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Under joist PIR... thoughts please


Carrerahill

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The extension now has floor joists - finally! 

 

The insulation is to be 100mm PIR between the joists - fine I have towers of the stuff sitting there at the ready.

 

However, I also have sheets of 50mm celotex, unfortunately they have already been cut to 325ish-mm x 1220mm. 

 

After I finished 95% of the joists last night, while soaked to the skin (coming too and from garage with the blocking for the floor during torrential rain), I had the brilliant idea that I could fix the 50mm celotex to the underside of the joists as an extra, taping the joints to create a big vapour barrier/insulation layer. This also gave me a home for this Celotex. So I dug out some 75mm screws and penny washers and then decided that as I was soaked, lying on the concrete to screw this in would probably not be a good idea. I came inside, got cleaned and opened a bottle of red wine instead.

 

About 5 minutes into my first glass of red I suddenly realised my plan was flawed. In effect the joists would become sort of sealed in and ventilation would be, well erradicated. So I am trying to work out if I am worrying too much, or if I have a valid concern here.

 

The floor is timber joists, with OSB on top, UFH pipes on top of the OSB, battened out above joists to clear the pipes with biscuit screed in-between and then some levelling compound over the lot then Karndean or Amtico. 

 

Thoughts, I could knock the 100mm PIR down the joist so it was sitting hard against the 50mm underside PIR, that would then form a 75mm gap above it, which could be left vented to the inside of the walls which may allow for a little ventilation, with the UFH pipes above it warming the OSB some heat would go down, the heat might start a convection cycle and convect heat up the wall (on the inside of the PIR cocoon) thus keeping it ventilated and dry? 

 

Or do I under-fix the 50mm PIR, then put in some battens to guarantee a gap, then bang the 100mm PIR down onto these battens so it sits flush with the top of the joists, then try and ensure ventilation through the joists? 

 

Do I just not bloody bother? I could always just put the cut pieces in on top of the 100mm PIR and foam the gap either side - I am then left with 150mm PIR effectively. I would have just done this but the sheets are too narrow for my joists centres (400mm centres), as said above, the Celotex is 325mm-ish wide so they are a little small. 

 

 

Edited by Carrerahill
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14 hours ago, Ed Davies said:

My feeling would be that it'd be unwise to seal up the bottom of your joists. Any water that gets into that area, for any reason, needs to get out as soon as possible.

That is the conclusion I have come to.

 

I am totally rethinking the insulation now.

 

I am thinking, breathable membrane (left over from roof) to the underside of the joists, some Knauf roll 44 in-between the joists, then a VCL over the joists, then my OSB.

 

This will give me a breathable joist with insulation and a barrier to stop warm moist air heading down through the floor.

 

I now have all this 100mm PIR - I was going to use it in the loft of the house, but I then kept realising I was going to form the same issue with condensation - the only room I can lose about 2 sheets into will be the bathroom as I will be pulling the ceiling in so I can VCL it before I refit then 100mm PIR can go in above.

 

 

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This is exactly what we did in our extension. 100mm PIR between joisest and another 50mm on the bottom of joists, plasterboard then on.

 

We have a ventilation gap between the 100mm insulation and the underside of the roofing membrane. Your scenario is a little different, but as you say you can leave a 50mm ventilation gap... Key thing would be, how much air flow would you get through it?

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41 minutes ago, Carrerahill said:

That is the conclusion I have come to.

 

I am totally rethinking the insulation now.

 

I am thinking, breathable membrane (left over from roof) to the underside of the joists, some Knauf roll 44 in-between the joists, then a VCL over the joists, then my OSB.

 

This will give me a breathable joist with insulation and a barrier to stop warm moist air heading down through the floor.

 

I now have all this 100mm PIR - I was going to use it in the loft of the house, but I then kept realising I was going to form the same issue with condensation - the only room I can lose about 2 sheets into will be the bathroom as I will be pulling the ceiling in so I can VCL it before I refit then 100mm PIR can go in above.

 

 

 

In this setup I am not clear what the breathable membrane achieves that eg garden netting would not. I wonder whether even a breathable membrane will inhibit your ventilation too much.

 

AIUI the underfloot joists need to have sufficient access to ventilation (which a breathable membrane may not provide), where the trad suggestion has been do not insulate below the bottom edge level and make sure your airbricks are clear, plus airtight the floor.

 

Or make sure that no moisture at all can get in by filling it all and seal it up completely. Which is where Leca or Pumped Beads between the underslab and the suspended floor apply.

 

F

Edited by Ferdinand
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34 minutes ago, Ferdinand said:

 

In this setup I am not clear what the breathable membrane achieves that eg garden netting would not. I wonder whether even a breathable membrane will inhibit your ventilation too much.

 

In a previous flat, I was advised by thermafleece that netting below the joist would be ok but they recommended a breather membrane to further reduce movement of cold air from the underfloor void through the insulation / any wee gaps.

 

This was with sheep wool insulation which I understand is better than e.g. rock wool as it sucks moisture out of the air / surrounding timbers. Perhaps the advice for synthetic wool would be different.

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On 13/08/2019 at 10:50, Ferdinand said:

 

In this setup I am not clear what the breathable membrane achieves that eg garden netting would not. I wonder whether even a breathable membrane will inhibit your ventilation too much.

 

AIUI the underfloot joists need to have sufficient access to ventilation (which a breathable membrane may not provide), where the trad suggestion has been do not insulate below the bottom edge level and make sure your airbricks are clear, plus airtight the floor.

 

Or make sure that no moisture at all can get in by filling it all and seal it up completely. Which is where Leca or Pumped Beads between the underslab and the suspended floor apply.

 

F

Hi Ferdinand,

 

I am reading your thoughts with interest, I am sponging up all the information at the moment and the conclusion I have come to is that there is no hard and fast rule, and for every "rule" I read, there is a contradictory one! 

 

The solemn area is well ventilated and I designed it to work well, even opening up a crawl space into the existing house which also helps to ventilate the whole house and the new extension. I nearly filled the lot in and concreted over it but went for a suspended timber floor for various reasons but also because I wanted access to the underside, particularly the kitchen area as I just see burying everything in concrete as a pest. 

 

My general concern is driven out of building something set to fail from day one, that is not me, my principle is that things get done properly and I just want to ensure the best for my property.

 

The interesting thing is that the previous owner used EPS throughout the entire ground floor between the joists and also the entire loft space - I am not noticing lots of damp or mould, it is a 60's house, very traditional with lots of breathing going on. The house is high on a hill so we are well drained too which helps. So am I worrying about something very theoretical? Do I just use the PIR I have sitting there and keep it high so the OSB is hard against it, this then exposes a decent portion of the joist for ventilation. But what happens if there is moisture trapped above this PIR - does it eventually get driven out with the heat from the UFH. It would need to come through 11/18mm OSB, 25mm biscuit screed, 5-6mm floor leveller, Amtico or Karndean on a HT adhesive... seems that moisture won't really have an easy route out.

 

Something like this:

 

As seen HERE: https://www.ecologicalbuildingsystems.com/post/ask-expert-thermally-upgrading-suspended-floors

 

The idea behind the breathable membrane, a bit like a roof, is that it creates a wind break, several articles I have read over the past 24 hours suggest that with the open fibre of a glasswool, rockwool, hemp, wool, whatever it be, has a reduced efficiency when exposed to a draught, so the breather membrane allows vapour to escape but cuts down on the gale blasting past. 

 

I have been speaking to Knauf this morning and their technical guy said not to fit the VCL above the floor as that would create an issue - that does make sense, but then again, are this lot then talking nonsense? Does anyone really know?

 

Solution: Cheap renewable energy and build buildings like we used to with big draughty cavities and lots of air bricks!

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3 hours ago, Carrerahill said:

The interesting thing is that the previous owner used EPS throughout the entire ground floor between the joists and also the entire loft space - I am not noticing lots of damp or mould,

 

EPS is considerably more vapour open than PUR.

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1 minute ago, Carrerahill said:

Do I have a concern here, or have I created it?

 

I think you're right to be concerned about getting this right.

 

5 hours ago, Carrerahill said:

I am thinking, breathable membrane (left over from roof) to the underside of the joists, some Knauf roll 44 in-between the joists, then a VCL over the joists, then my OSB.

 

I too think that's a good plan. My floors will be similar: 150 x 19 or 22 mm [¹]  sarking boards under the joists, full fill with mineral wool, VCL over then 22m chipboard and finish floor.

 

The warrant specifies a membrane across the bottom and in the middle of the mineral wool to prevent wind washing and support the mineral wool. I'm having second thoughts, though and may well just use 25mm EPS sheets taped at the edge. Given the reduction in the amount of mineral wool required it works out cost neutral and gives slightly better insulation and will be a little less fiddly to install, I think. I'll run that by the BCO when he comes to peer at my timber before I start adding insulation.

 

[¹] It varies. I started with 19mm but for the last batch I bought the timber place didn't have 19 to hand so I got 22.

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On 13/08/2019 at 11:57, Carrerahill said:

Hi Ferdinand,

 

I am reading your thoughts with interest, I am sponging up all the information at the moment and the conclusion I have come to is that there is no hard and fast rule, and for every "rule" I read, there is a contradictory one! 

 

The solemn area is well ventilated and I designed it to work well, even opening up a crawl space into the existing house which also helps to ventilate the whole house and the new extension. I nearly filled the lot in and concreted over it but went for a suspended timber floor for various reasons but also because I wanted access to the underside, particularly the kitchen area as I just see burying everything in concrete as a pest. 

 

My general concern is driven out of building something set to fail from day one, that is not me, my principle is that things get done properly and I just want to ensure the best for my property.

 

The interesting thing is that the previous owner used EPS throughout the entire ground floor between the joists and also the entire loft space - I am not noticing lots of damp or mould, it is a 60's house, very traditional with lots of breathing going on. The house is high on a hill so we are well drained too which helps. So am I worrying about something very theoretical? Do I just use the PIR I have sitting there and keep it high so the OSB is hard against it, this then exposes a decent portion of the joist for ventilation. But what happens if there is moisture trapped above this PIR - does it eventually get driven out with the heat from the UFH. It would need to come through 11/18mm OSB, 25mm biscuit screed, 5-6mm floor leveller, Amtico or Karndean on a HT adhesive... seems that moisture won't really have an easy route out.

 

Something like this: https://www.ecologicalbuildingsystems.com/post/ask-expert-thermally-upgrading-suspended-floors

 

On 13/08/2019 at 11:57, Carrerahill said:

 

The idea behind the breathable membrane, a bit like a roof, is that it creates a wind break, several articles I have read over the past 24 hours suggest that with the open fibre of a glasswool, rockwool, hemp, wool, whatever it be, has a reduced efficiency when exposed to a draught, so the breather membrane allows vapour to escape but cuts down on the gale blasting past. 

 

I have been speaking to Knauf this morning and their technical guy said not to fit the VCL above the floor as that would create an issue - that does make sense, but then again, are this lot then talking nonsense? Does anyone really know?

 

Solution: Cheap renewable energy and build buildings like we used to with big draughty cavities and lots of air bricks!

 

My last one was a vapour membrane on top of the existing floorboards, with rock wool between the joists and nothing below the rock wool except the ventilated space.

 

Then a further layer of Celotex .. 25mm .. above the vapour membrane in a traditional battened semi-floating floor, and my new floor on that. Cannot remember if we taped the Celotex .. we certainly foamed it all carefully. Generally I do not believe in ‘trusting the Belgians’.

 

That concurs with some of what you are doing, and disagrees with other bits.

 

All effectiveness will vary according to the level of draught, and I think Best is impossible to define.

 

I used rock wool cos I needed it out of the roof to empty the roof so they would install 270mm for me under the free programme, rather than the 100mm that was there, and because it is a hellavalot easier in a Reno than crawling around underneath.

 

What I do suggest is that you instrument it for temp and humidity to a moderate degree.

 

F

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