Hi, my first post after having tried to get logged on, but here now.
I am in the UK and started a garden music arts room project at the beginning of march 10n concrete cavity walls on block and beam floor.
The building is under permitted development and near boundary so max 2.5m height restriction.
my original design was for a warm roof rubber, 18mm OSB , 100mm cellotex , waterproof membrane, 18mm osb which projected over the outside wall , which has been built up to 10mm below the OSB, sealed with acoustic foam , then a layer of green glue bonding to another sheet of 18mm OSB which is sat on 175 x 50 joists , sat on the wall plate which is bedded on neoprene rubber strip , I was planning to concrete block in the gaps between the joists which are cut back to the edge of the plate and seal with acoustic foam and add 50mm rw5 between the joists , isolation clips then 1 layer of 15mm knauf sound board , MLV and 12.5mm knauf sound board.
Since pressing go and starting covid hit and my career as a function performer has ceased to be viable in the medium term so I have refocused on using the building for recording vocal against track, band rehearsal/ recording and use of the control room for video conference and voice over work, so the importance for the building to be sound proofed from passing jets from the airport, police helicopters and barking dogs is even more important , also the building needs to be capable of live band sessions with minimal sound spill rather than just the voice over track and mixing.
I want to add 50mm of Rw5 or rw3 or ideally 100mm between the joists , and with a view to adding an intensive 50mm green roof on top at a later stage should I need it/ can afford it.
But then CONDENSATION! I was aware that there is generally accepted balance of 2/3rds of the u value above and 1/3rd below, which although none of the manufactures seem to want to recommend a Hybrid roof anyway but adding extra U below the makes the risk of condensation very high apparently.#
I was sure I saw a roof either on here or in rods book about a vented flat roof made to meet code but can't seem to find it.
If it was vented , due to my almost parapet outer walls I was thinking of venting with mushrooms
So everything has ground to a halt due to the roof , with what to do , so
Does anyone have any reliably condensation safe warm roof advise designs or formula that can get me going again ?
If anyone could help I would really appreciate it
Thanks Paul
Urgent Flat warm roof design advice needed
Moderators: Aaronw, kendale, John Sayers
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PHolgate
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- Location: Chertsey Surrey England
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Waka
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Re: Urgent Flat warm roof design advice needed
Hi Paul,
Where are you located in the UK? Also could you include your location in your profile please (forum rules).
I've got a few thoughts for you. You say you've started building the studio, how far along are you? Any pics you can share?
If you haven't already built the roof I would stop immediately and rethink the roof plan.
You're almost certainly never going to get the head room required for a sound isolated studio in 2.4m outer height (I don't even think you get 2.5m under permitted dev near the border!).
It might be simpler for you to just get planning permission for a higher roof line (costs about £200).
Anyway on to the roof design. I didn't go with a "hybrid" warm roof for just the reasons you stated. You will need multiple layers of insulation below the roof line between the joists and another layer below the ceiling level to treat flutter echo and reduce decay rate. Not to mention probably a cloud above the listening position (for when recording), which usually also has insulation attached to it.
This has way higher a u value than above your roof deck.
I went for a traditional vented cold roof with 25mm2 continuous soffit vent strips along both long sides. The entire roof does almost nothing at all for sound isolation (being vented). So you need to board the bottom of the roof rafters/joists and bulk up everything below this level.
I went full room-within-a-room, which meant another set of studs and joists placed within the room. and therefore a 3 leaf roof design. This is the only viable way to isolate a vented roof.
I have a flat roof design, I have family I had to negotiate with in order to even get the height I got (Me and my wife, share a house with in-laws). My outer height to the roof deck is 3.2m sloping down to 3.1m. I have an inside out ceiling design to conserve acoustic height in the room and that puts the internal acoustic height at just over 2.5m and the internal visual height at just over 2.3m.
Can you now see how 2.4m outer height is crazy if you want isolation? You wont be able to stand in the room. Without a room in room design you can't go vented roof and still hope to achieve any isolation at all at low frequencies. You said a band rehearsal? My band puts out intermittently 100dB easily when rehearsing, those snares are harsh!
How far away from the neighbours, your own house is the studio plot? Mine is 8m from the nearest window (my own bedroom) and you can play 115dB rock songs in the studio and with the window wide open it sounds like a whisper. I'm only in reality getting 45dBs of isolation at very low frequencies right aoutside the doors, at mid to high frequencies the studio achieves 60dB isolation and up.
That's only with a timber framed studio. I couldn't think of a way to use bricks/blocks in the UK without having to vent the wall cavity (for condensation reasons, to meet building regs), meaning you would have to view a cavity wall (block inside and brick outside as a single leaf, and then build a third leaf within it.
You mention that you're sealing using acoustic foam, what do you mean? Is that like expanding foam? Because if so, it's really not great for what you're using it for. Sealing should be done using backer rod pushed into the gaps and then applying flexible silicone sealant (if you don't need to paint it) or an acoustic caulk/flexible acrylic sealant if you do need to paint over it.
Green glue will be an expensive waste of time if you don't have the required mass and sealed cavities all around the room (ceiling, roof, door included).
Do you have a plan for ventilation? Cooling/heating? A sealed room doesn't passively cool or ventilate, so all the condensation has nowhere to go, you need to actively ventilate the room (extractor fan and intake vent), these require specially designed and built huge silencer boxes.
The room will get HOT!, you almost certainly need a split air-con unit. You might be able to get away with a large ventilation fan for a while, but as soon as more than one person gets in the room (and 3-4 musicians!!), it will get boiling hot and stuffy in minutes. Opening the door to breath every 10 minutes seems to defeat the purpose of sound isolation right??
So all in all, I know this sounds like a huge list of things that need considering (it is!), unless you have thought about each one of the things above (and quite a few more!) you will be disappointed if you expect to get good sound isolation.
In short, I would do a three leaf cold roof, but as I described above, the roof is by far not the biggest of your concerns here.
Dan
Where are you located in the UK? Also could you include your location in your profile please (forum rules).
I've got a few thoughts for you. You say you've started building the studio, how far along are you? Any pics you can share?
If you haven't already built the roof I would stop immediately and rethink the roof plan.
You're almost certainly never going to get the head room required for a sound isolated studio in 2.4m outer height (I don't even think you get 2.5m under permitted dev near the border!).
It might be simpler for you to just get planning permission for a higher roof line (costs about £200).
Anyway on to the roof design. I didn't go with a "hybrid" warm roof for just the reasons you stated. You will need multiple layers of insulation below the roof line between the joists and another layer below the ceiling level to treat flutter echo and reduce decay rate. Not to mention probably a cloud above the listening position (for when recording), which usually also has insulation attached to it.
This has way higher a u value than above your roof deck.
I went for a traditional vented cold roof with 25mm2 continuous soffit vent strips along both long sides. The entire roof does almost nothing at all for sound isolation (being vented). So you need to board the bottom of the roof rafters/joists and bulk up everything below this level.
I went full room-within-a-room, which meant another set of studs and joists placed within the room. and therefore a 3 leaf roof design. This is the only viable way to isolate a vented roof.
I have a flat roof design, I have family I had to negotiate with in order to even get the height I got (Me and my wife, share a house with in-laws). My outer height to the roof deck is 3.2m sloping down to 3.1m. I have an inside out ceiling design to conserve acoustic height in the room and that puts the internal acoustic height at just over 2.5m and the internal visual height at just over 2.3m.
Can you now see how 2.4m outer height is crazy if you want isolation? You wont be able to stand in the room. Without a room in room design you can't go vented roof and still hope to achieve any isolation at all at low frequencies. You said a band rehearsal? My band puts out intermittently 100dB easily when rehearsing, those snares are harsh!
How far away from the neighbours, your own house is the studio plot? Mine is 8m from the nearest window (my own bedroom) and you can play 115dB rock songs in the studio and with the window wide open it sounds like a whisper. I'm only in reality getting 45dBs of isolation at very low frequencies right aoutside the doors, at mid to high frequencies the studio achieves 60dB isolation and up.
That's only with a timber framed studio. I couldn't think of a way to use bricks/blocks in the UK without having to vent the wall cavity (for condensation reasons, to meet building regs), meaning you would have to view a cavity wall (block inside and brick outside as a single leaf, and then build a third leaf within it.
You mention that you're sealing using acoustic foam, what do you mean? Is that like expanding foam? Because if so, it's really not great for what you're using it for. Sealing should be done using backer rod pushed into the gaps and then applying flexible silicone sealant (if you don't need to paint it) or an acoustic caulk/flexible acrylic sealant if you do need to paint over it.
Green glue will be an expensive waste of time if you don't have the required mass and sealed cavities all around the room (ceiling, roof, door included).
Do you have a plan for ventilation? Cooling/heating? A sealed room doesn't passively cool or ventilate, so all the condensation has nowhere to go, you need to actively ventilate the room (extractor fan and intake vent), these require specially designed and built huge silencer boxes.
The room will get HOT!, you almost certainly need a split air-con unit. You might be able to get away with a large ventilation fan for a while, but as soon as more than one person gets in the room (and 3-4 musicians!!), it will get boiling hot and stuffy in minutes. Opening the door to breath every 10 minutes seems to defeat the purpose of sound isolation right??
So all in all, I know this sounds like a huge list of things that need considering (it is!), unless you have thought about each one of the things above (and quite a few more!) you will be disappointed if you expect to get good sound isolation.
In short, I would do a three leaf cold roof, but as I described above, the roof is by far not the biggest of your concerns here.
Dan
Stay up at night reading books on acoustics and studio design, learn Sketchup, bang your head against a wall, redesign your studio 15 times, curse the gods of HVAC silencers and door seals .... or hire a studio designer.
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PHolgate
- Posts: 4
- Joined: Tue Jul 14, 2020 3:52 am
- Location: Chertsey Surrey England
Re: Urgent Flat warm roof design advice needed
Hi Dan
Thank you for your reply, really appreciate you taking the time.
Updated my location, know thanks for letting me know the rule, i'm in surrey.
Well, the cavity block walls are built, (I checked , no ventilation is required for regs just under the floor) internal leaf 1 built to plate with neoprene strip underneath plate, joists are on fitted flush top and the outside leaf to 5mm below the top of the joists all the way around ready to seal under the deck ( thanks for the tip about backer rod, wasn't aware of that).
Due to family time demands I need to get the building shell watertight in a few weeks so I am constrained within the permitted development 2.5m height (it is allowed within 2 meters) . I understand there will be compromises due to the height but I have no choice but to be pragmatic and live with the shortcomings but hope to maximise any advise i can get meanwhile.
The building is at the end of a 30meter garden backing on to another 25 metre ish garden before the houses behind so a reasonable spill outwards would be OK , the band sessions would probably more fun with the kids TBH, my days of screaming rock and pop are over , if I work with a band myself it will probably more jazz and swing , few horns maybe rather than anything too heavy, so I guess I just need to scale back my expectation to meet reality
Most of the commercial work I think will be voice against track plus some use for voiceover and video broadcast so probably more important will be the keeping out of noise from Heathrow air traffic , helicopters , m25 , dogs , kids.....than the noise we produce.
The building will be room within a room with the ceiling sealing with the inner block wall and the outer roof sealed with the outer wall leaf 2, floating concrete floor. I envisage a fair amount of acoustic treatment and traps , clouds, panels to make the rooms more pleasant.
Ventilation will be boxed and baffled fresh air inlet , fan and vented baffled exhaust , probably a split twin A/C further down the line but fresh air for now.
Roof wise given the fixed height, I am open to a vented cold roof but with the problem of spill from the vents land the problem of having a trimmer beam cutting off air flow due to the L shape this led me to the idea of a sealed hybrid warm roof with heavy deck and ceiling , (obviously loosing of 100mm plus of acoustically worthless cellotex) but sealed nether the less being the more practical option so far.
The decision seems what to put in the void that baffle and dissipate but not cause internal condensation due to the between joist insulation upsetting the dew point.
I saw a design somewhere , might have been one of Rons, but i cant seem to find it now , it was of a cold roof, vented filled with insulation and mass then a light 3rd leaf vented top, is something like that what you have ? . If there was a good cold roof design that would work with 350mm of space i could still be tempted to change if it would provide a better solution , failing that looks like i'm going to the dark side (warm room) . Paulus from wales has given me some great clarification and advise which I am looking to implement.
I have a few days left to collate any advise and then i'm going to have to bite the bullet one way or another.
Thanks again for your reply, really helpful getting everyones different view points and yours has been a reality checker on my expectations.
Kind regards
Paul
Thank you for your reply, really appreciate you taking the time.
Updated my location, know thanks for letting me know the rule, i'm in surrey.
Well, the cavity block walls are built, (I checked , no ventilation is required for regs just under the floor) internal leaf 1 built to plate with neoprene strip underneath plate, joists are on fitted flush top and the outside leaf to 5mm below the top of the joists all the way around ready to seal under the deck ( thanks for the tip about backer rod, wasn't aware of that).
Due to family time demands I need to get the building shell watertight in a few weeks so I am constrained within the permitted development 2.5m height (it is allowed within 2 meters) . I understand there will be compromises due to the height but I have no choice but to be pragmatic and live with the shortcomings but hope to maximise any advise i can get meanwhile.
The building is at the end of a 30meter garden backing on to another 25 metre ish garden before the houses behind so a reasonable spill outwards would be OK , the band sessions would probably more fun with the kids TBH, my days of screaming rock and pop are over , if I work with a band myself it will probably more jazz and swing , few horns maybe rather than anything too heavy, so I guess I just need to scale back my expectation to meet reality
Most of the commercial work I think will be voice against track plus some use for voiceover and video broadcast so probably more important will be the keeping out of noise from Heathrow air traffic , helicopters , m25 , dogs , kids.....than the noise we produce.
The building will be room within a room with the ceiling sealing with the inner block wall and the outer roof sealed with the outer wall leaf 2, floating concrete floor. I envisage a fair amount of acoustic treatment and traps , clouds, panels to make the rooms more pleasant.
Ventilation will be boxed and baffled fresh air inlet , fan and vented baffled exhaust , probably a split twin A/C further down the line but fresh air for now.
Roof wise given the fixed height, I am open to a vented cold roof but with the problem of spill from the vents land the problem of having a trimmer beam cutting off air flow due to the L shape this led me to the idea of a sealed hybrid warm roof with heavy deck and ceiling , (obviously loosing of 100mm plus of acoustically worthless cellotex) but sealed nether the less being the more practical option so far.
The decision seems what to put in the void that baffle and dissipate but not cause internal condensation due to the between joist insulation upsetting the dew point.
I saw a design somewhere , might have been one of Rons, but i cant seem to find it now , it was of a cold roof, vented filled with insulation and mass then a light 3rd leaf vented top, is something like that what you have ? . If there was a good cold roof design that would work with 350mm of space i could still be tempted to change if it would provide a better solution , failing that looks like i'm going to the dark side (warm room) . Paulus from wales has given me some great clarification and advise which I am looking to implement.
I have a few days left to collate any advise and then i'm going to have to bite the bullet one way or another.
Thanks again for your reply, really helpful getting everyones different view points and yours has been a reality checker on my expectations.
Kind regards
Paul
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Paulus87
- Senior Member
- Posts: 652
- Joined: Sun Mar 02, 2014 8:42 am
- Location: Wales, UK
Re: Urgent Flat warm roof design advice needed
I thought I'd reply on here instead of your message since it may be easier to keep track if everything is in the same thread.
Firstly, if you decide on a cold roof design then the open vents will not penetrate the double leaf assembly. The roof is a "third leaf" but not really, as it is full of holes and is not sealed up air tight, it is just a rain screen to keep your double leaf assembly dry.
You could design is like this drawing I've done, this is for a pitched roof but the same would apply for a flat roof except there would be no ridge vent obviously:
If you decide to stay with a warm roof design then that'll also work fine, you're limited with height anyway which means you really aren't going to be able to fit that much thick treatment in your ceiling. I would underfill the cavity between the two leaves in the roof/ceiling so that the cavity insulation doesn't thermally outperform the over-deck insulation. Then for your acoustic treatment inside the space you could hang a cloud over the mix position and call it good. Leave a few inches of space between it and your hard ceiling, and you could angle it if you have enough head height left so help deflect first reflections away.
Hope that helps,
Paul
Firstly, if you decide on a cold roof design then the open vents will not penetrate the double leaf assembly. The roof is a "third leaf" but not really, as it is full of holes and is not sealed up air tight, it is just a rain screen to keep your double leaf assembly dry.
You could design is like this drawing I've done, this is for a pitched roof but the same would apply for a flat roof except there would be no ridge vent obviously:
If you decide to stay with a warm roof design then that'll also work fine, you're limited with height anyway which means you really aren't going to be able to fit that much thick treatment in your ceiling. I would underfill the cavity between the two leaves in the roof/ceiling so that the cavity insulation doesn't thermally outperform the over-deck insulation. Then for your acoustic treatment inside the space you could hang a cloud over the mix position and call it good. Leave a few inches of space between it and your hard ceiling, and you could angle it if you have enough head height left so help deflect first reflections away.
Hope that helps,
Paul
Paul
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PHolgate
- Posts: 4
- Joined: Tue Jul 14, 2020 3:52 am
- Location: Chertsey Surrey England
Re: Urgent Flat warm roof design advice needed
Hi Paul , Thanks for the info and design, yes this looks like the kind of design I saw before.
OK so.. I could with a bit of faffing about with the wall heights arrange for a cold roof with a 50mm vented top ( i have a trimmer in the middle so with need to still vent at the top of the firing end I would guess . if I cross baton in 25 mm over 25mm that would allow ventilation across the vent space.
could i still use my 18mm OSB as the Top leaf mass ? ( i have enough green glue to put in between 2 of the sheet top leaf and 2 sheets of leaf 1)
Layers from in to out 12.5 sound block , green glue, 12.5 sound block , 15mm sound block on joists supported on leaf 1 walls , then 320mm void with insulation. 18mm oSB , green glue , 18mm OSB , 18mm OSB then 25mm baton and 25mm baton opposing , 18mm OSB and rubber .
I have put info into gregwor calculator for both warm roof and cold roof and I get the attached result
I would be easier to do the warm roof at this stage due to the block work being more or less the right height but if there was a definite benefit to the cold roof design I might switch
I assume I have entered correctly and do I also assume that the cold roof is better as it is a smooth graph? will the lower frequency dip as shown on the be that noticeable in reality? What is the significance of the dip , will it mean there will be a difference within the room or just means it will pass more of that frequency?
Would be interested in an expert interpretation.
Thanks Paul
OK so.. I could with a bit of faffing about with the wall heights arrange for a cold roof with a 50mm vented top ( i have a trimmer in the middle so with need to still vent at the top of the firing end I would guess . if I cross baton in 25 mm over 25mm that would allow ventilation across the vent space.
could i still use my 18mm OSB as the Top leaf mass ? ( i have enough green glue to put in between 2 of the sheet top leaf and 2 sheets of leaf 1)
Layers from in to out 12.5 sound block , green glue, 12.5 sound block , 15mm sound block on joists supported on leaf 1 walls , then 320mm void with insulation. 18mm oSB , green glue , 18mm OSB , 18mm OSB then 25mm baton and 25mm baton opposing , 18mm OSB and rubber .
I have put info into gregwor calculator for both warm roof and cold roof and I get the attached result
I would be easier to do the warm roof at this stage due to the block work being more or less the right height but if there was a definite benefit to the cold roof design I might switch
I assume I have entered correctly and do I also assume that the cold roof is better as it is a smooth graph? will the lower frequency dip as shown on the be that noticeable in reality? What is the significance of the dip , will it mean there will be a difference within the room or just means it will pass more of that frequency?
Would be interested in an expert interpretation.
Thanks Paul
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Paulus87
- Senior Member
- Posts: 652
- Joined: Sun Mar 02, 2014 8:42 am
- Location: Wales, UK
Re: Urgent Flat warm roof design advice needed
I am not completely following your proposed assembly, do you have sketch up? Could you draw it out and label it clearly?PHolgate wrote:Hi Paul , Thanks for the info and design, yes this looks like the kind of design I saw before.
OK so.. I could with a bit of faffing about with the wall heights arrange for a cold roof with a 50mm vented top ( i have a trimmer in the middle so with need to still vent at the top of the firing end I would guess . if I cross baton in 25 mm over 25mm that would allow ventilation across the vent space.
could i still use my 18mm OSB as the Top leaf mass ? ( i have enough green glue to put in between 2 of the sheet top leaf and 2 sheets of leaf 1)
Layers from in to out 12.5 sound block , green glue, 12.5 sound block , 15mm sound block on joists supported on leaf 1 walls , then 320mm void with insulation. 18mm oSB , green glue , 18mm OSB , 18mm OSB then 25mm baton and 25mm baton opposing , 18mm OSB and rubber .
I have put info into gregwor calculator for both warm roof and cold roof and I get the attached result
I would be easier to do the warm roof at this stage due to the block work being more or less the right height but if there was a definite benefit to the cold roof design I might switch
I assume I have entered correctly and do I also assume that the cold roof is better as it is a smooth graph? will the lower frequency dip as shown on the be that noticeable in reality? What is the significance of the dip , will it mean there will be a difference within the room or just means it will pass more of that frequency?
Would be interested in an expert interpretation.
Thanks Paul
As to the pros/cons of a warm roof vs cold roof, well they both can work fine, one is not necessarily better than the other acoustically in my opinion.
Paul
Paul
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PHolgate
- Posts: 4
- Joined: Tue Jul 14, 2020 3:52 am
- Location: Chertsey Surrey England
Re: Urgent Flat warm roof design advice needed
Thanks Paul
You have answered my question really, the walls are set up for an un vented warm roof and as there doesn't seem to be much in it for I'm going to go warm.
I'm taking your advice on going straight over the insulation , does the rubber still need to be glued to the insulation for the main bulk of the roof , ( I've got the foil faced stuff) or can I just lay it on and contact glue around the edges? If I use the 50mm Rw5 I have already there doesn't seem to be a condensation risk , would you recommend increasing the acoustic insulation more that that with the 230mm void? is there a rule of thumb. On the condensation calculator if looks like 75mm would still be just about ok apart from -8 outside which is pretty rare nowadays, but if I went to 100 I would need to increase to 120 celotex on top.
Again if you think 50mm of 75 mm would be ok I'll leave it at that and save the cost of the 120 stuff.
I've seen two different ways to lay the vapour barrier on the deck, one is the wrap the layer up and around the cellotex by about 6" and taped to the foil face and another guy just lay it flat on the deck and trimmed off at the edges , pretty sure the 1st way is correct but I would like to get it right.
I'm so glad I can get started, thanks for your help, just need to decide on the amount between the joists
Cheers Paul
You have answered my question really, the walls are set up for an un vented warm roof and as there doesn't seem to be much in it for I'm going to go warm.
I'm taking your advice on going straight over the insulation , does the rubber still need to be glued to the insulation for the main bulk of the roof , ( I've got the foil faced stuff) or can I just lay it on and contact glue around the edges? If I use the 50mm Rw5 I have already there doesn't seem to be a condensation risk , would you recommend increasing the acoustic insulation more that that with the 230mm void? is there a rule of thumb. On the condensation calculator if looks like 75mm would still be just about ok apart from -8 outside which is pretty rare nowadays, but if I went to 100 I would need to increase to 120 celotex on top.
Again if you think 50mm of 75 mm would be ok I'll leave it at that and save the cost of the 120 stuff.
I've seen two different ways to lay the vapour barrier on the deck, one is the wrap the layer up and around the cellotex by about 6" and taped to the foil face and another guy just lay it flat on the deck and trimmed off at the edges , pretty sure the 1st way is correct but I would like to get it right.
I'm so glad I can get started, thanks for your help, just need to decide on the amount between the joists
Cheers Paul
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Paulus87
- Senior Member
- Posts: 652
- Joined: Sun Mar 02, 2014 8:42 am
- Location: Wales, UK
Re: Urgent Flat warm roof design advice needed
Can I suggest you first check if building regs still allow the rubber to be stuck directly over the insulation without a solid surface on top? I know it certainly used to be ok but building regs often change, so double check that.PHolgate wrote:Thanks Paul
You have answered my question really, the walls are set up for an un vented warm roof and as there doesn't seem to be much in it for I'm going to go warm.
I'm taking your advice on going straight over the insulation , does the rubber still need to be glued to the insulation for the main bulk of the roof , ( I've got the foil faced stuff) or can I just lay it on and contact glue around the edges? If I use the 50mm Rw5 I have already there doesn't seem to be a condensation risk , would you recommend increasing the acoustic insulation more that that with the 230mm void? is there a rule of thumb. On the condensation calculator if looks like 75mm would still be just about ok apart from -8 outside which is pretty rare nowadays, but if I went to 100 I would need to increase to 120 celotex on top.
Again if you think 50mm of 75 mm would be ok I'll leave it at that and save the cost of the 120 stuff.
I've seen two different ways to lay the vapour barrier on the deck, one is the wrap the layer up and around the cellotex by about 6" and taped to the foil face and another guy just lay it flat on the deck and trimmed off at the edges , pretty sure the 1st way is correct but I would like to get it right.
I'm so glad I can get started, thanks for your help, just need to decide on the amount between the joists
Cheers Paul
Then I would suggest you get on youtube and search for an uploader called "Steve Roofer" he is a UK expert in warm roof installation. He has many videos on the subject is is very approachable if you have any questions.
Rule of thumb for cavity insulation for maximum isolation is a full fill but not compressed. In your case, just do as suggested by the condensation calculator. There is no point doing a full fill if it's going to risk condensation, and if this means putting in less than a full fill then so be it.
Paul
Paul