Carport under bedroom re-insulating project.

[Dustcap]

hi all, I'm new to the site and hoping for some advice please....

Me n the mrs moved into our new home back in Oct '16.
Over the coming months we discovered the master bedroom which is over a carport was freezing. You could feel the cold coming through the floor, even our mattress was cold from the cold coming up from below.
Anyway, after doing the front and back garden over the last few months it's now time to tackle this problem. I made an inspection hole in the underside of the ceiling in the carPort and found a 25mm wood cladding under a 5mm plasterboard, these were screwed to the underside of the joists, which were 220mm deep. Inside the void I found a 170mm thick layer of yellow glass fibre type insulation with a 50mm gap above it, then the chipboard of the bedroom floor.
(Measurements are approx but pretty close)

So my plan...
tear down the cladding and plasterboard. Remove the rubbish yellow insulation and replace with 200mm celotex or similar PIR insulation. Then under this, put fire grade (pink) 12.5mm plasterboard, then under this some new UPVC cladding.

Questions...

Should I leave an air gap somewhere (above/below the insulation so maybe apply a 150mm deep insulation)?

Should I foam the gaps where the insulation doesn't meet the wood exactly? (But shall try for a snug fit)

My main fear is a do the job and create moisture traps which then lead to rotted joists....

A friend of mine said it needs to be double plasterboarded to meet fire regs, is this correct?

He also mentioned an Intumescent fire matic, is this also necessary???

I'm hoping to get this started in the next couple of weeks so currently seeking useful/informed advice.

Thanks for any tips in advance,,,,
Appreciated.

Dustcap.

 

[Doggit]

I'd say you've got it spot on. The only wires in the floor void, will be for the garage lighting, so whilst you normally need to keep them clear of the insulation, I don't think the voltages concerened are enough to worry about; foaming any gaps wouldn't hurt. Double plasterboard for safety, but ignore the intumescent strip.

 

[JohnD]

170mm fibreglass is pretty good. Better insulation than an insulated cavity wall. So I will guess that you have a problem with draughts. Unless there are holes in the cladding and plasterboard, e.g. for pipes and cables, it's commonly where the joists have been slung into approximate holes in the walls. You can seal them up with expanding foam (I'd prefer the fire grade, though it is only rated for something like 8mm gaps).

You can also do it by stuffing extra mineral wool round the edges of the room, to muffle any gaps between plasterboard and walls, and under the skirting which is often gappy.

In your case I don't think you need special precautions against damp in the underfloor void.

Assuming you have carpet down, look for the tell-tale black marks where draughts carry fine dust which gets trapped in the pile.

 

[JohnD]

p.s.

cables can be under the insulation; or above it; but never buried between two layers. If they are draped above insulation, that counts as being in free air.

 

[Dustcap]

I'd say you've got it spot on. The only wires in the floor void, will be for the garage lighting, so whilst you normally need to keep them clear of the insulation, I don't think the voltages concerened are enough to worry about; foaming any gaps wouldn't hurt. Double plasterboard for safety, but ignore the intumescent strip.

Doggit, you mention double layer plasterboard, should these both be fire rated?

I assume cross the fittings so lines don't match?

Would the joints need taping on both the plaster board and the insulation?

Should I fill the entire space with insulation or leave a gap?

I shall look for draughts while I'm up there and fill any I see with foam...

Thanks again.

 

[Doggit]

You can use one piece of fire rated plasterboard, or 2 pieces of ordinary - which would add to the insulation. Yes, make sure you stagger any joints. You tape the joints on the plasterboard (top layer only needed) so they don't crack when plastered, but you tape any joints in the insulation so that moisture doesn't get through. If you're using PIR, then building regs would normall ask for 135mm, so you'd only fill the gap if you were using loft insulation.

 

[highleigh]

As has been said 170mm insulation is good so the cold must be going around/between it. If the air gap is above it, I.e. below the floor boards it sounds like an upside down warm/hybrid roof. ( I'm no expert so others may have other ideas)

If this is the case and there are drafts then you don't need to remove the nsulation but it may be worth removing all the cladding/ plasterboard, check the insulation covers the whole ceiling then add more nsulation below the joists to remove any cold bridging and make it air tight. Make good the carport ceiling and job done.

 

[Nozzle]

Don't get any expanding foam, should you use any, near your cables - it eats away at the PVC.

Be aware of cold bridging, and know that the advantage the matting has over solid insulation is that you can poke it into corners and close up small gaps in a way impossible with celotex. Tape over any resultant gaps and cracks to stop draughts bypassing your insulation.

Nozzle

 

[Dustcap]

Hi again guys.
So I started the project this weekend.

Down came the tongue & groove, down came the old plasterboard and are currently at tip. Down come the fibreglass insulation, waiting to go to tip.

Up went all our insulation and half of the plasterboard.

I have a query though.

While it's all down I thought I would add a 360 degree PIR sensor to the lights so it turns on when someone/thing moves under the carport!

I have wired these in before but this wiring diagram is confusing me
It appears to me it used the neutral wire as a switch opposed to the normal switched live!
Am I reading this wrong or what?
Cheers.

 

[Doggit]

The light is between the neutral, and the switched live (the LH one A I think)

 

[Dustcap]

So you think the left hand connection (yes it's an A) is the switched live? And the neutral is totally separate and should be wired as normal?

 

[Doggit]

Every light requires a neutral and a live connection, but the picture they show is a representation of how things work. The Live has an in arrow, and the A an out arrow, so this tells you the direction the lives going. The light is shown across the switch live and the neutral, but the connection to the neutral is further back, so it's telling you that it needs the neutral connection, but that this could be further back on the feed, so you could use a 3 core cable, take it to the light, and then take it onto the PIR, but you'd only connect the yellow cable from the light to the A connector on the PIR. Alternatively, you'd take a live (connected to A) and neutral cable from the PIR to the light switch. Hope I've made it sensible enough for you, but if you've done these before I suspect you get the drift.

 

[Dustcap]

Ok I pretty sure I get the gist.
I have preinstalled a length of 4 core from the switch to the pir and back to the 1st light fitting of the daisy chain. I shall use the brown live to the pir and the grey as a live from the pir and wire up neutral as normal.
I'm confident that shall do the trick.

 

[Doggit]

But did it go from the power, to the light, and then the pir, then you'd use 3 core, or from the power to the pir, and then to the light; in which case you'd only need twin and earth.

 

[Dustcap]

Power to Pir to light.

I think twin & earth would have done it but I had some 4 core knocking about so I used that!