U value

[JackofAll]

Have to decide on what type of insulation for vaulted ceiling, most likely 9x2 rafters though am looking to see if 12 x 2 are available. If 9x2 was thinking onmifit 225mm followed by insulated slab(mineral wool) 62.5mm. Does anyone know what kind if u value that would get. Engineer reckons sprayfoam would lend itself better all round not just the shape of the roof but Airtightness too. Have heard enough stories as to why one should avoid.

 

[JohnMo]

We spray foamed ours. We block house moisture from getting into the roof structure and if it does the foam is open cell, the roof outer membrane is vapour open so any moisture can escape.

 

Roof construction is

Slates.

Breather membrane.

Sarking boards

254mm posi rafter.

100mm underdrawn with counter battens.

Full fill spray foam - 354mm

Airtight, vapour block membrane

50mm counter battens 

Plasterboard.

 

Problem with spray foam reports is people putting it, where it is not appropriate.

 

 

 

[JackofAll]

14 hours ago, JohnMo said:

We spray foamed ours. We block house moisture from getting into the roof structure and if it does the foam is open cell, the roof outer membrane is vapour open so any moisture can escape.

 

Roof construction is

Slates.

Breather membrane.

Sarking boards

254mm posi rafter.

100mm underdrawn with counter battens.

Full fill spray foam - 354mm

Airtight, vapour block membrane

50mm counter battens 

Plasterboard.

 

Problem with spray foam reports is people putting it, where it is not appropriate.

 

 

 

Just curious, with full fill did you not have to counterbatten below the slate?

[JohnMo]

No the sarking boards have a gap, and the slates have a natural gap also.  It's not normal to batten below natural slate in Scotland, but we have to install sarking boards.

[JackofAll]

Looking at cellulose,has a better u value than the omnifit from what I can see, just not sure price wise, may be a better alternative to sprayfoam, any opinions?

[Mr Punter]

I would steer clear of spray foam insulation.  Mortgage companies and insurers are risk averse and scare stories have frightened them off.

[Iceverge]

2 hours ago, JackofAll said:

Looking at cellulose,has a better u value than the omnifit from what I can see, just not sure price wise, may be a better alternative to sprayfoam, any opinions?

 

This is where it's at. 

 

From memory about double the cost of mineral wool but it includes install. Similar to spray foam I think. 

 

Ask your engineer about decrement delay if you want to see their knowledge come to an abrupt end. 

 

Slates,

38mm slate battens. 

22mm counter battens along the rafters. 

Breather membrane taped at all joints. 

(Optional 11mm OSB sarking)

220x44mm joists full filled with cellulose @600cc.

47x44 battens run at 90deg and full filled with cellulose. 

Airtight membrane. 

47*44mm battened service void full fill with batt insulation. 

Plasterboard and skim. 

 

I value about 0.12-0.14 depending on whose cellulose you use. 

 

 

 

Edited December 28, 2022 by Iceverge

[JackofAll]

21 hours ago, Iceverge said:

 

This is where it's at. 

 

From memory about double the cost of mineral wool but it includes install. Similar to spray foam I think. 

 

Ask your engineer about decrement delay if you want to see their knowledge come to an abrupt end. 

 

Slates,

38mm slate battens. 

22mm counter battens along the rafters. 

Breather membrane taped at all joints. 

(Optional 11mm OSB sarking)

220x44mm joists full filled with cellulose @600cc.

47x44 battens run at 90deg and full filled with cellulose. 

Airtight membrane. 

47*44mm battened service void full fill with batt insulation. 

Plasterboard and skim. 

 

I value about 0.12-0.14 depending on whose cellulose you use. 

 

Thanks @Iceverge  that's approx 330 odd mm, I take it the service cavity battens run parallel to the first set containing the cellulose? Am curious as to the last sentence, "depending on whose cellulose you use" can you expand on that please, thanks again.

 

 

[Iceverge]

11 hours ago, JackofAll said:

I take it the service cavity battens run parallel to the first set containing the cellulose?

 Correct, this way they "pinch" the membrane nicely between the battens as well as  providing a good thermal break.

 

12 hours ago, JackofAll said:

Am curious as to the last sentence, "depending on whose cellulose you use"

 

Yes. Some providers have a k value of 0.038W/mK- 0.04W-mK. The one we used (Ecocel) has a NSAI cert of 0.034W/mK. 

 

In practice it's probably more important to ensure its installed correctly rather than the last 0.00X W/mK .  

 

[JackofAll]

12 hours ago, Iceverge said:

 Correct, this way they "pinch" the membrane nicely between the battens as well as  providing a good thermal break.

 

 

Yes. Some providers have a k value of 0.038W/mK- 0.04W-mK. The one we used (Ecocel) has a NSAI cert of 0.034W/mK. 

 

In practice it's probably more important to ensure its installed correctly rather than the last 0.00X W/mK .  

 

 

Will look them up, thanks @Iceverge