Ideal roof insulation…

[Simplysimon]

16 hours ago, matthyde83 said:

I’m really struggling to find a blown cellulose supplier and installer near me (Surrey).  Any suggestions? 

contact warmcell and they'll give you a list

[matthyde83]

21 hours ago, joe90 said:

All designed by a roofing company (in house S.E.).

Mind me asking who?

[joe90]

2 hours ago, matthyde83 said:

Mind me asking who?

Sorry all dealt with by my builder.

[matthyde83]

On 21/04/2022 at 08:20, joe90 said:

Sorry all dealt with by my builder.

So I think we’re looking 300mm I joists with rockwool between.  My architect has never used these before though so I need to tick a few boxes somehow.  
 

build up : OSB, 300mm rockwool between joists, air gap (50mm? - to avoid felt touching rockwool?), breathable felt, tiles

 

I can speak to the SAP people to confirm it’ll meet what I want in terms of insulation but how about condensation calcs to satisfy bc?

[Iceverge]

What U value do you need? 

 

In any case I would not ventilate below the membrane. Use battens and counter battens to and ventilate above the membrane with all joints taped. Windtighness is important, especially with batt insulation. 

 

With batts between the I joists you may have issue with the corners compressing in the flanges. I'm not sure how significant this is but green building store changed the ijoist/batt detail on Golcar passivhaus to fill the webs flush  with PIR. 

 

You'll need an airtight layer somewhere too. 

[Simplysimon]

3 hours ago, Iceverge said:

You'll need an airtight layer somewhere too

take a look at the medite site here for inner and outer layers

[JohnMo]

I just went spray foam 256mm posi- rafter counter battened inside with 2x 50mm battens and full filled with 350mm foam.  Vapour control airtight layer, further 50mm battens as service void then plasterboard.  We have a large section of 12 degree roof and the normal (ish) 45 degree.  12 degree, outside rafter is covered with 18mm external ply and breather membrane, then Sarnafil standing seam, 45 degree 22mm sarking boards and breather membrane then slates

[Iceverge]

35 minutes ago, Simplysimon said:

take a look at the medite site here for inner and outer layers

 

It's a nice system but more expensive than straightforward OSB outboard and membrane inside. That's what I'd do unless I was feeling "spendy!"

[Simplysimon]

8 minutes ago, Iceverge said:

 

It's a nice system but more expensive than straightforward OSB outboard and membrane inside. That's what I'd do unless I was feeling "spendy!"

but the medite mdf is vapour open whereas osb isn't so the panel can't breathe

[SteamyTea]

2 minutes ago, Simplysimon said:

medite mdf is vapour open

Do you mean this product, rather than any Medite product.

 

https://mdfosb.com/en/products/medite-vent

[Simplysimon]

5 minutes ago, SteamyTea said:

Do you mean this product, rather than any Medite product.

 

https://mdfosb.com/en/products/medite-vent

yup

[matthyde83]

5 hours ago, Iceverge said:

What U value do you need? 

 

In any case I would not ventilate below the membrane. Use battens and counter battens to and ventilate above the membrane with all joints taped. Windtighness is important, especially with batt insulation. 

 

With batts between the I joists you may have issue with the corners compressing in the flanges. I'm not sure how significant this is but green building store changed the ijoist/batt detail on Golcar passivhaus to fill the webs flush  with PIR. 

 

You'll need an airtight layer somewhere too. 

Im checking with SAP calc people to find out.  Original was 0.16 on some sections and 0.11 where there’s a loft.  So 0.14 on the whole thing may work.  (I’ve upgraded other areas too). 
 

Concern re air ventilation is felt touching the rockwool?  Builder is concerned about it becoming wet there like in other applications, you always keep the felt away. 
 

Airtight layer would be OSB on the inside. 

[Iceverge]

I can understand your builders concern but their thinking is based on old housing standards with no internal vapour control layer and impermeable membranes like bitumen felt. The moisture from the house would pass through the ceiling (almost entirely through air gaps not diffusion) and condense on the back of the felt. 

 

The way to resolve this was to allow the wind to blow behind the felt but this brings it's own problems. The wind also blows through the insulation making it perform poorly, think woolly jumper on a very windy day. With a plastic anorak and the zips open. Sweaty and cold. 

 

A better solution is a proper sealed vapour control layer internally (also airtight layer) and a sealed breathable windtight layer outboard like tyvek supro or protect vp400 or similar. Think a woolly jumper with a Gore-Tex anorak over the top on a windy day.  Warm and dry. 

 

Here's a screenshot from the Tyvek Supro agrément cert. Note figures 6-8 have the insulation touching the membrane. You need to incude a counter batten to avoid any moisture pooling behind the tile battens however. 

 

 

 

 

 

10 hours ago, matthyde83 said:

 Original was 0.16 on some sections and 0.11 where there’s a loft.  So 0.14 on the whole thing may work.  (I’ve upgraded other areas too). 

 

 

That's not to onerous. If sticking with I joists this would be a nice buildup. I'd be tempted to move the OSB racking to the outside and replace with a airtight membrane/vapour control membrane inside. It'd be easier to lay the OSB outside and would provide more protection against any critters that fancied making their home in your roof! 

 

 

 

 

 

 

 

Alternatively as I joists are more expensive than plain timber this would be cheaper for materials (but dearer for labour)  Stick build a 9x2 roof with OSB outboard. Counter batten inside initially with 2x2, (I can't show battens at 90deg  on the calculator). This would give extra insulation depth and thermally break the rafter cold bridge. Then a vapour membrane creating a 45mm insulated service cavity with battens laid over the top of the initial counterbatten. This would pinch the membrane between the two battens making it more robust. 

 

 

 

 

I've used cellulose throughout as it's a fantastic product, guarantees a full fill and really boosts airtightness and decrement delay (phase shift) and hand fitting mineral wool is not pleasant. 

[Renegade105]

On 12/05/2022 at 09:04, Iceverge said:

I can understand your builders concern but their thinking is based on old housing standards with no internal vapour control layer and impermeable membranes like bitumen felt. The moisture from the house would pass through the ceiling (almost entirely through air gaps not diffusion) and condense on the back of the felt. 

 

The way to resolve this was to allow the wind to blow behind the felt but this brings it's own problems. The wind also blows through the insulation making it perform poorly, think woolly jumper on a very windy day. With a plastic anorak and the zips open. Sweaty and cold. 

 

A better solution is a proper sealed vapour control layer internally (also airtight layer) and a sealed breathable windtight layer outboard like tyvek supro or protect vp400 or similar. Think a woolly jumper with a Gore-Tex anorak over the top on a windy day.  Warm and dry. 

 

Here's a screenshot from the Tyvek Supro agrément cert. Note figures 6-8 have the insulation touching the membrane. You need to incude a counter batten to avoid any moisture pooling behind the tile battens however. 

 

 

 

 

 

 

 

That's not to onerous. If sticking with I joists this would be a nice buildup. I'd be tempted to move the OSB racking to the outside and replace with a airtight membrane/vapour control membrane inside. It'd be easier to lay the OSB outside and would provide more protection against any critters that fancied making their home in your roof! 

 

 

 

 

 

 

 

Alternatively as I joists are more expensive than plain timber this would be cheaper for materials (but dearer for labour)  Stick build a 9x2 roof with OSB outboard. Counter batten inside initially with 2x2, (I can't show battens at 90deg  on the calculator). This would give extra insulation depth and thermally break the rafter cold bridge. Then a vapour membrane creating a 45mm insulated service cavity with battens laid over the top of the initial counterbatten. This would pinch the membrane between the two battens making it more robust. 

 

 

 

 

I've used cellulose throughout as it's a fantastic product, guarantees a full fill and really boosts airtightness and decrement delay (phase shift) and hand fitting mineral wool is not pleasant. 

 

Hi @Iceverge,

 

Sorry to be a pain as you are very clear, it's just that I am left quite confused with all of the options.

 

So, if using blown cellulose, could you please just speel out what you did/would do for a roof made from attic truss's, circa 225 deep timbers, spaced @ 600cc. and a pitch section circa 42 Deg, and a small flat roof section in the centre.

 

I am planning on having slate tiling.

 

Do you specify the attic truss differently to allow sealing the eaves a bit easier? and if so, what did you ask for exactly?

 

Finally, what brand/company for the cellulose did you go for too and did you get any issues getting insurance? I hear some people install the blown cellulose incorrectly and then can't seem to sell their house as the new buyer can't seem to get a mortgage or insurance on the house.

 

Many thanks!

[Dave Jones]

On 18/04/2022 at 09:39, joe90 said:

Breathable membrane under slates, as slates are flat there is no gap for wildlife to get access, my OSB is on the inside to a/ hold insulation in place b/ create an airtight layer. 

 

Again as slates are flat no gap for much wind to enter, we are in a windy location but never heard any flapping.

 

not an airtight layer.

 

Only a proper VCL can provide that. no taping, gluing etc etc

[joe90]

1 hour ago, Dave Jones said:

not an airtight layer.

Well the air test proved otherwise.

[Iceverge]

On 04/10/2022 at 13:46, Renegade105 said:

So, if using blown cellulose, could you please just speel out what you did/would do for a roof made from attic truss's, circa 225 deep timbers, spaced @ 600cc. and a pitch section circa 42 Deg, and a small flat roof section in the centre.

 

We had a cold trussed roof so much different. 

 

Trusses are slightly trickier to get the membrane right. A ridge beam and cut ratfers would be idea. 

 

I'm in Ireland and I don't think ecocel cover the UK. 

 

Do you have any roof section to post to give me a better idea? 

 

 

[Radian]

On 12/05/2022 at 09:04, Iceverge said:

Here's a screenshot from the Tyvek Supro agrément cert. Note figures 6-8 have the insulation touching the membrane. You need to incude a counter batten to avoid any moisture pooling behind the tile battens however. 

 

Why are Tyvek calling those "Warm roofs"? I thought the insulation had to be on top of the rafters to count as a warm roof.

 

[Temp]

17 hours ago, Radian said:

Why are Tyvek calling those "Warm roofs"? I thought the insulation had to be on top of the rafters to count as a warm roof.

 

 

Its the old confusion between..

 

"Warm Roof" or "Warm loft" - insulation between rafters rather than on the floor of the loft. 

 

and

 

"Warm Roof Construction" - meaning the structural members are on the warm side of the insulation.

 

 

Edited October 6, 2022 by Temp

[Temp]

Eg Valid options are..

• Cold Loft with Cold Roof Construction
• Warm Loft with Cold Roof Construction
• Warm Loft with Warm Roof Construction

[Dave Jones]

On 05/10/2022 at 13:51, joe90 said:

Well the air test proved otherwise.

stops vapour moving into it ? 

 

Its wrong and people shouldn't copy bad practice.

[joe90]

1 hour ago, Dave Jones said:

Its wrong and people shouldn't copy bad practice.

OSB4 otherwise known as smart ply as recommended in passive houses for a high “perm”

[Dave Jones]

On 06/10/2022 at 18:14, joe90 said:

OSB4 otherwise known as smart ply as recommended in passive houses for a high “perm”

 

still not a VCL.

 

For a warm roof a VCL MUST be used.