How much space to leave around the down light in kings

[compact]

Thanks for the advice unfortunately we can't lower the ceiling due to windows.
We're hoping to have angled spots set on dimmers so not too bright.
We've had to make the best of it.
Unfortunately limited insulation behind, will keep an eye on it for mold if we get condensation issues

 

[John D v2.0]

your problem will be interstitial condensation not condensation on the plasterboard. That means the cold timbers will be hit by warmer and damp air coming straight through the insulation layer. Unless you're going to pull the ceiling down and check the moisture in the timbers after the winter it's a bit of a non starter.
You can risk it, and there's every chance it'll be fine, but I wouldn't.
It's a fundamental principle of insulation (kingspan) and vapour control (plasterboard) that they are continuous and together. So blasting a hole is always going to be risky. I'd recommend as previous posters either a warm roof, drop the ceiling, or just surface mount the lights.

 

[ban-all-sheds]

Did you not tell the person who designed the roof that you wanted recessed lights in it?

 

[compact]

The builder said not a problem. Obviously is.
So now looking at maybe something surface mounted, where we can drill a hole, fish out the cables and pack the hole with rock wool.

Just have to find something that looks nice.

 

[John D v2.0]

A hole with Rockwell behind is not a vapour control layer. You'll need to make sure no damp air or vapour can diffuse through

 

[compact]

@John D v2.0 Where is the damp air or vapour diffusing from.
Is it from the room, through the insulation to the membrane?

Once I have packed rock wool insulation into the void we made, fished through the cable, then filled the hole, this should be an ok barrier?

 

[ban-all-sheds]

The builder said not a problem.

He thinks no insulation at all is not a problem. He's a clown.

Where is the damp air or vapour diffusing from.
Is it from the room, through the insulation to the membrane?

Yup.

The foil layer on the insulation stops it, as it is vapour-proof.

Or at least it would stop it if it were sealed to the frame at the edges.

Once I have packed rock wool insulation into the void we made, fished through the cable, then filled the hole, this should be an ok barrier?

Rockwool is not air-tight.

 

[John D v2.0]

Yes from the room, the air from inside can always condense if the surface is cold enough eg back of the fridge, single glazed window. Or where the air leaves the house through a hole near a cold timber.

You may be able to get sealing hoods or sealed down lights and fill all the gaps with silicone or similar.

I'm not an expert on condensation risk analysis but maybe someone else can help.

 

[Taylortwocities]

I would have thought that there is enough depth, in the set back area where the cables are, to put a layer of kingspan there. There should then be enough clearance (Clarance) to mount the downlight in front.
Of course the two levels of Kinspan would need to be sealed together to maintain a continuous thermal and moisture layer. This may be too much for the builder to comprehend.

 

[ban-all-sheds]

Of course the two levels of Kinspan would need to be sealed together to maintain a continuous thermal and moisture layer.

As per John D v2.0, I don't know how perfect that seal needs to be in practice.

 

[bobsuerita]

should you not have insulation under the rafters too
you look to have enough room there to form a pocket the insulation ignore the thinkness of insulation on that drawing i posted

 

[compact]

The rafters total 210 in depth. That would be 150 you guys advice + insulation + air gap by felt.

However with the differing advice and being up against time with plasterers we now have a tunnel gapy for front to back around the light. So I'm thinking the best I can do is put some insulation in the that tunnel with near the felt and have a , or fill all of it and surface mount.
It's been plastered know so unfortunately not the best situation but will have to make do and cross fingers.
We know we haven't the best vapour barrier, but if packed with some insulation it will stop the hot air getting all the way to the felt.
Not ideal but situation we are in.

 

[ban-all-sheds]

Why is the quality of housebuilding in this country so )(£&$&*%(^& poor?

We have some of the, if not the, worst housing in Europe - the lowest technical standards, the lowest space standards, the most shoddily built...

 

[compact]

Hi
So surface mounting is the better solution, can anyone recommend a nice plain white surface mounted dimable LED similar size to spots?
Cheers