Cold roof with no VCL? Can I have spotlights?

[Mattsville]

Hi All,

Wonder if anyone can shed some light on this. I'm no builder so please bare with me. We've had a single storey pitched roof extension built with a vaulted ceiling. The roof construction as done by our builder, is a cold roof as follows:

Slate Rood

Breathable Membrane

200mm Joists with ventilation gap and 100mm Celotex inbetween 

plasterboard backed with 40mm Celotex

I have sketched a profile - hope it all makes sense.

They have got as far as fitting the Celotex between the joists and following a conversation with the building control officer I have some concerns - I'd appreciate some advice.

1. Do we need a VCL? The 100mm  Celotex has a foil layer, as it looks like the plasterboard insulation does on either side of the insulation, but there has been no taping of joins and no plans to add a VCL. Is this ok?

2. We are having spotlights in the ceiling - the builder planned to leave gaps in the 100mm Celotex for them, but as building control has pointed out this means there are cold spots in the roof and the light is exposed to the vantilation gap and will get condensation on and rust. He advised cutting out halfway through the celotex to leave a space for the light but also 50mm of insulation above it. However this would mean cutting through the foil layer. Is this ok?

From the research I've done it appears that neither is acceptable. Is there any way we can add a VCL and still have spotlights in the ceiling?

Thanks very much for the help!

Matt

 

 

[ProDave]

Stop there and consult your BC inspector and talk to your builder.

 

It would make far more sense to me to put ALL the insulation above the joists in the roof space and tape and seal it.  Then the spaces between the joists would be inside the room in effect and then no problem with spotlights.

 

A cold roof like this is the very worst construction method possible for energy eficciency.

[markc]

So this has already been built! You really ought to go back to the architect. The bulk of your insulation is doing very little due to the joists sitting in 100mm of cold air. 

[Mattsville]

It's too late to put the insulation above the joists - the slate roof is already on and finished. Also the extra height would then mean blocking other windows on the house. 

The BC inspector seems happy enough with the construction, it is only the holes for the spotlights he has a problem with. Looking around it seems very common to use this type of cold roof construction and still acheive the U values required for buildings regs. Is this not right?

An online u value calculation is telling me this gives a value on 0.19... which whilst it may not be quite what it should be, if the BC inspector is happy with it then I am. Do you think this isn't right?

If we get 0.19 ish then that's fine by me, but I'm worried about no VCL causing condensation and rot, and also whether the spotlights will ruin all the insulation.

Thanks,

Matt

 

[ProDave]

Actual U values and energy efficiency rely on the details being right.  Cutting holes in the insulation for downlights compromises that.

 

I am not suggesting you put the insulation above the rafters, I am suggesting above the joists so up in the loft space of the extension all insulation laid across the joists on top of them in the roof space, not in between them.  Then you don't have to compromise the air tightness or insulation to fit downlights.

 

While this may be a common way to build a roof still, does not make it "good"

[Mattsville]

Thanks for the reply - it's a pitched roof with a vaulted ceiling, so there is no loft or joists.

Given the roof is already build, I'm willing to accept "acceptable" and not be too worried about "good" if I have to. What I'm not willing to accept is a roof that might rot in 20 years. It sounds like the roof construction is acceptable, but not if we put spotlights in. I believe the plasterboard can form the VCL if they tape the joints correctly, but then the downlights would ruin it. I'll have a look at surface mounted spotlights, but what about the cables to power them? I've been told it's ok to drill through the insulation to get power to spots, but what about the VCL?

Matt

[JohnMo]

So to clear you have vaulted ceiling, so head height not an issue.

 

The insulation is not in the joists, but between the rafters .

 

Assuming the insulated plasterboard has not been delivered, change the buildup.

 

If the 100mm PIR not fitted.

Insulation between rafter dense mineral wool, use whole sheets of 100mm PIR under the rafter.  Tape all joints or install vapour control layer.  Underdraw with 50mm battens, this will give you space for wiring, slim led down-lights and the control module.  Then plasterboard.  No holes required other than in plasterboard.

 

The above is way easier to fit well, our between rafters is a pain and not easy to do well.

[JohnMo]

Another thought is maintainability of vaulted ceiling lights, can you get to them once the house is furnished, without a scaffold?

[ProDave]

6 minutes ago, Mattsville said:

Thanks for the reply - it's a pitched roof with a vaulted ceiling, so there is no loft or joists.

Given the roof is already build, I'm willing to accept "acceptable" and not be too worried about "good" if I have to. What I'm not willing to accept is a roof that might rot in 20 years. It sounds like the roof construction is acceptable, but not if we put spotlights in. I believe the plasterboard can form the VCL if they tape the joints correctly, but then the downlights would ruin it. I'll have a look at surface mounted spotlights, but what about the cables to power them? I've been told it's ok to drill through the insulation to get power to spots, but what about the VCL?

Matt

Sorry I thought there was a flat roof underneath.

 

you will just have to accept the compromise BC have suggested.

 

For the benefit of anyone reading at an early stage, please do yourself a BIG favour and get the roof designed as a warm roof with the insulation above the rafters, so much easier to build, easier to detail and much better performance, not bothered by things like downlights.

[Mattsville]

Sorry everyone, I said joists when I meant rafters. 

Yes, the 100mm PIR is between the rafters. This is already fitted. Also the insulated plasterboard has been delivered, but they could send it back I assume.

Unfortunately ceiling height is still an issue - at the eaves the ceiling height is already a little low due to not wanting the roof to obstruct windows on the rest of the house, and also the roof is supported on oak trusses and it would look odd if we covered half the truss. I mean, I'd rather that than have a rotten roof in 20 years, but if we can avoid it by just having surface mounted spots then we will.

The compromise suggested by BC is to cut out holes in the insulated plasterboard and halfway through the 100mm PIR between the rafters, just leaving 50mm left above the spots. This sounds like it leaves us with no VCL at all. Is this OK?

As far as I can see other options we have are:

1. BC suggestion as above. Will this lead to a rotten roof as there's no VCL? Celotex say under no circumstances cut through the insulation, but if it just means a slight loss of insulation I'm not too worried.

2. Create VCL below 100mm PIR (either tape over rafters and joins or install a membrane) then cut holes in the insulated plasterboard only for shallow spots. We would still need holes in the VCL and 100mm PIR for power cables. Can LED spots be completely surrounded by PIR insulation?

3. 50mm batten below insulated plasterboard and then plaster below it creating a void for shallow spots. Should work fine but loses 62mm ceiling height and is a lot of work.

4. Keep roof construction as is and install surface mounted spots. This will mean cables have to pass through VCL. Is this acceptable? Celotex say drilling small holes for cables through PIR is fine.

 

If you could let me know which of these options is acceptable I'd be grateful. I think all would get passed by BC, but that's no good if the roof would rot. Replacing rusty spots in 20 years I can probably live with if it's not dangerous and doesn't make the room really cold.

Thanks for the ongoing help!

Matt

 

 

[JohnMo]

Install up/down lights at top of wall.  Have no lights in the ceiling.  They are usually too far from you to be much use anyway.

 

Sorted, make life easy not difficult. No fretting about roof life expectancy.

[Mattsville]

I'd never be happy with that, there'd be no light in the middle of the room. I'd rather have pendant lights (or bar mounted spots) at the apex but even those are going to need holes in the VCL for the cables. 

[ProDave]

8 minutes ago, Mattsville said:

I'd never be happy with that, there'd be no light in the middle of the room. I'd rather have pendant lights (or bar mounted spots) at the apex but even those are going to need holes in the VCL for the cables. 

At least you can seal small cable holes and they don't need a big ventilation gap to cool a downlight.

 

What pitch is your sloping ceiling?  for spotlights in the ceiling you probably want the ones that swivel to adjust the angle and there is a limit to how far they will tilt.  For our 45 degree vaulted ceiling, i used one pendant and some surface mount swiveling spotlights.

 

We keep banging on on this forum about a properly designed insulation and then air tightness layer and then INSIDE that a service void before the plasterboard goes on, so all the wires etc can be run without bothering the air tightness layer.  Self builders seem to understand this.  VERY FEW general builders have a clue about this and the rule of thumb seems to be the more houses they build, the less they know about building properly.

[Mattsville]

Roof pitch is 25 degrees.

I think you're probably right about the builders, however I think it's too late for me on this one to change the roof construction significantly. Just got to work out which of the options left to me is going to be acceptable. It seems like surface mounted spots with just holes for cables will be acceptable, and probably shallow flush mounted spots within the insulated plasterboard will be fine too. If we get fire rated spots then we don't need ventilation to cool them. I'm not really sure how essential a VCL is in any case - the ventilation above the rafters should clear any condesation that get through cable holes in the PIR shouldn't it?

 

[Jilly]

Did you get thin surface mounted downlights? I've got the same issue and don't want to chop into the insulation. 

[Nickfromwales]

9 minutes ago, Jilly said:

Did you get thin surface mounted downlights? I've got the same issue and don't want to chop into the insulation. 

I use these a lot, but the transformer does need some room. The fitting itself is great, and sits in the thickness of the plastered plasterboard, so can span joists etc.

 

https://www.cef.co.uk/catalogue/products/4574883-4w-led-downlight-neutral-white-4000k?gclid=Cj0KCQjwocShBhCOARIsAFVYq0gksXEWnigiRIZLYBRXTilqjSoTipTMBPcQpeBzoMVTPSGjwCORsewaAmyJEALw_wcB&gclsrc=aw.ds

[Jilly]

Thank you Nick! 😊

[Mike]

Forget ceiling downlights - there are more interesting ways to light a room, especially a room with a vaulted ceiling. The last one I built included spotlights suspended from tension wires hung from gable to gable, for example, as well as wall lights.

[Eric]

15 minutes ago, Mike said:

Forget ceiling downlights - there are more interesting ways to light a room, especially a room with a vaulted ceiling. The last one I built included spotlights suspended from tension wires hung from gable to gable, for example, as well as wall lights.

Have you got any photos of this? I will need lights in the near future for vaulted ceilings.

[Thorfun]

22 hours ago, Jilly said:

Did you get thin surface mounted downlights? I've got the same issue and don't want to chop into the insulation. 

Track lighting?

[Jilly]

The 10 wires are already dangling through the celotex, ready to go, but pre plasterboarding. 

[Thorfun]

5 minutes ago, Jilly said:

The 10 wires are already dangling through the celotex, ready to go, but pre plasterboarding. 

Fair enough!

[Mike]

3 hours ago, Eric said:

3 hours ago, Mike said:

Forget ceiling downlights - there are more interesting ways to light a room, especially a room with a vaulted ceiling. The last one I built included spotlights suspended from tension wires hung from gable to gable, for example, as well as wall lights.

Have you got any photos of this? I will need lights in the near future for vaulted ceilings.

My photo collection is on my main computer, which is currently in storage, however it was something like this (but more atmospheric): https://www.lightingstyles.co.uk/simple-adjustable-low-voltage-tension-wire-lamp

 

[Jilly]

On 08/04/2023 at 23:39, Nickfromwales said:

I use these a lot, but the transformer does need some room. The fitting itself is great, and sits in the thickness of the plastered plasterboard, so can span joists etc.

 

https://www.cef.co.uk/catalogue/products/4574883-4w-led-downlight-neutral-white-4000k?gclid=Cj0KCQjwocShBhCOARIsAFVYq0gksXEWnigiRIZLYBRXTilqjSoTipTMBPcQpeBzoMVTPSGjwCORsewaAmyJEALw_wcB&gclsrc=aw.ds

Hi Nick, do you manage to get these in without a ceiling void under the insulation?

 

 

[Nickfromwales]

13 hours ago, Jilly said:

Hi Nick, do you manage to get these in without a ceiling void under the insulation?

 

 

You can do, if you mount the transformer elsewhere, or just cut out a pocket in the insulation for the transformer to install into. Very few fittings will either be shallow enough, or are void of a transformer tbh.