Insulation below a warm flat roof

[scottishjohn]

 

2 hours ago, Moonshine said:

I spoke to a couple of PIR manufactures about it and they did a condensation risk calculation, which showed there was a risk of condensation in Jan - Mar, with a max of 0.00342 kg/m2  (Feb) for an exterior temp of -5 degrees. However the annual moisture accumulation is 0.00000 Kg/m² and any condensation will dry out over a 12 month period.

just not a sensible option -- you want no condensation at all in your calcs -  sounds like a time bomb

 as its not finished  what about lowering ceiling 100m by moving joists down ?  then you will have another 100 mm to play with 

 will it be that noticable  in that room  below 

Edited October 23, 2020 by scottishjohn

[revelation]

6 hours ago, PeterW said:

Not sure why the flush finish would be an issue as you just use adjustable legs to get the decking in line with the glazing ..??

 

When you say some has been done, what do you mean..?? Is all the deck down with insulation ..?? Or is some of the felt / membrane / GRP done ..??

Spray foam won’t be cheap and I don’t think will resolve your issues. 

 

 

Hi Peter

 

The decking will come above our doorways.  If we are doing it according to our original plan the decking comes close to flush with the door sills.    Right now its only the first layer of OSB and the Alutrix membrane that has been put down.  After reading al the advice I put a stop to any further work on the roof until we were sure of what we were doing.

 

I would bite the bullet and pay for spray foam to be done if it meant that we will get the balcony exactly how we want it.  Why do you think spray foam won't work?

 

 

[revelation]

5 hours ago, the_r_sole said:

The rule of thumb I've heard with hybrid roofs is you should have twice as much insulation on the outside...

If you're using adjustable feet to take the deck, have you looked at all the options for that? And/or can you not raise the finished level of the glass to accommodate a higher deck?

 

The problem is with our doorways, any raising of the deck will mean the deck coming higher than the door going out onto the balcony. We have a minimum amount of play. 

[Mr Punter]

Looking at the drawings I cannot see any firrings to create the required fall on the roof.

[revelation]

1 minute ago, Mr Punter said:

Looking at the drawings I cannot see any firrings to create the required fall on the roof.

 

Well noticed, after doing them I realised that I missed them off.  There are firing to create a fall to direct the water to the gullies and the gullies down to the drain.

[Mr Punter]

On a big roof you can get a lot of extra build up with the firrings.

[revelation]

Just now, Mr Punter said:

On a big roof you can get a lot of extra build up with the firrings.

 

I don't follow, the size of the roof is approx 8m x 5.5m

[Mr Punter]

10 hours ago, revelation said:

 

 

Is this a current photo?  Why not just do a proper warm roof?  You may need a lower door and higher upstands for the rooflights.

[Mr Punter]

Your firrings should give you a 1 in 60 fall, so if the high point is in the middle of the 5.5m it will be about 100mm higher than the lowest points.

[Moonshine]

1 minute ago, Mr Punter said:

Your firrings should give you a 1 in 60 fall, so if the high point is in the middle of the 5.5m it will be about 100mm higher than the lowest points.

 

I thought that it was that falls to be designed with minimum 1:40, to ensure a finished fall of 1:80. Where does the 1:60 come from?

[Mr Punter]

1 hour ago, Moonshine said:

Where does the 1:60 come from?

 

Lots of pre-cut firrings are set to this.  1:40 eats into a lot of sky on a flat roof if it is large.

[Temp]

Can you mark on this drawing where the floor sill height would be?  Currently it looks like you could replace the 15cm (?) raised decking with insulation.  It would also be nice to see a section through the door sill. 

 

[revelation]

1 hour ago, Temp said:

Can you mark on this drawing where the floor sill height would be?  Currently it looks like you could replace the 15cm (?) raised decking with insulation.  It would also be nice to see a section through the door sill. 

 

 

With the current door the sill would sit about level with the decking there maybe about 20mm of play but thats it.

 

If we get put in more insulation we could keep the deck at around the same level using short legs but the glass roof light would have to come up  so we wouldn't get a flush finish and it would create a trip hazard (especially having two young kids).

[the_r_sole]

2 minutes ago, revelation said:

If we get put in more insulation we could keep the deck at around the same level using short legs but the glass roof light would have to come up  so we wouldn't get a flush finish and it would create a trip hazard (especially having two young kids).

 

Clearly I'm missing something here - the sketch shows the glass flush with the deck, and 150mm legs holding the decking to that level - why can't you use smaller legs and use the space for insulation without changing any of the levels on the sketch?

[revelation]

3 hours ago, Mr Punter said:

 

Is this a current photo?  Why not just do a proper warm roof?  You may need a lower door and higher upstands for the rooflights.

 

I would love to just add some more insulation.  The door we have had that made already,  The problem is that the final finish would either finish higher than the doors or if we lower the decking then the glass roof lights will sit up higher than the decking so will create a trip hazard and aesthetically not look great.

[revelation]

8 minutes ago, the_r_sole said:

 

Clearly I'm missing something here - the sketch shows the glass flush with the deck, and 150mm legs holding the decking to that level - why can't you use smaller legs and use the space for insulation without changing any of the levels on the sketch?

 

The roof window does just sit on the curb it, it sits around the side so has some overhand that has to sit above the final felt or fibreglass layer.  

 

I have attached an image of the type of roof light.  I hope that clarifies 

[Temp]

6 hours ago, the_r_sole said:

why can't you use smaller legs and use the space for insulation without changing any of the levels on the sketch?

 

Or no legs at all. Insulation and walk on decking on top.

[Temp]

6 hours ago, revelation said:

The roof window does just sit on the curb it, it sits around the side so has some overhand that has to sit above the final felt or fibreglass layer.  

 

Have you already purchased them?

Can you raise them above deck level? Just means you just walk around them instead of over them.

[Temp]

How tall does the upstand for the roof light have to be?

[Temp]

Just a heads up.. is it this one...

 https://www.glazingvision.co.uk/rooflights/fixed-rooflights/flushglaze-walk-on-rooflight/

 

I was looking for their drawing of the upstand but I noticed it says keep 200mm clear around the upstand. How close is your upstand to the wall?  Perhaps ok but looks less than 200mm? I couldn't see a drawing that showed if it was 200mm from the inside or outside of the upstand.

 

 

Edited October 23, 2020 by Temp

[Temp]

14 minutes ago, Temp said:

How tall does the upstand for the roof light have to be?

 

Answering my own question i see it says 150mm. Possibly plus a 3 degree slope.

[revelation]

1 hour ago, Temp said:

 

Answering my own question i see it says 150mm. Possibly plus a 3 degree slope.

 

So essentially, if I raise the insulation I have to raise the curb, which leaves the glass roof light higher.  Considering the expense we have gone to to buy the glass so it fits flush, thats what we would ideally like.

 

[PeterW]

5 hours ago, revelation said:

 

So essentially, if I raise the insulation I have to raise the curb, which leaves the glass roof light higher.  Considering the expense we have gone to to buy the glass so it fits flush, thats what we would ideally like.

 

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 ..??

 

[pudding]

Raise the rooflight upstand, add more insulation to the same height on top of joists so everything is flush. Then, have a step up from the existing door to the new higher level. You'll have one small area of the step where the insulation isn't up to the rest then? 

[PeterW]

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.