Flat Roof Insulation Help

[richo106]

Hi All

 

I have attached some photos of my flat roof section which I will need to insulate internally.

 

I need to bring it down around 75mm to match up with my glass front so my plan was to have 25mm PIR over the rafters taped then 50mm battens to create a void for my downlights.

 

My idea at the minute is to put 100mm celotex between the joists pushed right up (50mm air gap will be maintained) then run my MVHR ducting and then put some left over omnifit on top of the PIR.

 

Does this sound an idea or not?😂 I am sure there are better ways to do this so open to any ideas. I have some spare 100mm PIR and knauf omnifit I would like to use ideally.

 

Also as you can see I have a roof lantern, I am really not sure what way to approach this in terms of air tightness or insulation so any ideas would be much appreciated 

 

 

[richo106]

[richo106]

I’ve just been looking through Buildhub at flat roof insulation and now I am even more confused!

[Nickfromwales]

15 minutes ago, richo106 said:

I’ve just been looking through Buildhub at flat roof insulation and now I am even more confused!

Relax, we promise to not let you feck this up

The worst cold bridging would be between the timbers at the top, and the plasterboard. Easy solution; use Marmox at 25mm thickness instead of the plasterboard and plaster it directly. Stuff everything with rockwool immediately surrounding the upstand, and Bobs ya Uncle. Not a single mention of Fanny, that would be rude!

[richo106]

11 minutes ago, Nickfromwales said:

Relax, we promise to not let you feck this up

The worst cold bridging would be between the timbers at the top, and the plasterboard. Easy solution; use Marmox at 25mm thickness instead of the plasterboard and plaster it directly. Stuff everything with rockwool immediately surrounding the upstand, and Bobs ya Uncle. Not a single mention of Fanny, that would be rude!

Something like this?

[Nickfromwales]

2 minutes ago, richo106 said:

Something like this?

Yup. Something like Marmox on the outside would help, but not essential. If you can get a 10mm board outside then do that too.

The only thing I'd add is some continuation over the upper timber of the upstand, as you don't want any (ideally) of that meeting room temp. The issue is exaggerated here because heat will rise, plus moisture, and they'll both be trapped here and fighting it out against anything 'cold'. 

More Marmox with a chamfered tuck into the angled section would be beneficial. 

[richo106]

14 minutes ago, Nickfromwales said:

Yup. Something like Marmox on the outside would help, but not essential. If you can get a 10mm board outside then do that too.

The only thing I'd add is some continuation over the upper timber of the upstand, as you don't want any (ideally) of that meeting room temp. The issue is exaggerated here because heat will rise, plus moisture, and they'll both be trapped here and fighting it out against anything 'cold'. 

More Marmox with a chamfered tuck into the angled section would be beneficial. 

The outside is already got rubber on

 

I have updated my drawing 

[richo106]

My next dilemma is how to insulate between the joists. I also need to run my MVHR ducts through this area 

[Nickfromwales]

7 minutes ago, richo106 said:

The outside is already got rubber on

 

I have updated my drawing 

Happy days. The external was an "if" so don't panic about that.

All looks good, but lets delete the assumptions

1. All voids in the actual upstand need to be packed solid with rigid PIR. Foamed and foil taped to leave zero gaps.
2. The joist depths are to be filled with rockwool / other?

 

[Nickfromwales]

Just now, richo106 said:

My next dilemma is how to insulate between the joists. I also need to run my MVHR ducts through this area 

Whack the ducts in, install a good quality airtightness membrane, hold the membrane in place with service battens at 300mm o/c's, and pump the bugger full of cellulose.

[Nickfromwales]

Just make sure you zip tie the ducts tight to the lower chord so the cellulose goes over the top of them, bringing them into the insulated envelope.

[richo106]

12 minutes ago, Nickfromwales said:

Whack the ducts in, install a good quality airtightness membrane, hold the membrane in place with service battens at 300mm o/c's, and pump the bugger full of cellulose.

Could I use knauf omnifit insulation instead on the blown cellulose? I am trying to DIY this plus I have some left over omnifit. 
 

Or I have 200mm standard Isover spacesaver loft insulation? 
 

Do I need to ensure there is an air gap?

[richo106]

[Nickfromwales]

Ah, just looked again and I think you will need to go the wool route + air gap as there isn't a lower OSB deck under the furring pieces. My bad, but I can blame the good people of Brewdog for their game-changing Punk IPA.

Why not go 50mm of PIR then 25mm service battens for cables, and use flat panel LED lights such as Xcite from CEF vs regular down / spot lights? You'll only need to scallop the PIR slightly to get the transformers in, which will be of little to zero detriment.

 

https://www.cef.co.uk/catalogue/products/4501996-8w-led-downlight-white-4000k?gclid=Cj0KCQjwmtGjBhDhARIsAEqfDEfRUXBbV-rtoRKAKUFI9HandcRD-tQnk9Gc_yo19B-gNVQTcpOgUWUaAgi1EALw_wcB&gclsrc=aw.ds

 

Those lights are the thickness of plasterboard, so you can span a joist without a problem (if you need lights to be where they 'need to go' and a joist prevents the OCD monster from being satisfied). I use them on most jobs and am super happy with them, as are the clients.

[MJNewton]

Howcome you didn't go for a warm roof design? 

[ETC]

And where’s the ventilation?

[Dave Jones]

7 hours ago, ETC said:

And where’s the ventilation?

 

this. you have a cold roof with no ventilation. will rot out in no time unless its addressed. Photo of the outside ?

 

eaves  and abutment vents, min 50mm gap maintained to allow an airflow etc

 

The lantern upstand needs the same amount of U value insulation as the rest but doesn't look like they have been designed deep enough (they never are).

 

Best stop and correct it now rather than have to rip it all out when the mould patches start appearing.

 

 

 

[richo106]

8 hours ago, MJNewton said:

Howcome you didn't go for a warm roof design? 

Due to the design and the glass front we are having we already needed around a 400mm Fascia so didn't want an even larger one

[richo106]

49 minutes ago, Dave Jones said:

 

this. you have a cold roof with no ventilation. will rot out in no time unless its addressed. Photo of the outside ?

 

eaves  and abutment vents, min 50mm gap maintained to allow an airflow etc

 

The lantern upstand needs the same amount of U value insulation as the rest but doesn't look like they have been designed deep enough (they never are).

 

Best stop and correct it now rather than have to rip it all out when the mould patches start appearing.

I have attached a couple of photos of the outside

 

I can ensure there is at least 50mm air gap above the insulation that is no problem, there is no vents in it that I am aware of

 

No the upstand is definitely not deep enough to get the same u value - made with 100 x 50mm battens. 

 

Yes I want to do it properly definitely and happy to do whatever to ensure there isn't any build up of damp/mould

[MJNewton]

1 hour ago, richo106 said:

Due to the design and the glass front we are having we already needed around a 400mm Fascia so didn't want an even larger one

 

I think you're going to struggle to ventilate a cold roof in that position, particularly at this stage. 

[richo106]

This doesn't sound promising!

 

Any suggestions how to go about this going forward?

 

Many Thanks

 

 

[MJNewton]

I wouldn't worry about the upstand - fairly commonplace for that to be thinner than what a normal roof or wall can accommodate insulation wise. 100mm of PIR for a relatively small surface are will be fine. 

 

It really is the ventilation of the cold roof though that concerns me. What's the detailing like at the junction with the pitched roof? I'm wondering if you can ventilate through the furring void, or does it taper to zero at the front? 

 

Who designed the roof?

[richo106]

43 minutes ago, MJNewton said:

I wouldn't worry about the upstand - fairly commonplace for that to be thinner than what a normal roof or wall can accommodate insulation wise. 100mm of PIR for a relatively small surface are will be fine. 

 

It really is the ventilation of the cold roof though that concerns me. What's the detailing like at the junction with the pitched roof? I'm wondering if you can ventilate through the furring void, or does it taper to zero at the front? 

 

Who designed the roof?

I will send some more pictures over later, ill take some tonight. Pretty sure the furring strips go to nothing

 

Ermm the roof was sorted by the builders with their roofing guy, I spoke to the roofer yesterday and he said as long as there is a 50mm air gap you are fine, obviously it doesn't seem as simple as this

 

 

[MJNewton]

The air gap needs to be ventilated though. 

[Mr Punter]

If you intend to insulate this between the joists with no airflow over the insulation it is very important you have a properly fitted VCL on the warm side of the insulation.  With your downlights and MVHR ducting this will be very difficult to achieve so the roof will be at high risk of interstitial condensation and premature failure.