Celotex PIR insulation over studs - is it REALLY necessary?

I'm DIYing my loft conversion (builder built the watertight structure and did roofwork, I'm doing all internals). It's hip to gable with a rear dormer.

Insulation wise, I have 90mm Celotex in between external studs. I am thinking whether or not I need to use 15mm OVER the studs too, or if it's overkill? Is it just to keep heat in, or is it to stop condensation from cold bridging on the studs? The roof is 120m between joists and rafters, with 25mm under, so definitely enough there. I'm also putting a vapour barrier (https://www.insulationshop.co/novia_1200g_vapour_control_layer.html?sp=1&search=novia) in the roof, but not the walls.

Bear in mind the house itself is only 3 years old with a lot of new build insulation as per regs, so already gets VERY hot in summer (almost unbearable in first floor bedrooms in summer).

If I'll be adding 15mm over the wall studs JUST to prevent heat loss, I don't see the point - in fact I'd rather the loft was cooler rather than hotter. As an example, before I put any roof insulation in, the loft was reaching almost 40 degrees last summer during the heatwave!

Building control (private) are fine with just 90mm between studs, but they seem quite relaxed on most matters so I thought I'd try and get a more technical answer here.

To add, the external walls are mainly tiles on OSB, with render on OSB on the gable wall

To add, the external walls are mainly tiles on OSB, with render on OSB on the gable wall

Bear in mind that a badly insulated loft conversion will be an oven in the summer. I'd put 50mm over the top if you're not struggling for head height.

garyo said:

Bear in mind that a badly insulated loft conversion will be an oven in the summer. I'd put 50mm over the top if you're not struggling for head height.

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I've already put the 25mm under rafters and ceiling joists...so am specifically after advice for the walls.

Although your post has thrown me off a little - why would a badly insulated loft be hotter than a well insulated loft? Doesn't the insulation keep the heat in?

So you think I definitely SHOULD put 15mm over the wall studs too then? House is already an oven as it's new build, won't this make it worse?

jamie765 said:

why would a badly insulated loft be hotter than a well insulated loft?

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Insulation is bidirectional, like a thermos that you can use for both icy drink and hot chocolate. In my current house the older 1960s loft areas are baking hot in the summer due to the heat soak off the black roof tiles and lack of insulation to protect the bedrooms from it.

If you're already struggling with too much heat in summer then I'd definitely over-spec what you're doing in the loft.

I have worked on lots of old loft conversions when I have been replacing the old skylights with new ones .
I quite regularly find no insulation between the rafters.
I always ask whats it like in summer and they say unbearably hot and cold in winter. The foil backed pir helps to keep your loft cool in summer, don't forget the vents.

jamie765 said:

Doesn't the insulation keep the heat in?

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Lol. Has no one explained to you how a fridge works?

Don't skimp on the insulation at this stage. Insulation across the studs is there to prevent cold bridging through the studs and it assists in the overall performance of both the insulation (and if done correctly) the vapour barrier.

You'd have to be mad not to go the whole 9 yards at this stage.