does it cannot get out and you will have mould/rot/mushrooms growing.

 

The main issue with a warm roof is having enough build height as they are a min of 150mm deep usually. obviously in an attic roof you wont have anything like this space available between the trusses so you have to go with a cold roof and copious amounts of ventilation above the insulation via the eaves and ridge.

 

 

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5 hours ago, ADLIan said:

Cold roofs should incorporate an AVCL to help prevent/minimise air movement and moisture vapour transfer through the system - what does get through can then removed by the ventilation above the insulation.


no this is bad advice . It will be a layer for moisture to be trapped on.

 

avoid any sort of hybrid roof they all allways bad.

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14 hours ago, Iceverge said:

 

Bad idea. Rotten roof territory. 

 

 

I seem to spend half my time on here telling people that this is a terrible plan. Out of interest where did you get the idea that it might be the way to go? It seems to be everywhere. 

 

 

What is your current roof build up, what level of performance are you hoping to achieve?  

 

@Iceverge I suspected bad idea hence question, thanks for confirming such.

 

current roof build up is bitumen felt and then tiles, no insulation. 4” rafters with a high collar. Plasterboard goes up rafters and across collar.

 

on stuff I’ve done before it’s been PIR cut between rafters. I hate doing this job. I’ve got the PIR on externally, counterbattened/battened. I wasn’t sure if I should have put the membrane in between counterbattens and battens, but I see from response above I’ve done correct build up.

 

Presumably I now have to fill in between the rafters. Does it have to be PIR or can I use a mineral wool? And then put a Moisture barrier underneath. How would you approach this? Thanks all for your help.

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17 hours ago, williamlincs said:

can i put further PIR 50mm insulation on ceiling,

 

No. Think about it this way...

 

Let's say the temperature inside and outside are constant so a temperaturs gradient exist between the two.

 

With a warm roof (insulation above rafters) the rafters will be on the warm side - so there is no risk of condensation on them.

With a cold roof (insulation below reafters) the rafters are cold but they can be well ventilated - so there is no risk of condensation on them.

 

If you add insulation between or below rafter in a warm roof you effectively "move" the rafters towards the cold side without providing the ventilation. This risks consensation foirming on them.

 

You can add some insulation between the rafters but I believe its limited to 1/3rd of the total and you should get a condensation risk analysis done. Insulation manufacturers will sometimes do these for you if you send them a proposed sketch ausing their insulation.

 

 

 

 

Edited by Temp
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if I can add my limited experience to this subject...

 

We have a balcony that is a 'link' between the main house and a flat roof room. like this:

 

588545894_Screenshot2021-11-18at12_22_58.thumb.png.1a862ce9bebfb59cf315aaebaade0572.png

 

the intensive wile meadow green roof if a warm roof with 190mm of PIR above the joists.

 

The balcony was designed as a warm roof with vacuum insulation panels (think Optim-R from Kingspan) but due to the exorbitant cost of those and also a mistake in the design we ended up with not having enough room for a warm roof.

 

We are now having a hybrid roof for the balcony section with 70mm PIR between the joists and 100mm PIR above the joists. This has been through a condensation analysis by Xtratherm and it has passed.

 

The important detail is, as has been said above, the VCL layer which is below the joists and any penetrations need to be sealed properly to reduce the risk of condensation.

 

So, a hybrid roof is a faff but it's definitely possible if the relevant calculations have been done and you have a document to prove it.

 

hope this different perspective helps.

 

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10 hours ago, Dave Jones said:


no this is bad advice . It will be a layer for moisture to be trapped on.

 

avoid any sort of hybrid roof they all allways bad.

 

Think some clarification is needed.

 

Its fine/recommended to put a VCL on the warm side of a cold roof construction. The VCL stops water vapour getting to the cold side and ventilation removes it if it does. Belt and braces.

 

+1 to hybrid roofs. A VCL may help reduce the risk of interstitial condensation but should not be relied on as are rarely perfect. 

 

If you design a warm roof properly you can put some insulation between the rafters but you should not put so much that it becomes a hybrid roof. See my previous post.

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I wouldn't put a vapour barrier in at this stage. 2 layers of impermeable materials in a structure is a recipe for decay.

 

Moisture will  get in. You have to make sure it can dry either to the inside or outside. The inside in your case. 

 

Having a brief look at UBAKUS you can put full mineral wool batts/roll between the rafters with no condensation risk. Then plasterboard with care to seal all penetrations. 

 

https://www.ubakus.com/en/r-value-calculator/

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Please read BS 5250 . Lack of an AVCL will allow moisture vapour from inside the house to pass through the insulation (or gaps if foil faced) and condense on any impermeable layer above the insulation. Even if you use a breather type membrane it may not be able to get rid of all the moisture vapour. I have seen a layer of condensation form on the underside of a breather membrane which then effectively renders it non breathable. The problem with using calculators like the one referenced is that you can get them to give any result that suits you!!

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9 minutes ago, ADLIan said:

Please read BS 5250 . Lack of an AVCL will allow moisture vapour from inside the house to pass through the insulation (or gaps if foil faced) and condense on any impermeable layer above the insulation. Even if you use a breather type membrane it may not be able to get rid of all the moisture vapour. I have seen a layer of condensation form on the underside of a breather membrane which then effectively renders it non breathable. The problem with using calculators like the one referenced is that you can get them to give any result that suits you!!

Hi @ADLIan,

 

in my situation we have the following buildup from inside to out:

 

VCL

Joists

70mm PIR between joists

Firrings to 1:60 fall

18mm OSB

100mm PIR

11mm OSB

fully adhesive single-ply membrane

 

This has passed all of the condensation risk analysis and the VCL (will be an AVCL actually as it'll be the same membrane I choose as the airtight layer for the rest of the house) will stop moisture from the house get in to the joists.

 

What happens to any moisture that does get in to the joists though? it can't go up as surely the single-ply waterproof membrane will stop it escaping above? I trust the condensation risk analysis but I'm just confused as to what will happen to any moisture that get's between the joists?

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13 minutes ago, Thorfun said:

Hi @ADLIan,

 

in my situation we have the following buildup from inside to out:

 

VCL

Joists

70mm PIR between joists

Firrings to 1:60 fall

18mm OSB

100mm PIR

11mm OSB

fully adhesive single-ply membrane

 

This has passed all of the condensation risk analysis and the VCL (will be an AVCL actually as it'll be the same membrane I choose as the airtight layer for the rest of the house) will stop moisture from the house get in to the joists.

 

What happens to any moisture that does get in to the joists though? it can't go up as surely the single-ply waterproof membrane will stop it escaping above? I trust the condensation risk analysis but I'm just confused as to what will happen to any moisture that get's between the joists?

hmmm....I wonder if one of those intelligent AVCLs would help here as it would let moisture back in to the house in the summer months, right?

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Hybrid roofs are generally discouraged. A hybrid pitched roof is often more forgiving as a breather type membrane is used. A hybrid flat roof is not so forgiving as the waterproof layer and ply/osb deck are both impermeable. To work there must be meticulous attention to detail in installing the AVCL ensuring all laps/joints/penetrations are very well sealed. Any imperfections may lead to problems in the longer term  - moisture vapour may get through the AVCL but liquid water is effectively trapped. BS 6229 gives more advice on flat roofs.

 

In a condensation assessment a lot will depend upon the vapour resistance value used for AVCL - does it assume the vapour resistance of the membrane, (which may be massively high if it includes a metal foil) or does the value reflect a more realistic 'in use' value including imperfections? As mentioned before its possible to get the CRA to say anything you want - it's knowing if correct and realistic values have been used.

 

Not sure an intelligent membrane would work - best check with the manufacturer and any independent certification they may have

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3 minutes ago, ADLIan said:

In a condensation assessment a lot will depend upon the vapour resistance value used for AVCL - does it assume the vapour resistance of the membrane, (which may be massively high if it includes a metal foil) or does the value reflect a more realistic 'in use' value including imperfections? As mentioned before its possible to get the CRA to say anything you want - it's knowing if correct and realistic values have been used.

does this help?

 

170390294_Screenshot2021-11-18at14_27_40.thumb.png.d5f587939779a41e75e5d5ef4931cf90.png

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19 hours ago, Dave Jones said:

there are only two ways to insulate a roof, warm and cold.

You can have a hybrid which I've used on a number of projects for various reasons (protected buildings with ceilings we had to retain, refurbishments working from one side, phased projects, etc).

 

26 minutes ago, Thorfun said:

does this help?

 

170390294_Screenshot2021-11-18at14_27_40.thumb.png.d5f587939779a41e75e5d5ef4931cf90.png

A WUFI test would be better as it takes into account the hygric behavior and moisture contained within the materials. eg the plywood deck and timber frame could have a moisture content of 20% if constructed over a wet winter.

 

We had a hybrid roof on one large roof (passive house) and got a WUFI test. It was a complicated timber frame roof that was designed to accommodate a sedum roof at a later date when funds became available. Anyway internally when the building was sealed (windows in roof on) we had to wait until the plywood deck and structural timbers dropped below a certain moisture content internally before we were allowed to add the vapour control layer. Later this void was pumped with cellulose. The roof had PIR on top of the plywood deck.

 

Most major suppliers of airtight membranes will provide a WUFI test if you're buying your vapour control membranes and tapes from them.

Edited by Dudda
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Values for the ply deck and the roofing membrane both look low. Nearer 1000 MNs/gm for ply and 800 MNs/g for the single ply (it's a continuous membrane with all joints very well sealed). These may be sufficient to alter the CRA. Problem with WUFI as I understand is that it is incredibly complex and there are no conventions for its use.

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Thanks all for contributing to this thread, I’ve learned masses and thankfully it’s stopped me doing something stupid.

 

I’ve read BS5250. Little confusing as it seems to me insulation infill between rafters is classified as warm roof in the paper?

 

anyhow, the PIR is coming off the roof tomorrow. This is my plan to insulate the space now.

 

1. 50mm PIR in between rafters up to collar height. A further 50mm PIR below this, I.e on underside face of rafters. AVCL. Held on with 25mm battens. Plasterboard.

2. Frame between collars and put 100mm PIR on underside face. AVCL and 25mm battens. Plasterboard.

 

this gives me 50mm gap between top of PIR and top face of rafter to allow for membrane drape. 5250 says about vent tiles, I’m using concrete double pantiles, VISTA membrane. Do I need vent tiles or will I get sufficient flow into roof void without?

 

Again, my thanks for all your help!

Edited by williamlincs
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19 minutes ago, williamlincs said:

Little confusing as it seems to me insulation infill between rafters is classified as warm roof in the paper?

 

No surprise.  If you have a flat roof, "warm" means all the insulation on top of the rafters.  With pitched roofs it tends to mean warm loft, with insulation between and sometimes above / below.

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43 minutes ago, williamlincs said:

this gives me 50mm gap between top of PIR and top face of rafter to allow for membrane drape.

please check with building control but I got rid of this 50mm requirement by having battens and counter battens on the rafters. this gives me 50mm of space between our fully filled rafters and the breathable membrane. I used Roofshield but it also seems that the Cromar Vent 3 Pro is breathable for warm roofs, and I'm sure there are other options as well.

 

but please don't take my word as gospel and get it checked by professionals. 

Edited by Thorfun
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5 hours ago, ADLIan said:

Please read BS 5250.

 

I've been looking for it. The old version has been superseded by the 2021 version but thats subsequently been withdrawn from the BSI store for some reason. 

 

Are you using the new version or the outdated one and do you have a link? 

 

 

 

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