Insulation gap and condensation

[Ben Blaukopf]

Every time I do something on this place, I find something new.

Got an insulated garage "professionally converted" before my time. I was planning on fitting a recessed consumer unit in one of the walls, but I think now this is going to be a bad idea.

Having carved a hole in the plasterboard, I find I've got

Single skin brick some polystyrene glued onto the brick about 50mm of rock wool an air gap - clearly well ventilated because I can feel the cold air, which probably explains why the room is not as warm as I think it should be. vapour barrier (plastic sheeting) plasterboard

1) Isn't that air gap in the wrong place? Shouln't it be on the cold side of the insulation? Not that I'm going to do anything about this right now, I'd have to rip out all the PB and insulation and start again. Next decade.

2) The socket and switch holes form holes in the vapour barrier.

If I recess the consumer unit, I'm going to be left with brick, polystyrene, and then not much else before the consumer unit.

I'm worried that I will be attracting condensation into the consumer unit, particularly as it will now be a cold spot. Am I overthinking this?

I have enough working space to get a vapour barrier wrapped round the back of the consumer unit, if that's desirable.

[Robin]

I've not done the sums* but the MCBs/RCDs/RCBOs in a CU generate some heat. It increases with the current carried. Might be enough to make a significant difference, especially if you seal the CU well (as you would anyhow in these fire conscious days).

*which is not to say I could if I tried!

[Andrew]

Which utterly defeats the point of the insulation. I'm afraid a total rip-out back to brick and re-doing it properly is the only solution.

Yes !!. With energy prices up 50%+ today, there is no time like the present.

Couldbe a problem in a timber framed house like Jims, but I wouldn't worry about it (especially with the ventilated gap behing the PBoard :-(

[Tricky Dicky]

Another issue you may have sinking sockets switches and the CU into the insulation is that the cables may come in contact with the polystyrene which over time will cause dangerous damage to the cables and another possible source of fire.

Richard

[Steve Walker]

Is that such a problem as people say? Apparently the cables lose plasticizer, which makes the insulation more brittle, but the plasticiser eats away the polystyrene, normally leaving a gap. It was raised as a problem decades ago, in regard to boats and caravans, but cables in houses don't generally suffer movement and vibration like they do, so slight embrittlement of the cable's insulation may not be such a problem in a house.

[Theo]

It depends on if it's outside air or not. If so, things are bad:

had to guess some thicknesses there, but for illustration)

If it's inside air it turns out not so bad in terms of U-value:

There is a condensation problem though. That could be solved by doing a 'cavity wall' fill into the gap, eg EPS beads: That site is ubakus.com (used in demo mode) - if the OP had a play with various stackups they might see what difference various interventions might have.

In particular, blocking the ventilation from the outside would seem more productive than filling the gap.

Also this is a garage. It's possibly not a habitable room, in which case the concern might be a lack of decrement delay (ie a low thermal mass), so when you go into the garage and turn on the heater it warms up fast. It also cools down fast, but that's ok for a space you aren't in for very long. You might not want somewhere well insulated but that retains heat, meaning you need to preheat it for hours before you go in (and it retains that heat for hours afterwards when you aren't there).

If you see the last stackup (with EPS beads) the phase shift goes up by 3.7 hours (meaning it takes longer to warm up) compared with an inside air gap, which might actually be counterproductive depending on the use case.

Theo

[Andrew]

Rip it all out and replace with ~75mm 'celotex' and PBoard.

Slap a layer of 50mm PIR on the wall without any gaps after making sure all electrical cables are in conduit or protected with capping and making slots in the PIR where it covers the conduit. Affix to wall with 30mm horizontal battens and frame anchors. Infill between the battens with 30mm PIR (Quinntherm do it in this thickness). Refix PBoard and sink electrical sockets into PBoard .

[Theo]

Link should be:

[RJH]

Excellent resource, thanks

[Animal]

If it were habitable room, ie heated all the time, that might be reasonable. For a garage 1" of polystyrene is plenty. As someone mentioned I'd look at where the air is getting in first, it might be that you can simply block the gap.