Roof insulation help

[saveasteading]

6 minutes ago, James Newport said:

makes you cough

The batts don't (in my experience) but are more expensive.

Also the cough ratio of wool varies by manufacturer too. I think it corresponds to cost pretty well.

Itchiness likewise.

 

11 minutes ago, James Newport said:

PIR is 5 out of 10 on the crappy jobs scale

Cough and itchiness to cost ratio: CICR, as we need even more acronyms (EMA).

We should include U values as that is what it is all about. Cost and Itchiness Ratio Compared to U values CIRCUs

[Evs]

I’m just calculating the amounts of materials I’ll need so I can work out some price comparisons. Would someone be able answer this for me please? 

 

I’ll just use celotex PIR for example.

I need to put 150mm between the rafters and then 50mm below. I know I need to do this in the vaulted area and all the pitched part of the roof in every attic room but do I need to do this in these area’s I’ve pointed towards in the drawing? Sorry for the poor sketch lol. 

 

The 300mm rockwool is what the architect has put on my plans for above the flat ceiling in the attic. I won’t need that much if I can take the 150mm PIR all the way to the apex of the roof. 

 

I’ve spoken to Knauf, they said to use 150mm of loft roll 32 between the rafters and 75mm PIR below. 

 

[Evs]

On 02/08/2021 at 23:15, Iceverge said:

Vapour membrane is vital for airtightness and to prevent condensation on your rafters causing the rafters to rot. 

 

PIR in the roof is a bad plan for loads of reasons. 

 

Torturous to install. Expensive. Bad decrement delay leading to overheating. Shrinkage of timbers and PIR causes drafts around the insulation making it underperform severely.

 

I have attached probably one of the cheapest options. No PIR. Better still if you can replace the glasswool between reg joists with blown densepack cellulose. Supplier installed.  Excellent for decrement delay, airtightness, toxicity, environmental credentials the list goes on. 

 

 

I’ve only only seen this reply somehow. Thank you for taking the time to do this. 

 

I’m still not sure whether I can fully insulate the trusses with 200mm of glasswool or I have to leave the 50mm air gap. 

Edited August 8, 2021 by Evs

[AliG]

On 04/08/2021 at 10:24, Evs said:

I need to put 150mm between the rafters and then 50mm below. I know I need to do this in the vaulted area and all the pitched part of the roof in every attic room but do I need to do this in these area’s I’ve pointed towards in the drawing? Sorry for the poor sketch lol. 

I did exactly what you did at the ceiling to save money.

 

However, we then found there was MVHR ducting above the ceiling and had to insulate it. So it only works if there is nothing that needs to be warm up there. Also there are air tightness issues with lights etc in the ceiling.

 

You can do it in either place on the walls at the side of the room. However, it is probably better on the roofline as you can then use the space and have it airtight.

 

Airtightness is actually the biggest issue here probably, you can get the u-values, but it is hard to make sure there are not draughts in the eaves, or where lights are in the ceiling.

 

I suspect keeping insulation at the roofline is best for this, but even then only if it is all foamed up, taped etc.

[Evs]

9 hours ago, AliG said:

I did exactly what you did at the ceiling to save money.

 

However, we then found there was MVHR ducting above the ceiling and had to insulate it. So it only works if there is nothing that needs to be warm up there. Also there are air tightness issues with lights etc in the ceiling.

 

You can do it in either place on the walls at the side of the room. However, it is probably better on the roofline as you can then use the space and have it airtight.

 

Airtightness is actually the biggest issue here probably, you can get the u-values, but it is hard to make sure there are not draughts in the eaves, or where lights are in the ceiling.

 

I suspect keeping insulation at the roofline is best for this, but even then only if it is all foamed up, taped etc.

I can’t use any of the roof space at either side of the attic rooms. Can’t cut the trusses and they’re at 400mm centres so can’t get anything in between them anyway. I’ve got a bit of room above the attic roof that I can use for storage. 
 

If I use PIR insulation, I was going to run in between the joists and then at right angles all the way through, then fit some battens behind the upright bits and put 100mm of rockwool behind the plaster board. 
 

I’ve got 15 spot lights in the big vaulted ceiling, I was wondering what do about the spots as I’d have to cut holes through the PIR if I use that. 

[AliG]

I think the best way to do the spots is to seal the insulation above and create a service cavity between it and the plasterboard.

 

Alternatively you could seal the individual lights, I’d like to hear from someone who abs tried this though.

 

At the sides I’d tape the PIR to the joists and make sure the plasterboard is sealed at the floor joint before you put down flooring or skirting boards.

[Evs]

Building control finally got back to me today. The BCO said that 120mm between the rafters and 20mm below will be sufficient and thats what he always says to install. I didn’t argue with him but on the celotex U value calculator is says 150mm between and 50mm below will be required to achieve a U value of 0.15. I could go ahead and do this but then if I have to prove that this will achieve the 0.15 U value, then I can’t. 

Edited August 12, 2021 by Evs

[DragsterDriver]

On 09/08/2021 at 08:40, Evs said:

I can’t use any of the roof space at either side of the attic rooms. Can’t cut the trusses and they’re at 400mm centres so can’t get anything in between them anyway. I’ve got a bit of room above the attic roof that I can use for storage. 
 

If I use PIR insulation, I was going to run in between the joists and then at right angles all the way through, then fit some battens behind the upright bits and put 100mm of rockwool behind the plaster board. 
 

I’ve got 15 spot lights in the big vaulted ceiling, I was wondering what do about the spots as I’d have to cut holes through the PIR if I use that. 

You can get very shallow led down lights  

[Iceverge]

On 08/08/2021 at 22:36, Evs said:

 

I’ve only only seen this reply somehow. Thank you for taking the time to do this. 

 

I’m still not sure whether I can fully insulate the trusses with 200mm of glasswool or I have to leave the 50mm air gap. 

Full fill between the rafters 

 membrane

batten 25mm

counter batten 25mm

for 50mm ventilation above the rafters.

tiles. 

[Ajn]

If your roof is on your solutions maybe limited. We used the Actis three layer system. It was the only way we could maintain the roof profile (smaller rafters) AND have the purlins visable on the inside. The outer blanket is the pinned to the rafters by 25mm batten, then a further 25mm tile batten giving a 25mm air gap to the tiles. Between the rafters the Hybris honeycomb product is the clean and simple to install. Not so good if the builder thinks 400mm is the same as 410mm...The inside then has another blanket fastened with more 25mm horizontal batten. Std plasterboard is used pinned to the battens. Cables can be run in the 25mm gap and behind the battens as required. Thin led spots can go in the plasterboard and still have an air gap around them. Lots and lots of tape is ued to seal the inner blanket. As for price, while they don't supply direct we did get a good deal with them through our local supplier. This also translated into us getting a lot of other stuff from the supplier as we ended up as a trade customer.

Another benefit is with all the aluminium foil involved in these profucts there is no chance of the alien space men getting you at night. I doesn't do mobile phone signals much good either.

[saveasteading]

6 hours ago, Ajn said:

Actis three layer system

I went to a presentation of this a few years ago, endorsed by local building control.... ie they provided the room and Actis provided sandwiches and perhaps more.

 

There was some discussion about whether it performs as suggested, as the method of testing may not be realistic. (forgotten now, but does 'closed box testing' mean anything?

 

I know from a major supplier of whole buildings that they have severe reservations about multilayer as a principle, as do I...I mean obviously it works, but how well? I always saw it as a remedy to pin inside a cold loft.

It seems to depend on the air gaps to allow the reflective surfaces to be allowed for in the calculations. (The plasterboard companies no longer say that foil backing helps insulation.)

But the honeycomb system Actis Hybris seemed  a sensible idea that clearly will work and will seal between the rafters.

 

Another BUT....it seems that you need the Hybris and then the multilayer, so this is going to get seriously expensive isn't it?

£10/m2 + £15/m2?

 

6 hours ago, Ajn said:

Not so good if the builder thinks 400mm is the same as 410mm.

 

Ajn....410mm being 18 "  c/c  less a 2" rafter...as we have. Can you be more specific? 400 cut from 1200 falls out of a 410mm space or leaves gaps?

And the reality is that spaces vary, especially with twisting of rafters, so can be 400 to 415 in the same section.