Air gap above insulation

Postby stephen.mackershan » Tue Feb 19, 2013 3:37 pm


I am looking for some advice on installing insulation in the eaves of my house. It's a bit of a complicated setup so will do my best to explain.

The ceiling below my loft space is not horizontal across the whole of the ceiling. Where the ceilings meet the outside wall there is a 45 degree slope of about 1 foot. Currently the loft space is insulated above the horizontal part of the ceiling only. This is causing issue in that the 45 degree section of the ceilings are not insulated, making them colder than the rest of the ceiling. Due to this, condensation is forming on them and they have mildew growing on them.

The problem I have is that the area above the 45 degree sections of the ceiling is hard to get access to and is a tight space. The space between the top of the lathing and the underside of the tiles is only the depth of the rafters (approx 100mm). I had previously read that an air gap is required at the eaves of 50mm between the top of the insulation and the underside of the tiles. I went and bought a load of spacers specifically to keep an air gap between some rockwool insulation and the underside of the tiles. However, due to the ceiling joists meeting the rafters right at the entrance to this small space I have found it practically impossible to get the spacers and insulation into place whilst maintaining the air gap.

I have since read that no air gap is required if the tiles have no felt on the underside (as is the case in my house) as the roof space is naturally breathable due to the small gaps between the tiles. So, after a long-winded explanation, I am wanting to know if anyone can recommend on whether this would be the case in my situation?

Thanks in advance for the advice.
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Simply Build It

Postby welsh brickie » Wed Feb 20, 2013 1:02 pm

keep an air gap, so use kingspan hardboard insulation on the angled areas it can be 50mm thick giving you 50mm air gap you can use ordinary fiberglass for the rest.
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Postby stephen.mackershan » Wed Feb 20, 2013 1:17 pm

I tried using insulation boards at first but due to the position of the ceiling joists joining the rafters it is impossible to get a solid board into place.
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Postby welsh brickie » Fri Feb 22, 2013 1:32 pm

welsh brickie
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Postby Perry525 » Mon Feb 25, 2013 6:55 pm

Air gaps are old tech and are not required as noted.
You could look at this from a different point of view.
Tiles are easy to remove and you can place the insulation from above.
Or you can pull down the lath and plaster, cut and fit polystyrene sheet between the rafters and then fit another 3 inch layer of polystyrene sheet below the rafters to close the heat bridge, finish with plasterboard. This will give you the best insulation possible within a confined space.

Well, except Aerogel, a 3/8 layer of Aerogel placed, where ever, will give you insulation the equivalent to 3 inches of fiberglass, it costs £2 a square foot, but you don't need much.
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