Insulation below a warm flat roof

[revelation]

1 hour 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 think our choices are effectively either the spray foam if I feel reassured by people that it won't cause any condensation issues.

 

Or to have a small step up next to the door ways and the rest of the decking to be higher but flush to the glass.  If we did this the max insulation we could get is 120mm, so we would still need to put some below I assume to get to building regs?

[revelation]

1 hour ago, PeterW said:

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. 

 

Peter, we have gone through all the measurements and maths on this.   We are restricted more than anything by the glass and its own frame around it, that has to sit around 80mm above the final roof finish.  So an increase in insulation means a raising of the glass.  The original plan of 50mm insulation was leaving us with the glass and decking all running flush (almost level with the door sill).   

 

Ideal Scenario is to be able to used closed cell spray foam underneath, what are your thoughts on this?

 

Next best thing, we have a small step up next to the doors and the rest of the decking sits higher but flush with the glass.  That will allow us to have around 120mm of insulation (We will probably still need more below?)

 

[PeterW]

7 hours ago, revelation said:

We are restricted more than anything by the glass and its own frame around it, that has to sit around 80mm above the final roof finish.  So an increase in insulation means a raising of the glass.  The original plan of 50mm insulation was leaving us with the glass and decking all running flush (almost level with the door sill).  

I think you’re missing the point here. 
 

You have 250mm, maybe even 270mm if the door has a cill as you can run the decking level will the cill. 
 

even if the glazing needs 80mm clear space then you have 170mm below your top surface at 250mm, or 190mm if you can get the deck up to cill height. 
 

That allows for 150-160mm of insulation above the deck as designed if you go with GRP and OSB as the surface, and loads space for your decking. 
 

The better way still would be go higher and slope the insulation using taper insulation toward the upstands and make them have channels around or near (not suggesting put them in the gutter ..!! ) and build a system of drainage across the roof in GRP. 
 

 

[revelation]

3 hours ago, PeterW said:

I think you’re missing the point here. 
 

You have 250mm, maybe even 270mm if the door has a cill as you can run the decking level will the cill. 
 

even if the glazing needs 80mm clear space then you have 170mm below your top surface at 250mm, or 190mm if you can get the deck up to cill height. 
 

That allows for 150-160mm of insulation above the deck as designed if you go with GRP and OSB as the surface, and loads space for your decking. 
 

The better way still would be go higher and slope the insulation using taper insulation toward the upstands and make them have channels around or near (not suggesting put them in the gutter ..!! ) and build a system of drainage across the roof in GRP. 
 

 

 

I don't think I am.  I have attached the drawing of the glass on-top of the curb, from the finished GRP to the top of the glass its 190mm (according to the manufacturers instructions). 

 

So 190mm + 50mm (insulation) + 10mm (approx for GRP)+ 18mm OSB = 268mm

 

268mm is pretty much level with the sill.  

 

We could make the curb a little shorter so the side of the roof light frame is sitting close to the GRP but that still wouldn't give us enough room to get enough insulation on top.  Thats why I have been exploring options to insulate below.

 

I hope that makes clearer sense now.

 

 

 

 

[PeterW]

Ok so that drawing helps. Do they also do one for the walk on units as a 3-5 degree slope on walk on glazing is going to drop it 50mm per metre run ..? Can the walk on be laid “flat”..?

 

I think you can build a gully around each of the kerbs  with 80mm PIR, and use this to your advantage in creating drainage channels across the roof to your roof drains. The remaining roof could be a second layer of 80mm on top, and the whole lot sheathed in OSB and GRP’d with a double layup around all the point areas and gullies to create one large waterproof structure. 
 

I’d be talking to two people  : 

- the rooflight manufacturer to see what the options are with the walk on units and the kerbs / minimum clearance. I’m assuming the 150mm is “splash proofing” which if you are decking the whole area becomes irrelevant to you as there will be no rain splash. 

- your building control officer as to what they will accept as minimum insulation, and even if spray insulation is acceptable as an alternative as it’s not being used in its certified method. Putting £2-3k worth of spray foam in place and then finding it has to come out would be a significant issue to overcome. 
 

I wish you good luck with this one. 
 

 

[revelation]

12 minutes ago, PeterW said:

Ok so that drawing helps. Do they also do one for the walk on units as a 3-5 degree slope on walk on glazing is going to drop it 50mm per metre run ..? Can the walk on be laid “flat”..?

 

I think you can build a gully around each of the kerbs  with 80mm PIR, and use this to your advantage in creating drainage channels across the roof to your roof drains. The remaining roof could be a second layer of 80mm on top, and the whole lot sheathed in OSB and GRP’d with a double layup around all the point areas and gullies to create one large waterproof structure. 
 

I’d be talking to two people  : 

- the rooflight manufacturer to see what the options are with the walk on units and the kerbs / minimum clearance. I’m assuming the 150mm is “splash proofing” which if you are decking the whole area becomes irrelevant to you as there will be no rain splash. 

- your building control officer as to what they will accept as minimum insulation, and even if spray insulation is acceptable as an alternative as it’s not being used in its certified method. Putting £2-3k worth of spray foam in place and then finding it has to come out would be a significant issue to overcome. 
 

I wish you good luck with this one. 
 

 

 

I have previously spoken to the manufacturer about the slope of the walk on and they said it can be laid without a slope if we wanted due to the strength of the glass we wouldn't get pooling.  But we are putting 1 degree on it.  

 

Its a good call speaking to the manufacturer about the 150mm to see if that can ben dropped due to the decking.  

 

Forming gullys around the roof light maybe possible but will be a pain as the roof light is in the middle and it will mean having to make further gullys off that to run to the edges. (I will speak to the builders about this one.

 

My building control officer is fine with spray foam as long as the product is certified etc...

 

 

[PeterTweeter]

Hi Revelation

I realise I'm a bit late to this party, but I've been browsing re. insulation. IMHO closed cell spray foam is a viable option. Throughout North America this is becoming the standard approach to retrofit attic roofs & cathedral ceilings. A variety of building science; government agencies have produced guidance about deep filling joist/rafter spaces. This is seen as the route to getting high R values ie low U values.

[revelation]

Hi Peter,

 

Thank you for your input, after much considering.  I had spoken to kingspan.  They had advised that they would be happy for a small amount of insulation below, I gave them the roof build up and they advised how much I could have below.