Builder recommending insulation in joists under flat warm roof

[swank]

We are doing a renovation of our ground floor bungalow which has an extension at the rear. The previous owner had extended it around six year ago.

 

 

Our building took out a spotlight in the flat extension roof to find there was no insulation, he mentioned that although this seems like a warm roof, there can still be insulation under it and he is recommending to do it to improve overall insulation. Since it is part of a larger renovation, there is no significant incremental cost to it, so I have the following question:

 

1. Based on my limited research, there is not supposed to be insulation within joists, so is there any benefit in doing it?

 

2. it seems there could be condensation issues if this is not done right, then is there a way to do it which gives benefits without causing condensation issues 

 

Overall I will be open to doing something because the cost is negligible and if there is a chance to improve insulation I want to do it, however if it doesnt give any benefit or worse can cause condensation then I would skip.

 

 

 

[Temp]

Yes with a warm roof all the insulation should be above the joists. It is possible to put some insulation between them as well but you should really get a condensation risk analysis done. I suppose some insulation manufacturers might do this free?

 

I've got it in my head that there is a rule of thumb which says no more than 1/3rd between, leaving 2/3rds above. So if there was 100mm above now, you shouldn't add more than 50mm between. However I cant remember where I got this from. 

 

Adding a vapour barrier before replasterboarding might be worth doing but don't penetrate it with downlighters etc.

Edited December 7, 2023 by Temp

[Redbeard]

10 hours ago, Temp said:

I've got it in my head that there is a rule of thumb which says no more than 1/3rd between, leaving 2/3rds above. So if there was 100mm above now, you shouldn't add more than 50mm between. However I cant remember where I got this from. 

 

Yes, there is a rule of thumb (2/3 vs 1/3), but note that it relates to R value, but thickness. 

[johnhenstock83]

absolutely no need, 100mm+ of celotex on top of everything will keep things toasty. we've got the same set up and the space in between the joists is nice and bone dry, warm as well, as are the upstands. everything freezes, but the upstands and the edge of the skylights never do, that's how warm things are kept.

[Redbeard]

I guess the difference (and the issue) here is that the OP does not know how much insulation is above the roof. If there's 'not much' then following the rule of thumb OP could only put 50% of 'not much' below the deck in any case (or schedule re-insulation for when the warm roof cover needs attention).

[FuerteStu]

So no extra for acoustic insulation? 

 

Warm roofs can transfer a lot of sound through them. 

[swank]

23 hours ago, Temp said:

Yes with a warm roof all the insulation should be above the joists. It is possible to put some insulation between them as well but you should really get a condensation risk analysis done. I suppose some insulation manufacturers might do this free?

 

I've got it in my head that there is a rule of thumb which says no more than 1/3rd between, leaving 2/3rds above. So if there was 100mm above now, you shouldn't add more than 50mm between. However I cant remember where I got this from. 

 

Adding a vapour barrier before replasterboarding might be worth doing but don't penetrate it with downlighters etc.

Thanks so we were thinking mineral wool insulation. So we leave two third space above and only fill one third of it, where does the vapour barrier go, below or above mineral wool?

 

We do have spotlights

[swank]

11 hours ago, Redbeard said:

I guess the difference (and the issue) here is that the OP does not know how much insulation is above the roof. If there's 'not much' then following the rule of thumb OP could only put 50% of 'not much' below the deck in any case (or schedule re-insulation for when the warm roof cover needs attention).

Yes that is right, we dont know what is above, and wanted to see if we can improve insulation without causing any harm. Seems like keeping half space above should be enough, and we need to figure out where to put vapour barrier. 

[pudding]

7 hours ago, swank said:

Thanks so we were thinking mineral wool insulation. So we leave two third space above and only fill one third of it, where does the vapour barrier go, below or above mineral wool?

 

We do have spotlights

Sorry. not quite. 2/3rds above - 1/3rd below refers to the insulation above/below joists, not air space. So if you had 100mm PIR boards above the joists, putting 50mm of mineral wool would be fine, push it all the way up next to the deck above. Then vapour barrier below joists, then plasterboard on top.

[swank]

56 minutes ago, pudding said:

Sorry. not quite. 2/3rds above - 1/3rd below refers to the insulation above/below joists, not air space. So if you had 100mm PIR boards above the joists, putting 50mm of mineral wool would be fine, push it all the way up next to the deck above. Then vapour barrier below joists, then plasterboard on top.

thanks, is there an easy way to find out how much above insulation exists ? We are planning to open the plaster board from

inside so don’t think we

will be able to determine it.

 

also a dumb question if mineral wool is

pushed below deck and then there is air gap and then vapour barrier, so how would the mineral

wool be supported by ?

[Mr Punter]

I would not suggest adding mineral wool because, with an unknown amount of insulation on top, you risk creating condensation, mould and rot.  If it is really cold, strip the roof covering, add more insulation and new membrane.

[Redbeard]

Note also that there is/should be a VCL in the existing 'sandwich' - probably on the first 'deck' above the roof timbers. If you add insulation below, and the VCL which should be on the warm side of that added insulation, you now have 2 VCLs. If they are both 100% (a rarity IMO) there may not be an issue. There still *should not* be an issue even if the new one is deficient, as the original VCL will not be cold enough (because of the 2/3 / 1/3 'rule') for condensation to occur. But that's why actually knowing how much is there now is really important. Is there a datum you can measure from, internally and externally? You'd still have to make assumptions re the thickness of the deck, but you should be able to get pretty close.

 

However, as @Mr Punter suggests, the safest way is to leave it as is - a Warm Roof, rather than making it a 'hybrid', with all the attendant uncertainties. You could still do the measuring as above. If you find it has 125mm (and you have to assume it's PIR) it's probably compliant (0.18W/m2K pre-June 2022 changes).

[Temp]

14 hours ago, swank said:

Thanks so we were thinking mineral wool insulation. So we leave two third space above and only fill one third of it, where does the vapour barrier go, below or above mineral wool?

 

We do have spotlights

 

No thats not what I meant at all. I'm not suggesting leaving a void.

 

You need to know how much existing insulation you have above the rafters. Then put no more than half that between them.

 

Suppose you had 100mm of PIR above the rafters already. Then you shouldn't add more than 50mm between them. That way the total is 150mm with 1/3rd (50mm) between and 2/3rds (100mm) above.

 

As others have said it should really be no more than 1/3rd of the total R value but 1/3rd of the thickness is probably close enough.

 

The vapour barrier would normally go under the rafters before you put the plasterboard back up but if you have downlights that are recessed into the plasterboard they would put holes in it. Consider replacing them with surface mounted LED lights and seal the holes where the wires come through.

 

 

 

 

[Redbeard]

On 06/12/2023 at 22:07, swank said:

The previous owner had extended it around six year ago.

 

OK, to try to avoid this one running and running... Was the extension signed off by Building Control? The 2010 Regs (in force 6 years ago) stipulated a flat roof U value of 0.18W/m2K. If it was done right and checked it will have a U value of 0.18 - probably 125mm PIR, or 150 if, like me, the builder was a bit of a pessimist. You *could* go further (and Passive House builders will be looking for 0.12 or even lower, but 0.18 is fairly respectable. If you risk possible moisture issues by going further why not stay where you are? Like I said, if you can find a datum inside and out you could measure.

[Mr Punter]

It is difficult to measure as you often have joists, OSB, firrings, OSB, VCL, insulation, OSB, waterproofing.

[Redbeard]

19 minutes ago, Mr Punter said:

It is difficult to measure as you often have joists, OSB, firrings, OSB, VCL, insulation, OSB, waterproofing.

True, and not all decks and sub-decks have to be the same thickness, but Total minus 18 minus 18 might yield something useful (measured at the lowest point to take out the firrings). By 'something useful' I mean that if the residual amount was 125 or 150 it could suggest that our estimation re the OSB thickness was right.