Roof insulation. Internal,external

[Russell griffiths]

Morning all. 

Looking for opinions on a roof,  ceiling build up

i need to install some form of insulation to cover my roof joists to stop any cold bridge 

so there are two options really, on top or inside. 

 

option 1. Soft batt type insulation between the joists( depth to be confirmed) with a semi ridgid ROCKWOOL type board over the top laid at right angles to the joist with a counter batten on top of that. 

 

 

Option 2. soft batt type insulation between the joists (depth to be confirmed) with a ridgid insulation board on the underside of the joists at ceiling level, 

 

i would prefer option 2 as I am concerned about raising the roof line, whereas I have masses of headroom underneath 

 

does one offer any benifits fits over the other? Or is it a case of if the same insulation was used in both options the combined thickness would be the same wherever you put it. 

 

Cheers russ. 

[Cpd]

I believe that the less permeable the insulation should be on the inside.  So ridged insulation on the inside and rockwool up above. 

[PeterW]

Option 2 needs you to get the VCL perfect... What is the inside finish and are there any penetrations planned or can you create a service void.?

[Russell griffiths]

Thanks @PeterW

more info, vcl on inside of inner insulation, taped to inner ICF wall. 

ceiling vaulted and completely flat so easy to wrap, 

90mm service void on underside of ceiling. 

Ceiling timber clad. 

 

if I thought it was going to be a pain to install the vcl over the inner insulation I could put up a layer of 9mm osb and install vcl flat on that. 

[Ed Davies]

What's going on top of all this? If it's somehow well ventilated the VCL becomes a lot less critical; i.e., still a good idea but lack of perfection isn't disastrous.

[Russell griffiths]

The finished roof is metal roof sheets, exact type not finalised yet, we are also undecided on if to have a ventilation gap under the sheets or not. 

Seems a debateable subject depending on if you feel it is a full warm roof or not. 

[A_L]

Now that you have said it is some sort of metal sheet roof I wood say that a ventilation gap on the cold side is essential. Well ventilated a VCL is a backup which I would install but is possibly not necessary. The boards in option two will have a higher vapour resistance than the between joist fill and will be les likely to need a VCL than option one where the rockwool batts will have a nominally higher vapour resistance than the joist fill but still a very low value. In option one the rockwool batts are not strong enough to take the external roof and you would require battens fixed to the joists to carry it.

 

Here is the BBA cert for a metal sheet roof, it has a ventilation gap and an optional VCL, page 7.

 

BBA Certificate for Colorcoat Urban Roof panel.pdf

Edited August 25, 2018 by A_L
remove block of white space

[Russell griffiths]

Thanks for that @A_L

the problem I have is I find that tata steel BBA diagram rather poor

why would you have an air gap on top of your insulation

this I believe lessons the effectiveness of the insulation dramatically as wind washing over the insulation decreases its effectiveness. 

I like the idea of an air gap

 

 

why are the also calling it a warm roof without having insulation over the joists, are they calling that foam pad insulation?  

[A_L]

@Russell griffiths , what is described as a 'breather membrane' in the diagram on page 7 is actually described in section 4.2 (page 4) as a 'vapour control layer' ! This and the relatively high vapour resistance of the OSB immediately below it means that there is a significant risk of condensation and a flow of air is needed to remove it. With insect proof ventilation at gutter and ridge and a air flow at right angles to the insulation I do not think there is to much of a problem, low air velocity.