Extension Insulation - advice plaese

[MrsGarner]

Good Afternoon,

We have recently had an extension built in order to keep costs down we are trying to do a lot of the internal finishing ourselves :-/

We have a double storey wrap around extension on a demi detached house - on the attached side we have a single storey element that is a pitched roof, below will be our dining room.

The ceiling joists in this area are 8x2 (assume pretty standard).

I would like to start insulating and celotex receommended 50mm space below roofing felt, then 150mm insulation between rafters with 12mm over rafters and then plasterboard.

As your not supposed to cut through the celotex I was wondering (maybe stupidly) where the lighting cables are supposed to run?...

Should we batten the over rafter celotex - this should allow enough space for cables and were are having normal pendants lights so no need to consider any downlight space here.

What I don't want to do is buy a ton of the 150mm celotex and wedge it into place to find that i should have run the cables above it (or something like that)

Thanks in advance.
Louise

 

[^woody^]

Are you insulating a flat joists or pitched rafters? You mention both.

Anyway, you can run cables through celotex if you sleeve it and plug the sleeve. It wont make any difference to the insualtion values. If there are massess of cables, then you'd design in a service duct.

 

[MrsGarner]

Thank you for your reply. Sorry I posted the above in a rush as kids were jumping around.

It is a pitched, tiled (white roofing membrane vent 3 classic) with a single large velux.

The roofing "timbers" are 8x2 in size as i was measuring them to ensure to work our what product i would need to buy from the celotex range and how much based on the timber spacing.

I was hoping to start insulating the ceiling before the electrics were done as the working area is so cold at the moment. There will be 4 lights coming through the celotex more is less in each corner of the velux.

When you say sleeve do you just mean like plastic conduit? (I have a ton of that lying around as we havent finished the electrics yet).

Do I run the electrics above all of the insulation and just pass through each layer using conduit?

I have no problems doing this would just mean waiting until my father in law wires the place up....means still working with my gloves on

Thanks again for your reply

 

[^woody^]

As you are insulating under the rafters, I'd suggest running the cable in the air gap above the insualtion (below the membrane) and then down to the fitting. This will avoid having two air gaps.

Potentially, and depending on the membrane, you could shove the insualtion tight up to the membrane with no air void and then have a service void between the two insualtions near the rafter bottom edge.

Another way is to have the cable on the bottom surface of the insulation in an inverted strip of cable capping slightly recessed into the insualtion.

Sleeves are bits of conduit or plastic overflow pipe or similar

 

[John D v2.0]

regarding the cables, you do have to derate them appropriately for their installation method, and insulation by its nature has a huge effect. If they are lighting cables on a 6A MCB, you probably don't need to worry full stop, but if there are any shower cables especially up there, you need to attach them ideally to a timber or masonry surface clear of the insulation.

 

[MrsGarner]

Thank you all for your replies, I don't "need" to insulate under the rafter but I wasn't sure if i should, there is a lot of information onthe celotex website and maybe a little too much as its not clear to me exactly what product I need to use, I will do some more research.

Thanks Louise