Cramped and awkward eaves space - best approach for roof insulation

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When I had my loft converted, the builders did not insulate the eaves, just the wall separating the inside of my house from the eaves space. Building control were fine with this. I think the builders did a good job, they just chose to insulate the easier surface (the wall) and at the time I didn't think it was worth the trouble to request they did the eaves as well. It's had building control sign off, I believe the insulation on the eaves floor is up to code and the builder said the building envelope upstairs would include the stud wall but not the eaves roof, so I assume that has been designed as a cold space

I was planning to use 100mm PIR board in between the rafters, which would leave an air gap to the membrane above (rafters are 140mm wide). However, this is now looking like a nightmare job for the following reasons.

1. The space is so small – it’s 850mm at the highest point. The access door is small (500mm by 800mm). Most normal techniques for getting the boards in between the rafters wouldn’t work. There isn’t much room to thump the boards into place, and I wouldn’t be able to fit a saw down the edge in a lot of places. I don’t think I will be able to get a tight fit between the rafters and there would be lots of trips to re-measure and re-cut the boards.

2. I could make very minor adjustments in the space but I expect I will have to do most of the cutting outside. My house is newly decorated and carpeted so cutting in the house isn’t an option. Which means a walk between measuring and cutting which will just make things even harder.

3. The shapes I’d need to cut would be complex. The rafters overlap at the top of the eaves because the builders could not get long enough rafters at the time (covid supply issues). The rafters are tripled where we have velux windows. And the old rafters weren’t sawn off properly at the bottom, so these poke up in between the spaces in random places.

Reason for insulating - There is some minor condensation in the eaves (which contains our security camera DVR), the builder said that insulating would fix this. The space is above some bedrooms and I think insulating would improve heat retention and enable us to have an extra storage space. At the moment the temperatures in the eaves are too extreme to store anything well.

I’m now considering over boarding the rafters with 30mm insulation board instead.

Any advice on the best way to insulate this eaves space?
 

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There is rockwool insulation under the eaves floor but I can't be sure of the depth. I don't think the floor space has a gap as big as 30cm though, it might be closer to 20cm. It's a 1960's property.
 
I've just checked the building notes from my architect and I think this confirms a 200mm gap as it specifies 50 * 200 C16 joists at 400 centres (subject to existing floor construction tbc by builders on site).

I don't think the eaves are a significant source of heat loss for the rooms underneath. The main issue is that without any insulation under the roof tiles and membrane, the eave space is freezing in winter and boiling in summer which makes it unusable.

From reading similar posts it seems that it is a common design for the eaves to be a cold space. If I did put in any insulation could there be any unintended consequences. I would definitely be leaving an airgap underneath the membrane.

Would using 30mm or even 50mm insulation make a noticeable difference to the temperature?
 
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Use cavity batts, far easier than PIR.
Thanks, after watching a few videos online this looks like a much quicker install.

Any tips on how to maintain the air gap above the insulation? If I buy 100mm and rafter is 140mm will it naturally leave a suitable gap or should I go for something thinner just to be on the safe side.

Wondering if strapping or stapling will be needed to hold these up or if the batts will pressure fit into place if they are slightly wider tham the gap
 

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