Insulation below a warm flat roof

[ProDave]

On 20/10/2020 at 09:12, Big Jimbo said:

I believe, but am happy to be corrected, If you have 100mm above, you can have 100mm inbetween, pushed up tight. I was told that you can't have more inside, than you have outside, and you should always try to have less. ie;100mm outside, 50mm inside. To be honest i thought it sounded strange, but was given this information from an engineer. I think it might have been BS to get over a problem.

Our warm roof has 100mm above the joists, and 200mm between the joists (full fill) and passed a condensation analysis.

[Moonshine]

1 hour ago, ProDave said:

Our warm roof has 100mm above the joists, and 200mm between the joists (full fill) and passed a condensation analysis.

 

What is the insulation and thermal performance? 

[PeterW]

1 hour ago, ProDave said:

Our warm roof has 100mm above the joists, and 200mm between the joists (full fill) and passed a condensation analysis.

Is there a sarking or OSB layer in there somewhere ..? 

[ProDave]

10 minutes ago, PeterW said:

Is there a sarking or OSB layer in there somewhere ..? 

The 100mm wood fibre on top of the joists is the sarking layer, then breathable membrane, counter battens, battens then tiles.

 

Full fill insulation between the joists, OSB racking layer on the inside, air tight membrane, service void, plasterboard.

 

An unusual build up with the racking layer on the inside (same with the walls) plenty of people told me I had put the frame up inside out.

[Temp]

Would it pass without the breathable membrane? I mean with it replaced by a walk on layer.

Edited October 24, 2020 by Temp

[Temp]

Could the OP "just" lower all the joists? Are the bolted oto or set into the walls?

[PeterW]

1 hour ago, ProDave said:

The 100mm wood fibre on top of the joists is the sarking layer, then breathable membrane, counter battens, battens then tiles.

 

Full fill insulation between the joists, OSB racking layer on the inside, air tight membrane, service void, plasterboard.

 

An unusual build up with the racking layer on the inside (same with the walls) plenty of people told me I had put the frame up inside out.

Ok so that is a different type of construction and not a classic warm flat roof. The OP has non permeable insulation as part of the build up. 

[ADLIan]

That’s the issue. Flat roof vs pitched roof, very different design scenarios.

[revelation]

7 hours ago, PeterW said:

But if you raise the deck then it is flush to the edge of the glass again ..??

 

You say the doors have already been

made. What’s the height difference between the bottom of the door cill and the top of the posi joists ..?? Assuming they are not sitting directly on those steels ..??

 

 

Hi Peter, Yes we can raise the deck but then it comes higher than the doorways, there is some brick/block-work to go on-top of the steels.  I would ideal love to have everything flush as soon as I walk out of the door.  So the current planned make up was giving us a perfectly flush finish.  All the suggested solutions of putting insulation below, including spray foam haven't had any reassuring feedback.  So it looks like we have no choice but to increase the insulation above.  Then have a small step as you come out of the door, ideally its not something we wanted to do though.

[revelation]

5 hours ago, PeterW said:

Just be wary of railing heights if that is an accessible roof. There looks to be 3 1/2 courses of blocks so that parapet is 780mm less any allowance for the insulation and decking. To meet regs it will need to be a minimum of 1100mm so you either need another 2 courses of blocks or some type of railing on the top of that wall. 

 

We will be getting a glass rail put in at a later date, we didn't initially apply for planning to use it as accessible roof.  So will look to have that done once that's approved.

[revelation]

3 hours ago, Temp said:

Could the OP "just" lower all the joists? Are the bolted oto or set into the walls?

 

Unfortunately all the joists have been set in place and bricked in, we also have set of large sliding doors in the kitchen below which are effectively floor to ceiling (approx 2500mm)  so even if we wanted to lower inside we couldn't really do that.

[PeterW]

1 minute ago, revelation said:

 

We will be getting a glass rail put in at a later date, we didn't initially apply for planning to use it as accessible roof.  So will look to have that done once that's approved.

OK so Building Control may require you to have lockable doors with a restraint bar if that’s the case as it will fail building regs. Better to get the planning approval now for use and then do it all in one go. 
 

 

[PeterW]

6 minutes ago, revelation said:

 

Hi Peter, Yes we can raise the deck but then it comes higher than the doorways, there is some brick/block-work to go on-top of the steels.  I would ideal love to have everything flush as soon as I walk out of the door.  So the current planned make up was giving us a perfectly flush finish.  All the suggested solutions of putting insulation below, including spray foam haven't had any reassuring feedback.  So it looks like we have no choice but to increase the insulation above.  Then have a small step as you come out of the door, ideally its not something we wanted to do though.

ok - can you fill in the gaps on this picture ..?

 

Depth to the bottom of the doors/step

Depth of upstand

Depth of Glazing 

 

[Temp]

This doc is interesting. I've just picked out one point below... It recommends putting insulation above the waterproofing layer which is a new one to me.

 

https://www.buildingscience.com/documents/insights/bsi-051-decks-roofs-you-can-walk-on

 

Where is, absolutely, the best place to add insulation? We know this answer already from the “Perfect Wall.”3  On the top of the structural deck and over the top of the water control layer, the air control layer and the vapor control layer. Take Max Baker’s Figure 1 and add the insulation in the right place and you get Figure 3.

 

 

 

[Temp]

And (although I think this is for the USA) it looks like spray foam might be the way to go after all...

 

Quote

 

What if you can’t insulate on the top of the deck and have to insulate underneath the deck? It happens, especially when you do not have space on the top of the deck for the insulation layer. Those darn architects again, not giving the rest of us space to make things work for them.

Snip

You have to go to spray foam.

Snip

With spray polyurethane foam the problem of condensation goes away. But, which type of spray foam? The open cell or the closed cell? The closed cell works pretty much everywhere. The open cell only in the South. So what is the definition of the South? Go to the International Residential Code (IRC) for their definition of South. South means Climate Zones 4 and lower. It has nothing to do with the Civil War. At least I don’t think it does.
 

 

 

 

 

I'd still put a good VCL below the joists before plasterboard.

 

 

 

Edited October 24, 2020 by Temp

[revelation]

7 hours ago, PeterW said:

ok - can you fill in the gaps on this picture ..?

 

Depth to the bottom of the doors/step

Depth of upstand

Depth of Glazing 

 

 

The step is approximately 250mm, I don't know the exact height of the upstand and the glazing, but where the glazing finishes is level with where the step comes (so approximately 250mm).   The glazing has attached frame which comes over the upstand so as the insulation increases so does the starting point of the upstand.  I have attached some images if the roof light on a previous post.

[revelation]

42 minutes ago, Temp said:

And (although I think this is for the USA) it looks like spray foam might be the way to go after all...

 

 

 

 

I'd still put a good VCL below the joists before plasterboard.

 

 

 

 

Thats really interesting, someone else had advised that closed cell spray foam may work on the bottom as it will expand and fill in any gaps therefore there isn't anywhere for condensation to form.  I have mentioned it as an option but so far no one else that has replied has said its a good idea or will definitely work.  

[Temp]

Worth thinking about how you would find leaks or dry out the roof if its all encased in foam.

[PeterW]

So I’m not sure of the issue ..?? 
 

12mm OSB Sheathing

Breather membrane

160mm PIR

18mm OSB T&G 

GRP 

 

That takes you to 195mm ish leaving 55mm or a little higher for decking and support. That could be as simple as 18mm Deckboards on  60x30 framing sat on 10-20mm adjustable deck pads. 
 

That gets you to min regs and a full warm roof build up. 
 

 

 

 

[scottishjohn]

18 minutes ago, revelation said:

 I have mentioned it as an option but so far no one else that has replied has said its a good idea or will definitely work.  

it will work   as its totally water proof 

 quite popular now to spray on insdie of old stone barns to make them water proof to the insdie and let it breath out tghrough the lime mortar .

also seen it used as water proofing on a basement on outside of walls 

 

Edited October 24, 2020 by scottishjohn

[Temp]

10 minutes ago, PeterW said:

So I’m not sure of the issue ..?? 
 

12mm OSB Sheathing

Breather membrane

160mm PIR

18mm OSB T&G 

GRP 

 

That takes you to 195mm ish leaving 55mm or a little higher for decking and support. That could be as simple as 18mm Deckboards on  60x30 framing sat on 10-20mm adjustable deck pads. 
 

That gets you to min regs and a full warm roof build up. 

 

That would require the rooflights to be raised up above the deck. They say the upstand  should be 150mm. Might be possible to have a "trench" around the roof light but it would need draining.

[Temp]

13 minutes ago, scottishjohn said:

it will work   as its totally water proof 

 quite popular now to spray on insdie of old stone barns to make them water proof to the insdie and let it breath out tghrough the lime mortar .

also seen it used as water proofing on a basement on outside of walls 

 

 

I've also seen dire warnings from conservation groups about problems with doing this to roofs.

[PeterW]

11 minutes ago, Temp said:

 

That would require the rooflights to be raised up above the deck. They say the upstand  should be 150mm. Might be possible to have a "trench" around the roof light but it would need draining.

I thought that but the units are 45mm thick so with an overlap they may need a small trench but that’s no bad think to move water off the deck. 
 

other option is reduce to 150mm or lose the 12mm sheathing and go direct onto the joists but that takes balls to work on top of as there is nothing to stop the boards breaking over the joists. 
 

if the doors have a standard threshold then there is another 35mm there to play with. 

[revelation]

1 hour ago, scottishjohn said:

it will work   as its totally water proof 

 quite popular now to spray on insdie of old stone barns to make them water proof to the insdie and let it breath out tghrough the lime mortar .

also seen it used as water proofing on a basement on outside of walls 

 

 

So if I have the amount of insulation I said in the original post and spray foam insulation underneath will that work as far as building regs go and no issues with condensation?

[scottishjohn]

1 hour ago, Temp said:

I've also seen dire warnings from conservation groups about problems with doing this to roofs.

that will be a roof that is not meant to be made sealed--like sarking boards  and slates

 we are dealing with a different animal here 

conservation groups don,t work with modern sealed  roof systems