Timber suspended floor - air gap between insulation and UFH deck?

[Glenn]

Hello, grateful for people's thoughts

 

I'm doing a farm building conversion which requires a suspended timber ground floor ie 150mm depth joists sitting on dwarf block walls. The ground floor structure includes a fast response UFH ie tubing running in grooved boards which form part of the floor structure. Nothing startling

 

My architect originally spec'd out 150mm PIR insulation that would fit between the joists and fill the space up the the underside of the floor construction. However the resulting U-value fell short of what was needed, and he has modified this to a double layer of 75mm running under the joists and 100mm between the joists. Both PIR. Which is good in that it nicely encloses the joists, but does leave a 50mm airgap between the floor deck and the top of the insulation between the joists

 

When I queried it, he says he has seen this airgap in low-profile UFH lierature, which indicates it won't be a problem. I instinctively don't like a layer/pockets of air being heated by the UFH and then cooling again. Am I being ridiculous?

 

Regards

Glenn

[Thedreamer]

I have 'modern' suspended floor.

 

I am surprised that 150mm PIR insulation fell short.

 

My joists are filled with insulation but the bottom parts of the joists are exposed for airflow, I would not feel comfortable with having the joists covered underneath by PIR. 

[ProDave]

Fitting PIR under the joists is a tricky job to detail well.

 

Why not just go up to full fill 200mm joists?

[TonyT]

I have 200mm pir and suspended floor.

staggered layers sealed per layer.

in between joists

[Glenn]

Thank you all for your thoughts, some interesting ones in there:

1. A problem to enclose the joists by having insulation underneath and between? I need educating! Isn't that like an upside-down warm roof with EWI and insulation between rafters?

2. I will check with various U-values etc, but my question was more about that air gap between the floor and the top of the insulation. Any problems anyone can see with that?

 

 

Regards

Glenn

[ProDave]

12 minutes ago, Glenn said:

1. A problem to enclose the joists by having insulation underneath and between? I need educating! Isn't that like an upside-down warm roof with EWI and insulation between rafters?

Yes, you try slotting sheets under the joists, then crawling on your back in a very confined space to screw them upwards into the bottom of the joists with those big plastic caps.  Then doing same to tape the joints.  A miserable job with lots of scope to get it wrong.

 

I want to see how you fix the very last one.......

 

A LOT easier to just use thicker joists and full fill the gaps.

[Thedreamer]

3 hours ago, Glenn said:

Thank you all for your thoughts, some interesting ones in there:

1. A problem to enclose the joists by having insulation underneath and between? I need educating! Isn't that like an upside-down warm roof with EWI and insulation between rafters?

 

 

Most modern roofs incorporate a breather membrane but the PIR sheet would be a vapour control layer. What would be the point of the airbricks etc if the joists cannot be ventilated?