100mm insulation between 5x2 rafters too much?

[Ian H]

Would that leave me too small of an air gap between the insulation and felt?

 

[Doggit]

Should be okay if done carefully, or you could add an inch batten to play safe.

 

[white_noise]

What sort of insulation are you using, if 100mm mineral wool, how about 75mm PIR board as will have about the same thermal qualities.

 

[Ian H]

It's Ecotherm PIR board. I was looking for 75mm but found a bulk of 100mm for a good price.

 

[white_noise]

I would and have done in the past battened out to allow the ventilation gap of 40-50mm. Its unlikely you will get exactly 52mm timber to match the existing rafters, so you will have a bit of a gap either side of the PIR board at the face, that if you want to fill will expanding foam. I didn't have to as i was lucky and able to machine wood down to 52mm.

 

[geraldthehamster]

Is your felt the modern breathable type, or the older impermeable type? If the latter, you need 50mm, with continuous ventilation (e.g. by means of soffit and ridge vents). If the former, in theory you need no gap, though some gap is often advised

 

[Ian H]

It will be new breathable stuff. Is that because the space ontop of the felt/under the tiles will become the vent?

The wood is new too so I think it will really be 44ish mm.

9 pieces of 100mm only just fitted it my van.

 

[geraldthehamster]

It will be new breathable stuff.

In which case the answer to your original question is "No"

 

[Doggit]

Crap spatial skills Ian. Cut some of the boards to 10mm wider than the joist gap, and you can get a few more boards in there.

 

[Ian H]

That's all the guy had!!!

 

[John D v2.0]

you don't need an air gap below the membrane, but you do need a space for the water to run down on the outside behind the battens so it doesn't build up and end up soaking through the nail holes etc. Pretty sure even 10mm is enough for that though. Alternative is counter battens on top.

 

[Ian H]

They are matching the neighbours roof so can't really go ontop. The bit that will be insulated is about 3mtrs so I think I'll add an inch on the underside to be sure.

The condensation on our fancy scaffold is a pain. I just sheeted over the rafters to catch tomorow mornings drips.

 

[Doggit]

you don't need an air gap below the membrane, but you do need a space for the water to run down on the outside behind the battens so it doesn't build up and end up soaking through the nail holes etc. Pretty sure even 10mm is enough for that though

But you tape the foil faced PIR, or install a VCL to stop any moisture getting through. The air gap up and over the insulation is then a double measure to make sure the rafters don't rot. 25mm for a breather membrane (which Ian's got) or 50mm with felt.

 

[John D v2.0]

But you tape the foil faced PIR, or install a VCL to stop any moisture getting through. The air gap up and over the insulation is then a double measure to make sure the rafters don't rot. 25mm for a breather membrane (which Ian's got) or 50mm with felt.

I'm not sure about double measures, the air gap would have to be ventilated to do anything useful, but with a breathable membrane over you'd struggle to need it in the first place.
Of course do the condensation risk analysis and if it says there's a risk then redesign the buildup.

 

[Doggit]

The air gap is ventilated at the eaves, and in theory, a breathable membrane would allow any moisture through an out, but in practice, leave an inch gap between the PIR and the membrane.