Spray foam insulation

[Russell griffiths]

Can anybody spread some light on the different types of spray foam insulation 

for instance with some of the closed cell types would you still need a vapour control layer on the warm side of the insulation? Or does this completely seal everything thing up

do you still need a breathable membrane on top of your rafters under your counter battens or is there now no moisture that needs to escape 

 

trying to work out the associated costs that go with various different insulation types

its ok thinking one thing is a good price but then add in £1000 worth of vapour control layer and it all adds up. 

Cheers

confused as Normal. 

[D Walter]

Hi Russell, I have no technical knowledge on this but we are using closed cell spray foam as our topmost insulation layer in a ventilated build up under a metal (Tata) roof.  One of the reasons we decided to use foam is to assist in getting an airtight seal around the roof plate as we will have an MVHR system.  Airtightness seems to be one of the main reasons closed cell spray foam is so popular in new builds. 

 

We did use a breather membrane above the rafters (under the battens) partly because you need something to spray against from underneath and OSB is not acceptable from a condensation calcs perspective.  BASF can provide some evidence by way of test results that the heat from the spray foam will not damage the breather membrane (I think the test results are for open cell foam but they say the heat impact is equivalent).  I wanted to spray against PIR fixed above the rafters (not foil backed) but, illogically to my mind, this threw out the conensation calcs.

 

I believe there is an argument that you would not need a VCL under closed cell spray foam insulation but BASF does say the foam will allow vapour transfer, albeit at a very slow rate.  One point to note is that it is easier to spray thinner layers of insulation (we are using 154mm) rather than the thicker layer we would need to achieve our target U values.  Spray foam is also more expensive than most alternatives.  Accordingly, we are going to have semi rigid batts under the spray foam and PIR under the rafters, with a VCL under that.

[A_L]

3 hours ago, Russell griffiths said:

for instance with some of the closed cell types would you still need a vapour control layer on the warm side of the insulation? Or does this completely seal everything thing up

 

no, unless there is a layer more vapour resistant than the insulation on its cold side,  good ventilation on the cold side would reinforce this. What is the thermal conductance and vapour resistance of the insulation?

 

 

3 hours ago, Russell griffiths said:

do you still need a breathable membrane on top of your rafters under your counter battens or is there now no moisture that needs to escape 

 

a (non-tenting) water vapour permeable membrane at this point is really there to act as a backup should the final layer (i.e. slates etc.) have a fault and let water through