Full fill Kingspan K103 Insulation Condensation Risk

[Gizzydash]

Hi All,

For a couple of reasons (radiator pipes, ceiling height, avoiding blocking air vents), and to avoid a lot of faffing about with all those things, I'm stuck with 100mm joists in a suspended timber floor. From what I can tell the only insulation that will achieve the u-value I need (0.25) is Kingspan K103 PIR (100mm).

I'm a bit concerned about essentially enveloping the joists entirely with the exception of the bottoms (floor across top of joists, insulation down full height of joists all way along), with PIR not being breathable. From what I can tell none of the breathable insulation will meet the 0.25 u-value so it's PIR or nothing.

Should I be worried about condensation risk? not sure how the joists will dispel moisture if any builds up in the void, given near 90% of their surface will be covered. There will be ventilation front and back but even with that worried it might not be enough.

I don't want to introduce problems where there are none and from what I can see the PIR is very expensive.

Any thoughts or experiences with PIR would be appreciated.

 

[tony1851]

As long as there is sufficient sub-floor ventilation, covering the depth of the joists with the PIR will not be a problem.

The recommended venting is 1500mm2 per m run of wall, or 500mm2 per sq. m of ground floor area, whichever is the greater.

 

[^woody^]

If this is controlled works, then the inspector would expect insulation to cover the bottom of floor joists to achieve the required u value.

There is no condensation risk. But you need to ensure that there is sufficient ventilation to the floor void

 

[tony1851]

If this is controlled works, then the inspector would expect insulation to cover the bottom of floor joists to achieve the required u value.

Tecnically, yes, because the 100 PIR would only be covering aound 87% of the floor area. But getting it underneath the joists as well? A real pain.

 

[Gizzydash]

Just when I thought I had a full understanding of requirements

My understanding was 100mm of PIR in-between the joists would be under the required u value of 0.25 - according to the Kingspan u-value calculator 90mm of PIR would be 0.24, and 100mm 0.22. Where I am the required u value is 0.25.

Why would I have to board under the bottom of the joists?

 

[^woody^]

Why would I have to board under the bottom of the joists?

For the same reason you would need to insulate over or under roof joists! The joists are thermal bridges,

U values relate to the whole component (ie the floor), so you would calculate the insulation requirement for the whole floor, not just the bits in between joists.

 

[Gizzydash]

Ok, thanks, I get the point about the joists themselves being thermal bridges. However, with a suspended timber floor supported by in my case 4 sleeper walls, it's not possible to board end to end across the room under the joists with no thermal bridges. Each sleeper wall will create a thermal bridge. Are you saying in all cases irrespective of joist depth you have to insulate under the joists or is this because i'm using 100mm joists?

 

[tony1851]

Strictly speaking the underside of the joists would need strips of PIR underneath (probably about 75 thick) to compensate and to give you the equivalent of 100mm over the whole floor area.
But it would be a very pedantic and officious inspector who would ask you to insulate under the joists as well!

 

[Gizzydash]

I thought calculators provided by insulation companies take account of the joists thermal conductivity and offset this against the insulations thermal conductivity inbetween the joists, giving almost an average for the total floor (obviously not though as their setup is in-between the joists and no mention of underside). I'll just have to hope the inspector doesn't request that or I'd be better just cracking on with no insulation given the hassle that would take to install and still leave thermal bridges at the sleeper walls anyway.
There's a fireplace in the room, sleeper walls go out around the hearth which the joist ends rest on. What would be the inspectors expectations around how that is detailed? Only option is to basically end the insulation at the the sleeper wall butted up against the slab on the underside of the hearth (expanding foam if any small gaps).

 

[Notch7]

I thought calculators provided by insulation companies take account of the joists thermal conductivity and offset this against the insulations thermal conductivity inbetween the joists, giving almost an average for the total floor (obviously not though as their setup is in-between the joists and no mention of underside). I'll just have to hope the inspector doesn't request that or I'd be better just cracking on with no insulation given the hassle that would take to install and still leave thermal bridges at the sleeper walls anyway.
There's a fireplace in the room, sleeper walls go out around the hearth which the joist ends rest on. What would be the inspectors expectations around how that is detailed? Only option is to basically end the insulation at the the sleeper wall butted up against the slab on the underside of the hearth (expanding foam if any small gaps).

insulation in between the joists will still result in a massive improvement over none.

Are you doing this under building regs? If you are ask the inspector to approve what you want to do and if not, just do what suits you.


On a practical note, you can cut the insulation tight, but with a tapered cut so they drop in fine - but you will need to measure exactly and measure each piece to fit. Or you can cut it loose and foam up, but you would need stops to prevent insulation dropping through.

I would foil tape all the joints and over each joist to join the foil face, then cover with a VCL.

 

[Gizzydash]

Intending to do it under building regs despite several builders/architects saying to just crack on. If I do go down the route of doing just what suits me, and inspector doesn't approve, what are the consequences of that? assume problems for me down the line if I want to sell the house and have completed unapproved works?

 

[jeds]

Something not quite right here. 100mm PIR between suspended floor boards would give a u-value something less than 0.20. Why are the boards 100mm? Do you have adequate ventilation to the floor void? Breathability isn't a factor - why do you think it is?

 

[Gizzydash]

The setup i'd proposed (for 100mm joists) was 100mm kingspan k103 (inbetween joists) that would give u value of 0.22 (0.6 PA ratio). There is ventilation to the void so I thought that was all good and the insulation proposed would meet u value requirements. However, steer i've got from comments above is that the joists themselves will act as thermal bridges and as such will need insulated from below (to strictly meet BC requirements). So setup i'd end up with is 100mm kingspan between joists and 50/70mm below joists (how i'd fix that i'm not sure).

 

[Mr Chibs]

Think this is storm in tea cup.

if you are used to living with an uninsulated suspended floor, then any insulation you put down there will be an improvement.

I did 150mm rockwool on my floor, and pir around the air vents. Cost mainly.

Yes the undersides of the joists are uninsulated, but so are the ends, and where contact is made on a sleeper wall or hanger... and you can’t do anything about that.

If the thermal efficiency is that important , then it’s time to get the concrete out and make it a solid floor.

 

[Mr Chibs]

Thinking out aloud, if you have no floor height established, perhaps you could set the joists lower, insulate then over board The whole floor in pir and float your flooring.