Tight buildup - tapered insulation?

[SuperPav]

So the builders have (very slowly) started building up the walls of our bungalow extension (upwards).

 

The house is a U shape with a flat roof in the "middle" of the U, which is higher than the main ceilings of the house.

 

What i'm finding now is that I only have approx 200mm to play with in the U-section as the distance between the top of the flat roof joists, and the bottom of the 100mm deep stone cill of a window facing the flat roof. If we install firrings (running across the joists) that'll be borderline in terms of the space available for insulation (100-110mm at a push with two 18mm OSB decks).

 

The length of the flat roof is approx 3.7m, (by about 3.2m wide), and the window is in one of the side walls, about half way along.

 

Trying to get as much insulation in there as possible, and respecting the falls required, I am now looking at just getting sheets of Kingspan 30-50mm (1:60) insulation. It's a small enough area that the cost of two packs (~£500) isn't too much of a concern (at this stage we've spent far more than that fixing much more avoidable builder's problems). This would mean that we'd have 190mm down to 130mm in that middle section.

 

Just looking for a bit of a sanity check that the following build up makes sense and that I've not made some stupid assumptions:

 

Liquid roof (brand tbc!)

18mm OSB3 TG4 deck

tapered insulation: 50-30mm PIR. 50mm + 20mm + 20mm at the "inside/top end", 30mm at lower end

100mm celotex

VCL

18mm OSB3 TG4 subdeck

PS10 Posijoists

12mm plasterboard

 

 

ETA the bit further down in the image where the joists run perpendicular is also flat roof, but with 8x2 (cantilevered over the steel) so a smaller depth, so no build-up challenge there can can use firrings.

 

Edited December 13, 2021 by SuperPav

[redtop]

4 hours ago, SuperPav said:

So the builders have (very slowly) started building up the walls of our bungalow extension (upwards).

 

The house is a U shape with a flat roof in the "middle" of the U, which is higher than the main ceilings of the house.

 

What i'm finding now is that I only have approx 200mm to play with in the U-section as the distance between the top of the flat roof joists, and the bottom of the 100mm deep stone cill of a window facing the flat roof. If we install firrings (running across the joists) that'll be borderline in terms of the space available for insulation (100-110mm at a push with two 18mm OSB decks).

 

The length of the flat roof is approx 3.7m, (by about 3.2m wide), and the window is in one of the side walls, about half way along.

 

Trying to get as much insulation in there as possible, and respecting the falls required, I am now looking at just getting sheets of Kingspan 30-50mm (1:60) insulation. It's a small enough area that the cost of two packs (~£500) isn't too much of a concern (at this stage we've spent far more than that fixing much more avoidable builder's problems). This would mean that we'd have 190mm down to 130mm in that middle section.

 

Just looking for a bit of a sanity check that the following build up makes sense and that I've not made some stupid assumptions:

 

Liquid roof (brand tbc!)

18mm OSB3 TG4 deck

tapered insulation: 50-30mm PIR. 50mm + 20mm + 20mm at the "inside/top end", 30mm at lower end

100mm celotex

VCL

18mm OSB3 TG4 subdeck

PS10 Posijoists

12mm plasterboard

 

 

ETA the bit further down in the image where the joists run perpendicular is also flat roof, but with 8x2 (cantilevered over the steel) so a smaller depth, so no build-up challenge there can can use firrings.

 

Build up is what we did

 

[Mr Punter]

Sounds better than firrings.  What fixings are you using for the top sheet of OSB?

[Gordo]

I would be worried that the cold plywood deck between the weatherproof coating and the insulation will suffer from condensation and rot out.

[Mr Punter]

6 minutes ago, Gordo said:

I would be worried that the cold plywood deck between the weatherproof coating and the insulation will suffer from condensation and rot out.

 

Where would the moisture causing the condensation come from?  I have seen some roof insulation where they bond a thin ply layer to take membranes etc and they have BBA.

[Gordo]

33 minutes ago, Mr Punter said:

 

Where would the moisture causing the condensation come from?  I have seen some roof insulation where they bond a thin ply layer to take membranes etc and they have BBA.

Condensation from interior sweats on underside of a cold impermeable layer. Some membranes are slightly breathable to an extent.  Do as you wish. I have seen this practice also, doesn’t meet its right. I have also seen the cold deck plywood rot out.  I wouldn’t do it

Edited December 13, 2021 by Gordo

[Mr Punter]

31 minutes ago, Gordo said:

Do as you wish. I have seen this practice also, doesn’t meet its right. I have also see the plywood rot out.  I wouldn’t do it

 

What would you use in this circumstance?  Some sort of cement board as the top layer?

[Gordo]

5 minutes ago, Mr Punter said:

 

What would you use in this circumstance?  Some sort of cement board as the top layer?

Cement board fully bonded to insulation would be my initial thoughts. I haven’t researched it much but seems to be a thing. To date I have  always gone with pvc roofing mechanical fixed to insulation. But acknowledge risk of insulation being compressed if subject to ANY traffic even window cleaning. So if I am ever in that position I will then fully consider alternatives to plywood. Id welcome anyone’s thoughts

[MasonC]

Is there any reason you have chosen to go with a liquid system? I would eliminate the ply above the insulation an go for a single ply membrane bonded straight to foil face(better U value) PIR insulation(you may be forced to use tissue faced board if the brand doesn't use a contact adhesive compatible).

PIR insulation will be good if subjected to light traffic and would only become spongy after quite a few years. Mineral wool insulation however goes very spongey very fast if walked on. When we used mineral wool on high traffic areas like maintenance walkways on hospitals we put a galvanised metal sheet(wrapped in fleece) on top of the membrane and attached membrane over the top to spread the load. 

[Iceverge]

Siga have a lot of confidence in their compact roof. 

 

https://shop.siga.swiss/gb-en/blog/how-to-build-a-moisture-safe-flat-roof

 

 

“In a compact roof assembly featuring Majrex®200, and using a high quality rigid insulation with a lambda value of < 0.026 W/mK, you may only need as little as 60 mm of insulation above the roof deck to ensure a safe construction. We see this as a safe solution, and are happy to write a warranty for our product, and to perform condensation risk calculations for our customers.”

 

“All of our calculations even assume that the membrane is installed much more poorly than it should be in practice — this means that even if in a worst case scenario, you can rest assured knowing that your compact roof construction is safe.”

 

[Gordo]

1 hour ago, MasonC said:

Is there any reason you have chosen to go with a liquid system? I would eliminate the ply above the insulation an go for a single ply membrane bonded straight to foil face(better U value) PIR insulation(you may be forced to use tissue faced board if the brand doesn't use a contact adhesive compatible).

PIR insulation will be good if subjected to light traffic and would only become spongy after quite a few years. Mineral wool insulation however goes very spongey very fast if walked on. When we used mineral wool on high traffic areas like maintenance walkways on hospitals we put a galvanised metal sheet(wrapped in fleece) on top of the membrane and attached membrane over the top to spread the load. 

I’d run a mile from a liquid waterproofed lol. I haven’t seen anyone actually use mineral wool in a warm deck but have seen it specified. Which type are they using? maybe of the slabs like drytherm. Just interested as I would have expected it to be way too compressible for even infrequent light traffic.

[SuperPav]

20 hours ago, Mr Punter said:

Sounds better than firrings.  What fixings are you using for the top sheet of OSB?

 

That is the important question! 

 

I am open to suggestions... on another flat roof I had done, we had 120mm PIR over firrings, fixed with 200mm long screws, and that is not something I'd like to repeat - the roofer's hit rate of aiming the screws into the firrings through that build up was somewhat less than 100%. Which then starts to make a bit of a mockery of the VCL, before even mentioning thermal bridging. 

I'd consider using the sunken "top hat" screw fixings so that a much shorter screw can be used. It doesn't massively help with aim, but fitting onto 75mm posijoists without firrings should be easier as they can be marked out quite accurately before driving the screws.

 

I'd prefer an adhesive of some kind.. I guess something like TT47 can be bonded to most things? 

The roof will most likely end up having sedum trays, or smooth round cobles on the top for aesthetics anyway, so I don't think it'll be going anywhere!

 

Ref other comments on why use liquid roof over a deck - simply I've not been happy with EPDM on two roofs, especially one that is under a pitched roof, where a blown tile landed and punctured the EPDM, resulting in rain water seeping in through the crack and making a right mess. And the patch didn't look too hot either! So I don't think I'd ever consider a roof with EPDM laid directly over PIR if there was ANY chance of anything standing or landing on it.

[Mr Punter]

Perhaps you can just ignore the joists and fix through to the lower OSB.  18mm OSB has a pretty decent pull-out rating.  Just make sure you have the correct screw lengths for the differing insulation thicknesses as the screws will only be threaded at the end 60mm,  The tube washer type fixings look good as the screw will not pop through the waterproofing if the insulation is compressed.

[MasonC]

18 hours ago, Gordo said:

I’d run a mile from a liquid waterproofed lol. I haven’t seen anyone actually use mineral wool in a warm deck but have seen it specified. Which type are they using? maybe of the slabs like drytherm. Just interested as I would have expected it to be way too compressible for even infrequent light traffic.

 Rockwool is the architects specification of choice it seems. its perfectly fine to walk on to start with very similar to PIR but I become spongy quickly, we found every normally by the end of the build. Iko all do their own brand of mineral wool though. 

Liquids not too bad if installed correctly and the application is suitable. it was our preference for all hight traffic areas (balconies etc.) to be done in liquid but the m2 cost is just stupid compared to felt which is perfectly fine for high traffic areas. Some people preferred Hot melt but there is the big H&S push towards flame free so liquid wins out there, however felt manufactures have create flame free systems, basically self adhesive felt. 

[MasonC]

2 hours ago, SuperPav said:

 

That is the important question! 

 

I am open to suggestions... on another flat roof I had done, we had 120mm PIR over firrings, fixed with 200mm long screws, and that is not something I'd like to repeat - the roofer's hit rate of aiming the screws into the firrings through that build up was somewhat less than 100%. Which then starts to make a bit of a mockery of the VCL, before even mentioning thermal bridging. 

I'd consider using the sunken "top hat" screw fixings so that a much shorter screw can be used. It doesn't massively help with aim, but fitting onto 75mm posijoists without firrings should be easier as they can be marked out quite accurately before driving the screws.

 

I'd prefer an adhesive of some kind.. I guess something like TT47 can be bonded to most things? 

The roof will most likely end up having sedum trays, or smooth round cobles on the top for aesthetics anyway, so I don't think it'll be going anywhere!

 

Ref other comments on why use liquid roof over a deck - simply I've not been happy with EPDM on two roofs, especially one that is under a pitched roof, where a blown tile landed and punctured the EPDM, resulting in rain water seeping in through the crack and making a right mess. And the patch didn't look too hot either! So I don't think I'd ever consider a roof with EPDM laid directly over PIR if there was ANY chance of anything standing or landing on it.

 

If you're going to cover the roof in a ballast or sedum trays I would consider a single ply or felt system.

While both single ply and EPDM both have the con of being prone to puncture Single ply at thicker gauges i.e. 1.8 is extremely resistant and if your covering in ballast you will be required to have a protection layer over the single ply which coupled with your ballast or green roof would make it bomb proof to punctures. 

 

Alternatively just use a felt system which would give e you the robustness you want a high longevity and it would eliminate the 18mm ply. Felt systems like Bauder and Icopal are tired and tested but everyone is doing them now Sika has a system and so do Iko it depends on your budgetary requirements. Bauder also do their own green roof systems which then fall under their guarantee as well though. 

[Gordo]

On 14/12/2021 at 13:18, SuperPav said:

 

I am open to suggestions... on another flat roof I had done, we had 120mm PIR over firrings, fixed with 200mm long screws, and that is not something I'd like to repeat - the roofer's hit rate of aiming the screws into the firrings through that build up was somewhat less than 100%. Which then starts to make a bit of a mockery of the VCL, before even mentioning thermal bridging. 

The fixings are just to hold PVC roof membrane and insulation down against wind uplift. Therefore I’m pretty sure it is adequate to screw it just to plywood deck. It shouldn’t really be necessary to screw to solid grounds. Correct me if I am wrong.

 

I read somewhere that the the fixings through VCL are considered self sealing. Sounded plausible enough.

[RobLe]

On 13/12/2021 at 18:17, Gordo said:

Cement board fully bonded to insulation would be my initial thoughts. I haven’t researched it much but seems to be a thing. To date I have  always gone with pvc roofing mechanical fixed to insulation. But acknowledge risk of insulation being compressed if subject to ANY traffic even window cleaning. So if I am ever in that position I will then fully consider alternatives to plywood. Id welcome anyone’s thoughts

 

While the osb adds strength, it is damaged by water - I’ve not seen a BBA cert for osb below epdm, or am I not looking in the right place?  Not that there should be any water, but it makes me nervous too, wood in that cold location.  As I’ve done quite a bit of ewi-ing, what about this layup:

 

(Outside)

Epdm

Ewi basecoat+glass mesh, around 4mm thick total

TR27

VCL

 

Clearly it’s more work than osb3tg4.  It’s cheaper and can’t rot, not as tough as osb but I think tougher than 4mm cement board due to mesh, and can give a step free finish.  There will be a week delay, waiting for the basecoat to fully dry prior to epdm.  
Thoughts?