warm flat roof connected to cold pitched roof = confused
Discussion
wolfracesonic said:
I'm even more confused now
Does the soffit venting allow external cold air to circulate under the Celotex OP or have they isolated the flat roof from the pitch roof somehow? The plot thickens!
Existing joists are connected directly into the cold pitched roof, so your assumptions are correct about the cold air under the celotex. Does the soffit venting allow external cold air to circulate under the Celotex OP or have they isolated the flat roof from the pitch roof somehow? The plot thickens!mikebradford said:
Is the pic an indication of what the builder believes you are intending doing? after adding additional insulation partially between the flat roof rafters (indicated green)?
It looks that way.When the builder was initially engaged it was on the premise that the ceilings would remain in situ, it's only as a consequence the old roof being removed and the great british weather causing the old boards to get wet and fell in. So the builder should have accounted for the VCL as part of his work to create a warm flat roof.
As things stand right now, the only insulation in these room will be via plasterboard, or if I go to the additional expense and time of celotexing between the joists.
No BC involved, though I think it's the only next logical step in getting the builder to understand what he's built, a waterproof cold roof, with plasterboard for insulation.
Edited by rfsteel on Monday 3rd October 19:21
Ultimately its not the end of the world.
Your in a relatively easy position to make this work, as you have no plasterboards to the underside of your flat roof.
You appear to know the principles of how both a cold and warm roof work. So hopefully all goes well for you from this point on.
Your in a relatively easy position to make this work, as you have no plasterboards to the underside of your flat roof.
You appear to know the principles of how both a cold and warm roof work. So hopefully all goes well for you from this point on.
mikebradford said:
Ultimately its not the end of the world.
Your in a relatively easy position to make this work, as you have no plasterboards to the underside of your flat roof.
You appear to know the principles of how both a cold and warm roof work. So hopefully all goes well for you from this point on.
Thanks, I've spent the weekend head scratching and learnt alot about roofs this weekend.Your in a relatively easy position to make this work, as you have no plasterboards to the underside of your flat roof.
You appear to know the principles of how both a cold and warm roof work. So hopefully all goes well for you from this point on.
Looks like I've got the law on my side also, so hopefully the builders next suggestion is a viable one;
planningportal.co.uk said:
First, if a person carrying out building work contravenes the Building Regulations, the local authority may prosecute them in the Magistrates' Court where an unlimited fine may be imposed (sections 35 and 35A of the Building Act 1984). Prosecution is possible up to two years after the completion of the offending work. This action will usually be taken against the person carrying out the work (builder, installer or main contractor).
https://www.planningportal.co.uk/info/200128/building_control/38/building_regulations/3Thanks again for your help guys.
Have a look at the following...
www.trada.co.uk/techinfo/.../How%20to%20design%20a...
What the builder has done is created a half arsed warm roof, but with no VCL and the deck above the insulation rather than below. He has done this as an underlay for the GRP, but the OSB deck shouldn't be on top of a warm roof setup.
If going GRP, he should have fitted all insulation between joists (and possibly some below), furring to create a minimum 50 mm vented void then deck and GRP. A good VCL is fitted below the joists and then plasterboard to finish. This vented void could then have been linked to the pitched roof void.
If going warm flat roof, he should have had plasterboard, joists and furrings, deck then VCL, insulation and membrane fitted to the insulation. No wood should be above the VCL as there is a risk of it being exposed to condensation. In a normal warm flat roof, some condensation could form within the insulation layer, but as it is foam there isn't an issue. With OSB in there the OSB could absorb that moisture/condensation. If its moisture content exceeds 20% it is at risk of decay and delamination.
As a quick and dirty fix, as mentioned above by a few posters, you could insulate between and below the joists leaving a 50mm void between the insulation between and insulation above. This is then vented. That should probably be OK, but in essence the insulation above the vented void wouldn't really contribute to the overall performance (due to cold air moving through the void). So you end up paying twice for insulation and only getting the benefit once.
www.trada.co.uk/techinfo/.../How%20to%20design%20a...
What the builder has done is created a half arsed warm roof, but with no VCL and the deck above the insulation rather than below. He has done this as an underlay for the GRP, but the OSB deck shouldn't be on top of a warm roof setup.
If going GRP, he should have fitted all insulation between joists (and possibly some below), furring to create a minimum 50 mm vented void then deck and GRP. A good VCL is fitted below the joists and then plasterboard to finish. This vented void could then have been linked to the pitched roof void.
If going warm flat roof, he should have had plasterboard, joists and furrings, deck then VCL, insulation and membrane fitted to the insulation. No wood should be above the VCL as there is a risk of it being exposed to condensation. In a normal warm flat roof, some condensation could form within the insulation layer, but as it is foam there isn't an issue. With OSB in there the OSB could absorb that moisture/condensation. If its moisture content exceeds 20% it is at risk of decay and delamination.
As a quick and dirty fix, as mentioned above by a few posters, you could insulate between and below the joists leaving a 50mm void between the insulation between and insulation above. This is then vented. That should probably be OK, but in essence the insulation above the vented void wouldn't really contribute to the overall performance (due to cold air moving through the void). So you end up paying twice for insulation and only getting the benefit once.
rfsteel said:
Can he fix it ?
Or should I let him ?
Not really.Or should I let him ?
As I said earlier, a 'better' 'solution' would be to make it an independent warm roof. This would involve making the flat roof eave detail insulated and airtight. Tape the joints in the Celotex with suitable foil faced tape, then insulate/seal the other end where the main roof rafters and flat roof joists interface. You would then need tile vents to vent the main roof.
The above would IMO be better than trying to vent through the flat roof
however, as posted above there should be a deck below the Celotex with a VB over. I can only imagine the Celotex will compress and lose integrity over time whatever you do (short of starting again)
No way building regs passed it either as it stands
ks' your builder quotes from.227bhp said:
roofer said:
Builder has put insulation underside because he wouldn't have been able to GRP onto Celotex. That's why he's a builder. Roofers don't use GRP.
You can get Kingspan with ply bonded onto it for roofing straight onto.roofer said:
227bhp said:
roofer said:
Builder has put insulation underside because he wouldn't have been able to GRP onto Celotex. That's why he's a builder. Roofers don't use GRP.
You can get Kingspan with ply bonded onto it for roofing straight onto.227bhp said:
roofer said:
227bhp said:
roofer said:
Builder has put insulation underside because he wouldn't have been able to GRP onto Celotex. That's why he's a builder. Roofers don't use GRP.
You can get Kingspan with ply bonded onto it for roofing straight onto.You can't use heat straight onto it, it's bonded with minimal pu and buckles .
further response from my builder;
I've just sent the following links back to the builder and asked him to reconsider his position;
http://www.bauder.co.uk/assets/b/a/bauderflatroofd...
myBuilder said:
The insulation (foil) is your VCL. Polythene sheeting is another form of VCL which can be used independently or in addition to foil backed insulation or foil backed plasterboard. It's more often used along full filled insulated stud walls (i.e. in roof dormers or in the office section of your upcoming garage build) as it's easier to seal between the studs and corners this way and impossible to adequately ventilate the walls, or where insulated stud walls are built against cold solid brick walls and it's vital condensation doesn't build in the cavity. It would also have been required had we installed a cheap polystyrene insulation board such as Jablite / Jabroof (which wouldn't conform to current Building Regs).
My suggestion of a polythene VCL below the rafters is a simple solution to act as a draft seal for next to no effort or cost. Maybe calling it a VCL isn't the correct term for this application... Bare in mind you will have a draft not only from the soffits but from the existing attic space as well. It's not mandatory for your warm roof but I would install it myself now that you have an opportunity to do so. This will not leave you with a cold roof. A cold roof would have insulation installed between and/or below the rafters and they would themselves need to be ventilated above.
So it looks like no separate VCL has been installed, and the builder is relying on the foil back insulation boards, even though the boards are not butted together, nor have the joins been foiled taped.My suggestion of a polythene VCL below the rafters is a simple solution to act as a draft seal for next to no effort or cost. Maybe calling it a VCL isn't the correct term for this application... Bare in mind you will have a draft not only from the soffits but from the existing attic space as well. It's not mandatory for your warm roof but I would install it myself now that you have an opportunity to do so. This will not leave you with a cold roof. A cold roof would have insulation installed between and/or below the rafters and they would themselves need to be ventilated above.
I've just sent the following links back to the builder and asked him to reconsider his position;
http://www.bauder.co.uk/assets/b/a/bauderflatroofd...
bauder.co.uk said:
Moisture vapour is also present in all the permeable materials
of a building, including the roof construction, and as the vapour
pressure inside a building is continually changing, there is a
constant flow of water vapour in and out of the roofing materials.
In cold weather the temperature under the waterproofing
will fall and can create a zone in the roof structure where the
temperatures are below the dew point. Moisture will condense
in this zone to form interstitial condensation.
The incorporation of adequate insulation and ventilation in the
roof void of a cold roof construction, or sufficient insulation
and vapour barrier in a warm roof construction, will prevent the
formation of interstitial condensation in the normal range of
environmental conditions experienced.
http://www.buildingregs4plans.co.uk/guidance_flat_...of a building, including the roof construction, and as the vapour
pressure inside a building is continually changing, there is a
constant flow of water vapour in and out of the roofing materials.
In cold weather the temperature under the waterproofing
will fall and can create a zone in the roof structure where the
temperatures are below the dew point. Moisture will condense
in this zone to form interstitial condensation.
The incorporation of adequate insulation and ventilation in the
roof void of a cold roof construction, or sufficient insulation
and vapour barrier in a warm roof construction, will prevent the
formation of interstitial condensation in the normal range of
environmental conditions experienced.
buildingregs4plans.co.uk said:
Cross-Ventilation
To help disperse the moisture vapour, the building regulations require cross ventilation to be provided in the form of a 50mm air gap between the deck and the insulation and continuous gap of about 25mm at the eaves.
This can be difficult to achieve where roofs abut external walls and propriety mushroom vents to provided the equivalent 25mm continuous ventilation are available.
Vapour control layer for a cold roof
However the cross ventilation does not completely remove the moisture vapour in the ceiling void and a vapour control layer sealed at joints and penetrations is required under the insulation and over the plasterboard to provide a barrier against moisture vapour rising up from the room below.
Alternatively metalized polyester lined plasterboard can be used.
Vapour control layer for a warm roof
A vapour layer should be positioned under the insulation or, in some situations, immediately below the roof covering to minimise water vapour condensing beneath the membrane.
Vapour control layer may be formed using any of the following:
A polythene sheet membrane loose laid and restrained by mechanical fasteners or nailed to the deck (timber decks) all laps should be sealed with an appropriate adhesive.
A reinforced bitumen sheet. - one layer of BS 747 Type 5U felt, fully bonded or mechanically nailed to the deck.
Two layers of Type 5U felt fully bonded in hot bitumen. check - all laps must be sealed with bitumen.
For single-ply membranes, the VCL should be either polythene or reinforced aluminium foil.
Designers often specify readymade ‘composite’ decking that combines together plywood, insulation, vapour control layer and felt covering into one product.
To help disperse the moisture vapour, the building regulations require cross ventilation to be provided in the form of a 50mm air gap between the deck and the insulation and continuous gap of about 25mm at the eaves.
This can be difficult to achieve where roofs abut external walls and propriety mushroom vents to provided the equivalent 25mm continuous ventilation are available.
Vapour control layer for a cold roof
However the cross ventilation does not completely remove the moisture vapour in the ceiling void and a vapour control layer sealed at joints and penetrations is required under the insulation and over the plasterboard to provide a barrier against moisture vapour rising up from the room below.
Alternatively metalized polyester lined plasterboard can be used.
Vapour control layer for a warm roof
A vapour layer should be positioned under the insulation or, in some situations, immediately below the roof covering to minimise water vapour condensing beneath the membrane.
Vapour control layer may be formed using any of the following:
A polythene sheet membrane loose laid and restrained by mechanical fasteners or nailed to the deck (timber decks) all laps should be sealed with an appropriate adhesive.
A reinforced bitumen sheet. - one layer of BS 747 Type 5U felt, fully bonded or mechanically nailed to the deck.
Two layers of Type 5U felt fully bonded in hot bitumen. check - all laps must be sealed with bitumen.
For single-ply membranes, the VCL should be either polythene or reinforced aluminium foil.
Designers often specify readymade ‘composite’ decking that combines together plywood, insulation, vapour control layer and felt covering into one product.
Edited by rfsteel on Tuesday 4th October 08:29
mikebradford said:
Is the pic an indication of what the builder believes you are intending doing? after adding additional insulation partially between the flat roof rafters (indicated green)?
The pink line here deserves more credit!(Following with interest, as the flat above mine has a warm roof space that is connected to the cold roof space over the rest of the property, exactly as mikebradford describes.)
Though to be fair, they have gone the extra mile and instead of having a single board ceiling, they have a fancy wooden slatted ceiling with gaps big enough to see through.
Finally a positive response from my builder, thanks Mike, looks like your diagram spurred on this action;
myBuilder said:
As a start, PIR insulation is the worst form of sound insulation imaginable. It is only a very effective thermal insulator, which is the job it is doing on your roof.
In regards to the VCL, this is covered by bullet point 4 of the second screenshot you have attached.
However, having analysed your roof sketch, particularly the pink annotation, I admit that I can now see the design detail which is incorrect and assume is what you are trying to get across (is it?). Whilst concentrating on future proofing your pitched roof ventilation, we're inadvertently funneling warm air from the lounge into it. So, we need to remove the ventilation detail from the flat roof completely and as and when you decide to insulate the pitched roof, you'll need to vent from the lower end of this itself.
A schoolboy error and I really should have picked up on this earlier seeing as it's not a detail I've ever installed, seen, or even took into consideration whilst quoting (otherwise you would have seen 'vented soffits on rear wall' as part of the quotation.)
So, seeing as your ceilings are still down, lets install 80mm celotex boards (slightly thicker than the width of your cavity walls) between the rafters from the top of the wall plate (or where there should have been one when the extension was built!) up to the warm roof boards. These can then be foamed and taped in place. The same can be undetaken on the inner wall.
This will seal the entire extension space and form a true warm roof as opposed to the hybrid we've ended up with. It will also remove the need (and would in fact cause condensation in that void if fitted due to no air flow) for any draft seal sheet below the rafters which I previously suggested.
Although the roof boards are butted up tight, we can tape the joints from below for extra piece of mind.
I have to admit, I'm feeling like a bit of a wally right now, but happy to hold my hands up and correct the detail. Lesson for the future though, don't try to kill too many birds with one stone whilst in the middle of a job, something somewhere will get overlooked!
No doubt you've put a tonne of research into realising this error yourself and have delayed getting the ceilings back up. I'm happy with the above solution as it's a standard detail (and not a hybrid which I dreamt up for your case!). But please make sure it also now makes sense in your mind so you have piece of mind going forward.
PS. Just incase this looks to you like I was cutting corners in someway (?), I most definitely was not (quite the opposite making a conscious effort to purchase different soffits). Hopefully you can see I take much pride in the work I undertake and do my best to think ahead of potential future issues or uses. Obviously this is somewhat an exception and an embarrassing one for myself having a client pointing out a very obvious flaw!.
Hopefully a positive outcome, I've also asked that the builder undertakes a condensation risk analysis to ensure the proposed solution will work.In regards to the VCL, this is covered by bullet point 4 of the second screenshot you have attached.
However, having analysed your roof sketch, particularly the pink annotation, I admit that I can now see the design detail which is incorrect and assume is what you are trying to get across (is it?). Whilst concentrating on future proofing your pitched roof ventilation, we're inadvertently funneling warm air from the lounge into it. So, we need to remove the ventilation detail from the flat roof completely and as and when you decide to insulate the pitched roof, you'll need to vent from the lower end of this itself.
A schoolboy error and I really should have picked up on this earlier seeing as it's not a detail I've ever installed, seen, or even took into consideration whilst quoting (otherwise you would have seen 'vented soffits on rear wall' as part of the quotation.)
So, seeing as your ceilings are still down, lets install 80mm celotex boards (slightly thicker than the width of your cavity walls) between the rafters from the top of the wall plate (or where there should have been one when the extension was built!) up to the warm roof boards. These can then be foamed and taped in place. The same can be undetaken on the inner wall.
This will seal the entire extension space and form a true warm roof as opposed to the hybrid we've ended up with. It will also remove the need (and would in fact cause condensation in that void if fitted due to no air flow) for any draft seal sheet below the rafters which I previously suggested.
Although the roof boards are butted up tight, we can tape the joints from below for extra piece of mind.
I have to admit, I'm feeling like a bit of a wally right now, but happy to hold my hands up and correct the detail. Lesson for the future though, don't try to kill too many birds with one stone whilst in the middle of a job, something somewhere will get overlooked!
No doubt you've put a tonne of research into realising this error yourself and have delayed getting the ceilings back up. I'm happy with the above solution as it's a standard detail (and not a hybrid which I dreamt up for your case!). But please make sure it also now makes sense in your mind so you have piece of mind going forward.
PS. Just incase this looks to you like I was cutting corners in someway (?), I most definitely was not (quite the opposite making a conscious effort to purchase different soffits). Hopefully you can see I take much pride in the work I undertake and do my best to think ahead of potential future issues or uses. Obviously this is somewhat an exception and an embarrassing one for myself having a client pointing out a very obvious flaw!.
With regard to the main pitched roof, id be happy as long as you have some form of soffit vent on opposite sides of the roof.
As my diagram below sketch 1.
The vents dont need to be on the same side as the flat roof.
You simply need them to be opposite each other such that the positive and negative air pressure means that air is drawn through the roof structure.
Depending on the insulation in the existing pitched roof, their may be a little bit of hassle ensuring the vents are not blocked by any insulation you have in there.
Alternatively, you can have traditional soffit vents on the front of the house? ( opposite the falt roof side) and then some form of tile ventilator on the pitched roof side just above the flat roof itself. Sketch 2
In the future if you want to change the pitched roof to a habitable space, and make the roof insulated at rafter level, you may not need the vents, depending on how you insulate. But for now if you have quilt type insulation at ceiling level in the pitched roof area of the house, you do need air flow over the top of it.
As Wolf stated below, it does appear you builder has realised his error, so seems reasonable to allow him to rectify his work and move your project along.
As my diagram below sketch 1.
The vents dont need to be on the same side as the flat roof.
You simply need them to be opposite each other such that the positive and negative air pressure means that air is drawn through the roof structure.
Depending on the insulation in the existing pitched roof, their may be a little bit of hassle ensuring the vents are not blocked by any insulation you have in there.
Alternatively, you can have traditional soffit vents on the front of the house? ( opposite the falt roof side) and then some form of tile ventilator on the pitched roof side just above the flat roof itself. Sketch 2
In the future if you want to change the pitched roof to a habitable space, and make the roof insulated at rafter level, you may not need the vents, depending on how you insulate. But for now if you have quilt type insulation at ceiling level in the pitched roof area of the house, you do need air flow over the top of it.
As Wolf stated below, it does appear you builder has realised his error, so seems reasonable to allow him to rectify his work and move your project along.
When the builder replaced the pitched roof soffits and gutters he also installed vent and felt support trays, so the pitched roof should be taken care of, will just need the vent tiles to the rear of the pitched roof.
Mistakes can be made, you can tell a true professional by how they deal with there mistakes.
Mistakes can be made, you can tell a true professional by how they deal with there mistakes.
mikebradford said:
With regard to the main pitched roof, id be happy as long as you have some form of soffit vent on opposite sides of the roof.
As my diagram below sketch 1.
The vents dont need to be on the same side as the flat roof.
You simply need them to be opposite each other such that the positive and negative air pressure means that air is drawn through the roof structure.
Depending on the insulation in the existing pitched roof, their may be a little bit of hassle ensuring the vents are not blocked by any insulation you have in there.
Alternatively, you can have traditional soffit vents on the front of the house? ( opposite the falt roof side) and then some form of tile ventilator on the pitched roof side just above the flat roof itself. Sketch 2
In the future if you want to change the pitched roof to a habitable space, and make the roof insulated at rafter level, you may not need the vents, depending on how you insulate. But for now if you have quilt type insulation at ceiling level in the pitched roof area of the house, you do need air flow over the top of it.
As Wolf stated below, it does appear you builder has realised his error, so seems reasonable to allow him to rectify his work and move your project along.
IME that design doesn't work well. You have the stack vent effect which results in stagnant areas at eaves level on the opposing unvented eaves. Given the OP has vents on the other three sides it makes sense to add vents on the blocked sideAs my diagram below sketch 1.
The vents dont need to be on the same side as the flat roof.
You simply need them to be opposite each other such that the positive and negative air pressure means that air is drawn through the roof structure.
Depending on the insulation in the existing pitched roof, their may be a little bit of hassle ensuring the vents are not blocked by any insulation you have in there.
Alternatively, you can have traditional soffit vents on the front of the house? ( opposite the falt roof side) and then some form of tile ventilator on the pitched roof side just above the flat roof itself. Sketch 2
In the future if you want to change the pitched roof to a habitable space, and make the roof insulated at rafter level, you may not need the vents, depending on how you insulate. But for now if you have quilt type insulation at ceiling level in the pitched roof area of the house, you do need air flow over the top of it.
As Wolf stated below, it does appear you builder has realised his error, so seems reasonable to allow him to rectify his work and move your project along.
Gassing Station | Homes, Gardens and DIY | Top of Page | What's New | My Stuff