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Ensuring vaulted ceiling won't get hidden condensation


Jilly
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The builder is taking a rest... I've lost trust and am checking everything as it's all down to me now..

 

The rafters are 200mm.

The original spec of the vaulted being was 200mm of sheeps wool, BUT we had to then change to F1 bituminous roofing membrane, non breathable, because of the bats and so was encouraged to change to PIR 150mm and a 50mm gap. Fair enough.

However, I need to understand the detailing.

On the SE diagram the steels are outside the thermal envelope, except for the one resting on the internal wall. (NB the vaulted ceiling will be the whole area, the diagram is slightly wrong as the design was amended, it seems to me that the vertical steel could go in or out of the thermal envelope).

See photo: the steel is bolted to the wooden ‘beam’ of the roof ridge. This means non breathable membrane stops at the top there, so the gap is not contiguous with the ventilation things in the ridge tile.

Unless I’m missing something, there’s an 50mm air gap but no actual air circulation. There’s nowhere for the air to be drawn in at the top.

 

Surely non breathable membrane and 'plastic' insulation are a potential problem, even if there is a vapour barrier in the plaster board? I feel it needs air to be drawn over it somehow and not being able to inspect it bothers me. 

Do I need to change tack and think of a breathable house as its a conversion with all the associated compromises?  

 

Steel detail.pdf Steel in situ.pdf

Edited by Jilly
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15 minutes ago, Jilly said:

Unless I’m missing something, there’s an 50mm air gap but no actual air circulation. There’s nowhere for the air to be drawn in at the top.

Does this 50mm air gap have eaves ventilators? If so, is there a continuous gap to provide flow across the whole roof? From the photo, I'm suspect the ridge beam prevents this? In that case you do need to add ventilators.

 

In the photo it looks like the steel beam is inside the insulated envelope?

 

I wonder whether your only other option would have been to look at using woodfibre insulation but that would have requires a re-design of the roof buildup (as this system typically uses an external insulation sarking board). However, I don't know how this stuff works with bats.

 

I think your easiest option is to ensure that the 50mm ventilation gap between your pir and the F1 membrane has sufficient cross-flow and then focus on breathability around the rest of the house.

 

 

 

 

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17 minutes ago, SimonD said:

Does this 50mm air gap have eaves ventilators? If so, is there a continuous gap to provide flow across the whole roof? From the photo, I'm suspect the ridge beam prevents this? In that case you do need to add ventilators.

 

In the photo it looks like the steel beam is inside the insulated envelope?

 

 

 

 

 

 

There should be ventilation 'things' in the soffit board, I'll have to double check.. I can see now that the lads who have been chopping the insulation have got confused at the apex under the beam as they didn't understand the detail either. As long as insulation isn't packed into the apex their should be a gap (I think they were going to stuff it with rock wool).

 

I can see its difficult to keep the insulation off the roofing felt and not pushed up too far against the F1, which could accidentally close the gap in places. 

 

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Just now, Jilly said:

There should be ventilation 'things' in the soffit board, I'll have to double check.. I can see now that the lads who have been chopping the insulation have got confused at the apex under the beam as they didn't understand the detail either. As long as insulation isn't packed into the apex their should be a gap (I think they were going to stuff it with rock wool).

 

I can see its difficult to keep the insulation off the roofing felt and not pushed up too far against the F1, which could accidentally close the gap in places. 

 

You are meant to install battens to create the required gap near the membrane, to stop the insulation pushing up too close

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The modern breathable roofing membranes are polyester and it fluffs up and tangles in their feet so they can't escape from it. Bitumen roofing membrane is very common in old buildings, but before this PIR stuff came along. 

 

Ahh, battens makes sense. No sign of them. 

 

So there needs to be something to create a chimney effect to draw air up and out, which I'm guessing might be present in the roof tile ridge but isn't actually joined because of the ridge beam?

Edited by Jilly
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@Jilly   , here's a schematic for our COLD roof - it's flat - not sure yours is, but the principle is the same. Imagine the illustration to be a cross section of your vaulted ceiling....

The image shows how to allow for ventilation, and where the VCL (dashed line at the bottom of our PIR ) is. 

 

flatroof.jpg.f7490fd533f53e7b878f282c84dcfa8a.jpg

(Copyright  Sam Edge, Architect)

There would be an equivalent breather gap at the other end of the roof ( in this image - the right hand side - not illustrated here)

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53 minutes ago, Jilly said:

The modern breathable roofing membranes are polyester and it fluffs up and tangles in their feet so they can't escape from it. Bitumen roofing membrane is very common in old buildings, but before this PIR stuff came along. 

 

Ahh, battens makes sense. No sign of them. 

 

So there needs to be something to create a chimney effect to draw air up and out, which I'm guessing might be present in the roof tile ridge but isn't actually joined because of the ridge beam?

Ahh OK, is that just because bats are common in your area? Or you have them in parts of your house? Not sure i'd want any pet bats at this particular point in time ?

 

Yes, you will need to use the chimney effect, soffit ventilation and then ridge ventilation, this can be achieved with a dry ridge (which is ventilated all along), and then you will have to cut the roof membrane right at the ridge to allow air through, but all depends on whether there is space to do so without risking issues with weatherproofing.

 

For me I think i'd be using this stuff: https://www.lbsbmonline.co.uk/tlx-bat-safe-breather-membrane Expensive but, it does the job

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We have a pitched roof with 200mm deep rafters. Its a breathable membrane so only needed to allow 25mm for it to drape. Think our insulation is around 160mm PIR. Vapour barrier below and plasterboard. Wish we had more insulation, mainly because it gets too hot in summer in those rooms. 

 

You might investigate putting another 50mm layer of PIR or insulated plasterboard below the rafters.

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46 minutes ago, ToughButterCup said:

taped ?  As in airtightness tape applied to the joins?  Or is there a different sort of PIR ( taped PIR ?)

 

Celotex / Kingspan / Xtratherm style foil faced boards, joins taped with self adhesive aluminium foil tape as a vapour control layer.

 

https://www.toolstation.com/aluminium-foil-tape/p81953

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2 hours ago, Jilly said:

The modern breathable roofing membranes are polyester and it fluffs up and tangles in their feet so they can't escape from it. Bitumen roofing membrane is very common in old buildings, but before this PIR stuff came along. 

 

Ahh, battens makes sense. No sign of them. 

 

So there needs to be something to create a chimney effect to draw air up and out, which I'm guessing might be present in the roof tile ridge but isn't actually joined because of the ridge beam?

Yes, chimney effect with vents at the eaves and vents and the ridge. With low pitched roofs it is sometimes okay to use vents on both of the eaves and rely on pressure differences across the house to create to necessary air flow. In this instance you need to ensure continuous flow so need a 50mm gap at the ridge. Here's a good illustration:

RoofWithSoffitRoofVentAndBaffle.png

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1 hour ago, MikeGrahamT21 said:

Ahh OK, is that just because bats are common in your area? Or you have them in parts of your house? Not sure i'd want any pet bats at this particular point in time ?

 

Yes, you will need to use the chimney effect, soffit ventilation and then ridge ventilation, this can be achieved with a dry ridge (which is ventilated all along), and then you will have to cut the roof membrane right at the ridge to allow air through, but all depends on whether there is space to do so without risking issues with weatherproofing.

 

For me I think i'd be using this stuff: https://www.lbsbmonline.co.uk/tlx-bat-safe-breather-membrane Expensive but, it does the job

I think that according to some bat experts, this new kind of membrane is unproven/insufficiently tested in the field hence why the bitumen is still recommended.

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1 hour ago, Temp said:

 Wish we had more insulation, mainly because it gets too hot in summer in those rooms.

 

This is common with PIR and other light weight insulations and is probably more down to it's lack of thermal mass and decrement delay rather than it's thickness.

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Is the roof on yet?  What is above the membrane in the photo?

 

If roof not on why not fit external insulation above the rafters and make it into a warm roof (very much like my own)?

 

Then you have eaves vents and a ridge vent to vent the gap between the breathable membrane and the tiles, and are you really telling me it is possible for a bat to enter through the standard eaves vent profiles? (e.g the OV10 that I used)  If the bats can't get in the type of membrane wrt bats is not an issue.

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@Jilly you need to insure no moisture enters the roof area the only way to do this is with a robust method of air sealing, if you put the 50mm pir under the rafters tape all the joints together with foil tape, I fitted each sheet to each other with expanding foam fist and then applied the tape. 

The boards will need to be taped to the walls to make a complete airtight box. 

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1 hour ago, NickK said:

@ProDave would you mind sharing your roof build up with insulation above the rafters? Some sort of visualisation always helps me to understand.

Outside to in:

 

Concrete tiles

tile battens

counter battens (forming a ventilated cavity vented at ridge and eaves)

Breathable membrane

100mm wood fibre board acting as sarking board.

200mm rafters, insulated with Frametherm 35 (fitted from inside)

11mm OSB racking layer

Air tight membrane

25mm battens forming service void

12.5mm plasterboard, plaster and paint.

 

Frametherm insulation in ready for OSB to to on.

 

roof_insulation_8.thumb.jpg.b82b60ba3de044783fc37968d21f85a5.jpg

 

Roof from outside, membrane over wood fibre board, counter battens going on.

roof_10.thumb.jpg.a98f9f34aac7ec46158f4c8ea0a131f5.jpg

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4 hours ago, Ian said:

I wasn't sure if you've already installed the felt but recently breathable membranes have become available that are suitable for use with bats eg:

http://www.tlxinsulation.co.uk/tlx-batsafe.aspx

Thats another long post. Its not actually 'Batsafe'  (its still tanglefoot polyester) and there are several lawsuits pending. My builder ordered it by mistake, but luckily I checked with the ecologist and we could send it back. 

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2 hours ago, ProDave said:

Is the roof on yet?  What is above the membrane in the photo?

 

If roof not on why not fit external insulation above the rafters and make it into a warm roof (very much like my own)?

 

Then you have eaves vents and a ridge vent to vent the gap between the breathable membrane and the tiles, and are you really telling me it is possible for a bat to enter through the standard eaves vent profiles? (e.g the OV10 that I used)  If the bats can't get in the type of membrane wrt bats is not an issue.

Yes, the roof is on, clay tiles on top of the Bitumen felt. I don't have a section of what the roofer did, but I don't think it communicates with the ridge tile, as in the diagram someone has posted. 

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3 hours ago, SimonD said:

Yes, chimney effect with vents at the eaves and vents and the ridge. With low pitched roofs it is sometimes okay to use vents on both of the eaves and rely on pressure differences across the house to create to necessary air flow. In this instance you need to ensure continuous flow so need a 50mm gap at the ridge. Here's a good illustration:

RoofWithSoffitRoofVentAndBaffle.png

So... my ridge beam seems to block the exit to the ridge tile ventilation. On the SE's diagram the steel is encased in the insulation, but that was for sheeps wool with and a breathable membrane. This issue with bats was overlooked when the BC drawings were submitted. My fault. 

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Ok so you can get round this. What sort of tiles have you got @Jilly..?

 

Prior to ridge venting the simplest way was to use tile vents - you’ll need to replace a tile every 3m or so in the top 3 rows of tiles. It’s easy done  - they push up the tile above to allow access to any fixing and then swap the tile for a vent. They are made to match the tiles. 
 

Alternative if you have a planning issue or non standard tile is to use lead vents - lovely looking but eye watering expensive if they are non standard sizes. 
 

These are the vents - about 3 times the price of plastic but lovely to look at ..!!

 

https://www.justlead.co.uk/product/roof-void-vent/

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