Loft storage PIR - 5cm between rafter + 10cm between joists

[ozasco]

Hi, I'd like to board my loft and use it for storage and I'd like to move my combi boiler from the bedroom to the loft, to gain some space.
I'd like to replace the current loft insulation with PIR to gain some height: 191cm is the highest point from the top of the joists to the rafters.
I'd also like to use PIR between rafters, to reduce the coldness a bit up there.
Is it ok to do so?
Will it increase the risk of condensation (currently I don't notice any at all)?

For the 127mm joists I'd use 100mm PIR plus reuse 27mm of the current insulation rolls, and then board it with moisture-resistant OSB.
For the 100mm rafters, I'd use 50mm PIR and leave 50mm gap for the felt ventilation, and then board it with a foil-backed plasterboard.

I'm an intermediate-level DIYer but have never done anything up there. It's a semi-detached house. Images attached.

Other questions:
I'm still not sure if I'll go ahead with moving up the combi boiler (Greenstar 30i ErP), but I guess it can interfere with the condensation/acclimatisation up there?
I got this unknown "device" on the gable's end, is it some kind of air vent?

Please and thanks

 

[pcaouolte]

Your unknown device on the gable end is easy, so I'll take that question. It is a sounder and strobe for an intruder alarm system. It might be a dummy or it might be real. It is quite old and may have been superseded by the Yale alarm.

I'll leave the insulation questions to others who will be better qualified than me.

 

[robinbanks]

For the 127mm joists I'd use 100mm PIR plus reuse 27mm of the current insulation rolls, and then board it with moisture-resistant OSB.
For the 100mm rafters, I'd use 50mm PIR and leave 50mm gap for the felt ventilation, and then board it with a foil-backed plasterboard.

Your proposals are inadequate for current regulations and I dare say would represent a worse result than the insulation you already have from the wool

I'm still not sure if I'll go ahead with moving up the combi boiler (Greenstar 30i ErP), but I guess it can interfere with the condensation/acclimatisation up there?

You can't move a boiler yourself. For least worry/hassle, keep the boiler within the heated envelope of the house. If installing it outside the heated envelope, be sure to insulate well all the pipework exposed to freezing temperatures

 

[ozasco]

@robinbanks which part is inadequate, please?
10cm PIR would be roughly equivalent to 20cm wool.
But then I got additional 5cm on the rafters, 15cm total, which is equivalent of 30cm wool.

Yes, I forgot to explain that the boiler will be done by a proper professional, thanks for the advice, and yes, pipes would be insulated.

 

[sparkymarka]

It’s a regal safe bellbox ..

 

[robinbanks]

which part is inadequate, please?

An appreciable percentage of your loft is insulated by wood only, which is thermally poorer than PIR. I'd consider moving the PIR completely to the ceiling, 50mm between and 100mm as a continuous layer under the rafters

 

[cdbe]

I would leave the 100mm of mineral wool between the joists, lay 150mm PIR on top and lay 9mm osb on top of that - this will form a perfectly adequate and well insulated storage loft floor - in the eaves where you won't be going, pile up all that spare mineral wool - aim for 400mm (while maintaining air flow).

I would avoid insulating the rafters and considering it as a component of the of the ceiling insulation (technically I suppose it is, but only after you've heated a huge empty loft space at great expense).

Those pipes and tank will be part of the old heating system and can be removed. The boiler is "room sealed" and will not affect the air in the loft. It will have a frost protection and so can be suitably used in the loft - but there are regulations relating to access/flooring/lighting for servicing purposes which will need to be met.

It's likely those dormers have no insulation in the walls or roof and you should consider upgrading this. The walls are fairly straightforward, the roofs should ideally be converted to "warm roofs" when next renewed.

 

[ozasco]

Thanks guys, I will probably just board it, provide ladder/lighting, and move the boiler up (leaving an easily accessible boiler switch on the 1st floor).
I will still decide which insulation to use under it (probably: current wool with full 270mm height).
Does a telescopic ladder nearby suffice, or does it need a drop-down attached to the hatch?

@cdbe you have just opened my mind to the lack of insulation in the walls, I'm posting some photos along. On the front dormer wall, next to the skirting board, it's measuring 11°C when my smart TRV, 20cm apart, actually measures 22°, and it's -3 outside.
You can see that even my double glazed window has better insulation.
The surface temperature of the ceiling is not a big issue though. It's measuring between 16 to 19 depending where I measure it, even if it's more on the flat roof part. I had to reboard one of the bedroom's ceiling, and there was wool there already.

 

[ozasco]

So, I guess the easiest/cheapest way to improve my walls is to make a 30x60cm ish hole in the wall and just insert some wool there to both sides (trying to avoid squeezing it if possible), right?
Then just replaster/paint the small hole

2 years ago when I bought the house, I had to do a small fix on a crack/hole on the dormer's front wall (plasterboard), and I did notice I could even see the sunlight through the hole, before I patched it.
I came from a tropical country and didn't realize at that point that this was a very bad sign, and should have been addressed back then, without rework now.

BTW, the reason I installed these smart TRVs is that sometimes I felt (still feel) it's cold inside, and I think I now understand that it's not just because of the British weather/way, but rather my walls.

It's me, my wife, and my 9 year daughter at this 3 bedroom house, and we spend 2k/year on gas/electricity. Now it all adds up to the reason why.

 

[ozasco]

What about bead insulation for the dormer wall?
Then I'd just need to drill some smaller holes and apply (does it require an air gun?). I can see that some companies do it with free funding, e.g.:

Does bead cavity wall insulation work? - Dyson Energy Services

Does bead cavity wall insulation work? We invite ThermaBead, a leading manufacturer to discuss the features and benefits.

dysonenergyservices.co.uk

 

[ozasco]

I've been reading a bit, I think I'd better replaster it with a vapour barrier and let some gap between the new insulation and the exterior felt/tiles so that it ventilates out any condensation, right?