Air gap needed if roof has no membrane?

[BadgerParty]

Hi,

Recently purchased my first house, and I'm in the process of sorting out the loft. The plan is to insulate the roof to <0.18U, so that in the future it can be converted to a livable space once I can afford it. This is a mid-terrace house built in the 1930s.

The roof construction is slate tiles onto 20x50mm wooden battens, the battens are nailed directly onto 20x80mm wooden sarking boards with no membrane or sarking felt. The sarking boards are nailed to 90x45mm rafters on 350mm centres, and the rafters are supported by purlins that run the length of the house, and are braced onto the party walls using diagonal struts. Please see image attached, which shows the slates exposed from the underside from where someone cut through the sarking boards.

My plan is to fit PIR insulation between the rafters, ideally fully filling them to 90mm depth so that it bottoms out directly against the sarking boards, and then attach a minimum of 60mm PIR insulation underneath the rafters, and finish with 12.5mm plasterboard. According to the calculators I've used, this should give me 0.18U

The roof is in good repair, with no evidence of damp or rot to the woodwork.

My question is, is it necessary to have an airgap between the insulation and the sarking boards? I've been trawling the forums trying to find an answer, and my repeated calls and emails to building control have gone unanswered, but I just seem to be going around in circles. The way I see it, there is already a 20mm air gap from the battens, and the sarking boards have gaps between them, so air is able to move around and escape.

Any advice would be appreciated, as I want to go ahead and order the insulation, and I can't seem to get building control to respond.

Thanks

Phil

 

[Bonni]

A roof with or without felt needs ventilation/air gap. Do not insulate between those rafters in the picture if you don't have heating in the loft, you will suffer major condensation.

 

[mikeey84]

I'm really not sure that creating a space that is probably 1/2 the volume of your 1st floor to heat is a great idea?

All you will do is have a large warmish loft that sucks all the heat from your house, and then there is a fair chance you may have to rip half of it out when you do the full loft conversion so that building control can see the make up of the roof?

 

[BadgerParty]

Ok so I managed to get hold of a nice guy at building control, and he confirmed that a 20mm air gap between the insulation and the sarking board will be sufficient, as the roof is already breathable.

I have the pre-works inspection on Tuesday, so after that I'll order materials and crack on