Insulating loft with loft conversion

21 May 2015
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United Kingdom
We have an old bungalow with two rooms in the roof. We are not sure exactly when the rooms were added, but it was done a long time ago. The rooms are uninsulated, and the loft space needs proper insulation, so I'm planning to insulate the remaining loft space and walls.

I'm not sure how to handle the void between the ground floor ceiling and the loft rooms. The rooms in the roof are on metal I-Beams and large wooden joists, with air free to circulate - you can see from one end to the other between the joints, and there is no insulation at all. As a result ground floor has only a lath/plaster ceiling with no insulation and is losing a lot of heat.

My current plan is to use some plastic sheeting to seal the gap between ground floor and then insulate between the joists in the open loft, then have the wall insulation butting up against the joists before adding the extra 170mm layer in the open loft. We will still have to do something for the loft room ceilings, but this should be a good start.

However I have some concern about potential condensation in the void space between the floors. It should be a warm space, but not very well ventilated, and I would appreciate any advice if this is likely to be an issue, or if I should simply omit the plastic sheet to allow some breathability.
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Am i reading this right that the void your refering to is between the downstairs (heated) and the conversion upstairs (also heated)?
If it helps i have exactly the same, except I am about to rip out the current loft job and do it properly. The architect drawings specify celotex or similar between the roof rafters ie enclosing the roof and structure in insulation and soundproofing between the loft floor joists.
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In principle, this is what you need to be doing (also including a vapour barrier on the warm side of the insulation in the loft).
Well blimey...I was just about to create a new thread on almost the exact same subject.

I was wondering which of the above images would be the best solution so I am presuming that the middle image is the one to aim for.
We are also thinking of removing all the existing work and redoing it because it will be much easier than trying to do it with the rooms as they are. Shame really but needs to be done.

As you can see its not a bad job just a shame they did not insulate when it was done 20 years ago. Also need to have a good look at the rafters and see if they need attention as it has already had some reinforcing done to some of them so might be ideal time to inspect and sort them also.

It has had leaks in the past but what looks likes wet in the above images isn't. They are dry.
Well blimey...I was just about to create a new thread on almost the exact same subject.
It would be better to start your own thread and leave this one for mutant64 I would say even if it does cover a similar topic. (y)
Use rafter trays to prevent insulation obstructing air flow at eaves. Principle generally as Tony1851 shows but I would also include 100mm Rockwool insulation for acoustic insulation between joists of loft floor.
The point is that the OP will never come back to us to let us know what they have done. So it is a dead thread.


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