Flat roof detail - warm or cold?

[G and J]

1 hour ago, SBMS said:

A 150mm upstand for skylight? As I understand it that’s the minimum for a rooflight. But your PIR depth isn’t governed by the height of your upstand. Your 150mm upstand is from the finished surface of the roof (ie above your insulation). 

I can move the skylight up to preserve the 150mm upstand around the skylight if I thicken the flat roof, what I can’t do is raise the parapet as that risks us contravening our planning permission. So I guess knowing how big the upstand (watertight layer to parapet) has to be helps me understand what I’ve got to work with.

[SBMS]

2 layers of superfoil on top of joists and 90mm PIR is 0.11 u value. You can reduce the PIR if you’re willing to accept a reduced u value. I think you’ll get that without having to modify your parapet wall. You might have to lift your roof light upstand a bit. That roof (like mine was) will be difficult to ventilate. I switched to a warm roof for this reason.

[G and J]

1 hour ago, SBMS said:

2 layers of superfoil on top of joists and 90mm PIR is 0.11 u value. You can reduce the PIR if you’re willing to accept a reduced u value. I think you’ll get that without having to modify your parapet wall. You might have to lift your roof light upstand a bit. That roof (like mine was) will be difficult to ventilate. I switched to a warm roof for this reason.

I have looked at Superfoil.   I’m guessing you mean the Superfoil laid directly onto the top of the joists, taped to be your airtight layer, then PIR on top, then ply, then furring strips, then ply, then EPDM?

[G and J]

4 minutes ago, G and J said:

I have looked at Superfoil.   I’m guessing you mean the Superfoil laid directly onto the top of the joists, taped to be your airtight layer, then PIR on top, then ply, then furring strips, then ply, then EPDM?

Like this I mean….

[SBMS]

3 minutes ago, G and J said:

Like this I mean….

Example roof makeup, no ventilated cavity needed,

similar to your makeup diagram I think

 

From https://ybsinsulation.com/wp-content/uploads/2022/03/SuperQuilt-Data-Sheet-V3-Feb-2019.pdf

 

there’s also a detail with 4 layers of super foil between and over which hits 0.11. It indicated that a non ventilated cavity works on that as well which would mean your roof height wouldn’t change at all? Worth a chat to confirm with YBS that no ventilation is required?

 

also superfoil acts as a vcl when taped. 

 

[G and J]

22 minutes ago, SBMS said:

Example roof makeup, no ventilated cavity needed,

similar to your makeup diagram I think

 

From https://ybsinsulation.com/wp-content/uploads/2022/03/SuperQuilt-Data-Sheet-V3-Feb-2019.pdf

 

there’s also a detail with 4 layers of super foil between and over which hits 0.11. It indicated that a non ventilated cavity works on that as well which would mean your roof height wouldn’t change at all? Worth a chat to confirm with YBS that no ventilation is required?

 

also superfoil acts as a vcl when taped. 

 

Ah I see, thank you.   I can understand the ply to distribute the weight (of, in our case, a few pigeons and a very occasional window cleaner’s feet), but doesn’t that mean that the ply layer cannot breath at all?  Does that matter?

 

Also, are those clips standard things?  I’ll be onto Superfoil on Monday (lucky Superfoil lol).

[JohnMo]

I did similar on our garden room, but with 200mm of EPS in top instead of PIR.

[JohnMo]

1 minute ago, G and J said:

Also, are those clips standard things?

Clip can be purchased from super foil, I didn't bother, as they would fit our fat joists. So just stapled onto the vertical face of each joist. On a big roof the clips would be better and quicker.

[SBMS]

4 minutes ago, G and J said:

Ah I see, thank you.   I can understand the ply to distribute the weight (of, in our case, a few pigeons and a very occasional window cleaner’s feet), but doesn’t that mean that the ply layer cannot breath at all?  Does that matter?

 

Also, are those clips standard things?  I’ll be onto Superfoil on Monday (lucky Superfoil lol).

I dont think the ply sub deck needs to breathe (or rather, is susceptible to condensation and therefore needs airflow to ventilate) because it’s on the warm side of the insulation. 

[G and J]

6 minutes ago, JohnMo said:

Clip can be purchased from super foil, I didn't bother, as they would fit our fat joists. So just stapled onto the vertical face of each joist. On a big roof the clips would be better and quicker.

That sounds good.  I’m not sure how thick the top chord of our posijoists will be, so that’s another bit of info on my info shopping list.  I think Superfoil only needs 20mm unventilated airgap to achieve max. R value so that should be ok. 
 

We’re lucky it’s only a tiny area - 3m x 5m with about a third of that taken up by a skylight, so I won’t even need that many staples.

[G and J]

5 minutes ago, SBMS said:

I dont think the ply sub deck needs to breathe (or rather, is susceptible to condensation and therefore needs airflow to ventilate) because it’s on the warm side of the insulation. 

So is this an example of the one third/two thirds rule?  In which case I need a goodly bit of PIR to keep the ply warm.

 

Which superfoil variant did you use?

[saveasteading]

I wouldn't use superfoil at all, let alone multiple layers.

The figures are theoretical. It has to be loose. You show mineral insulation somehow placed around it.

I might use it on a garden shed, but more likely would use bubblewrap.

You are better off with filling the whole void with pir or mineral wool.

 

[SBMS]

3 minutes ago, saveasteading said:

I wouldn't use superfoil at all, let alone multiple layers.

The figures are theoretical. It has to be loose. You show mineral insulation somehow placed around it.

I might use it on a garden shed, but more likely would use bubblewrap.

You are better off with filling the whole void with pir or mineral wool.

 

Challenge here is ventilating a cold roof makeup? We found it difficult to detail with a parapet wall hence switched to warm roof. I’m not sure what you mean when you say the figures are theoretical? All figures are theoretical. Don’t think he’s planning on mineral wool in between rafters as well? I wouldn’t, Not needed to achieve u value and becomes a hybrid roof then

[G and J]

1 minute ago, saveasteading said:

I wouldn't use superfoil at all, let alone multiple layers.

The figures are theoretical. It has to be loose. You show mineral insulation somehow placed around it.

I might use it on a garden shed, but more likely would use bubblewrap.

You are better off with filling the whole void with pir or mineral wool.

 

Hmmmmm.  If the joists are only 202mm and I fill the void with mineral wool I’ll not get a good u value at all.

 

The context is that we also have a large set of patio doors in that area, so I’m having a fret that that area will be the coldest bit of the house.  

[saveasteading]

1 minute ago, G and J said:

I’ll not get a good u value at all

The foil figures are imaginary. Forget them. Pir above and wool in the gaps is good...what makes you think otherwise?

The saving on superfoil will go a long way to your heating bill.

[G and J]

23 minutes ago, saveasteading said:

The foil figures are imaginary. Forget them. Pir above and wool in the gaps is good...what makes you think otherwise?

The saving on superfoil will go a long way to your heating bill.

So build up from top would be….

 

EPDM

Ply 

PIR (tapered to save the thickness of furring as it is only a small area.

VCL - i.e. airtight layer

Ply

Posi-joists

Mineral wool stuffed in everywhere I can get it inc. around MVHR ducts and plenums.

And the theory is that as long as I get enough PIR on top without too much cold bridging then the VCL will stay warm enough to avoid any form of condensation.

 

Am I close?

[Iceverge]

9 minutes ago, G and J said:

So build up from top would be….

 

EPDM

Ply 

PIR (tapered to save the thickness of furring as it is only a small area.

VCL - i.e. airtight layer

Ply

Posi-joists

Mineral wool stuffed in everywhere I can get it inc. around MVHR ducts and plenums.

And the theory is that as long as I get enough PIR on top without too much cold bridging then the VCL will stay warm enough to avoid any form of condensation.

 

Am I close?

 

That'll be fine. 

 

Personally I would always prefer a ventilated gap above a drainage plane but what you've got will work. 

 

 

[G and J]

9 minutes ago, Iceverge said:

a drainage plane

Drainage plane!  Isn’t that the EPDM?

[Iceverge]

Yes. 

 

However a single skin layer for water proofing is more vulnerable to long term damage than a system which deals with 99% of the bulk water with a primary external layer, then a ventilated gap and then a secondary impervious layer that will allow any drips drain away by gravity.

 

A cavity wall, a slated membraned roof would be examples. 

 

Relying on a single waterproofing layer such as an EDPM over a warm roof, rendered external insulation, or a bituminous flat roof is less robust long term. The waterproofing layer is assembled on site, generally relying an adhesive of some sort to cure correctly. 

Moreover the elements will be directly pummeling the only waterproofing layer increasing the lightly hook of leaks. 

 

 

 

[G and J]

1 minute ago, Iceverge said:

Yes. 

 

However a single skin layer for water proofing is more vulnerable to long term damage than a system which deals with 99% of the bulk water with a primary external layer, then a ventilated gap and then a secondary impervious layer that will allow any drips drain away by gravity.

 

A cavity wall, a slated membraned roof would be examples. 

 

Relying on a single waterproofing layer such as an EDPM over a warm roof, rendered external insulation, or a bituminous flat roof is less robust long term. The waterproofing layer is assembled on site, generally relying an adhesive of some sort to cure correctly. 

Moreover the elements will be directly pummeling the only waterproofing layer increasing the lightly hook of leaks. 

 

 

 

OK, I can understand all that.  Is there something I can do to my flat roof though?  Or is the simple fact that it is a flat roof such that I just have to accept the short lifespans and risk of water ingress?

[G and J]

PS.  Designing out the flat roof is not an option for us, we did think long and hard about that at an early stage.

[Iceverge]

1 minute ago, G and J said:

OK, I can understand all that.  Is there something I can do to my flat roof though?  Or is the simple fact that it is a flat roof such that I just have to accept the short lifespans and risk of water ingress?

 

I might try to sketch something later if I have a chance

[G and J]

1 minute ago, Iceverge said:

 

I might try to sketch something later if I have a chance

Fab.  Thank you.

[SBMS]

We didn’t end up using superfoil. We went with 180mm PIR because we’ve got the height. I was trying to solution something that minimally raised your roof. Given the option I would use a pure PIR warm

roof on deck. But if you are restricted I don’t know why a PIR/superfoil isnt a good option?

 

I’d also look at GRP instead of EPDM. I discounted rubber due to detailing around the roof lights; you might want to consider it.

[G and J]

25 minutes ago, SBMS said:

I’d also look at GRP instead of EPDM. I discounted rubber due to detailing around the roof lights; you might want to consider it.

I haven’t thought that far ahead, but I note that EPDM appears to be the standard approach at the mo.  
 

28 minutes ago, SBMS said:

But if you are restricted I don’t know why a PIR/superfoil isnt a good option?

And I appreciate the help.  I suspect PIR and Superfoil is a good option but my habit is to try and get my head round all sensible options before going for what seems best.