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Posi-joists on suspended ground floor with large air gap.

Postby danielshepherd » Tue May 11, 2021 7:39 pm


I have an extension built on my home which my Dad, who has since passed away, built in 2011.

The extension has always been very very cold compared to the rest of the house. The rest of the house is built on solid floor. We had tight site access so my Dad decided to do a suspended floor rather than a solid thinking it would be easier to get materials on site.

I always suspected the floor was the problem as the air gap underneath is about 600mm deep due to the ground level that connects it to the rest of the house.

But also upon opening the floor I found it was built on posi-joists. The architects drawings I found stated that it needed to have 100mm of PIR board between each joist, which they did install - but at the top of the joists. As the joists are about 200mm deep this mean it left the bottom 100mm open and allowed the cold air to directly reach the top chord, which is 47mm thick. Effectively this would be like having your outside wall made of 47mm thick wood and then just having plasterboard directly on top. No wonder it was cold!

As shown:
Inside, showing the air gap mistake

So now I need to figure out a 'safe' way to insulate the joists. What I wanted to do is push the PIR board to the bottom so it was level with the bottom chord - possibly use gapotape too (though that might be unnecessary in this idea) effectively creating an air tight seal at the bottom. Then I was going to put sheep's wool insulation inbetween the metal fixings of each joist - making them full. Finally a sheet of SuperQuilt on top to act as a vapour barrier, an air tight seal for the room and foil to reflect the underfloor heating back - it would also have an air gap between the top of the PIR board and the bottom of the quilt and from the top of the quilt and the chip board subfloor.

My biggest question is that in doing this the top chord of the posi-joist would effectively be sealed off from the fresh air void below by the PIR boards. Could that cause any issues with damp/rot? Or does it just mean the top chord is now part of the warm room? (though sealed off from the main room by the vapour barrier)

If so I don't see any real way to fully insulate away the issues but also keep the top chord access to the fresh air. I think the only way I can think would be to move the PIR board up so there's a gap just above the bottom chord, but keep it full of sheep's wool so that it's got nearly 200mm of insulation before meeting the top chord - it at least will be in theory then "breathable".

The top and bottom chords on the joists are 122mm wide and 47mm thick on 600mm centres.

Side profile of the joist.

I suspect there will be a school of thought in the best way to go about this. I could find little to nothing regarding insulating posi-joists online with such a large cold air gap below. The architects specified Wolf easi-joist WS200 - I asked them this exactly insulation/breathability question and included the photo. That appeared to be a mistake as apparently whilst identical, these are not official Wolf easi-joists, so he wouldn't answer the question, even on an "in theory with this type of top/bottom chord split".

Look forward to hearing thoughts.
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Simply Build It

Postby stoneyboy » Fri May 14, 2021 10:25 pm

Hi danielshepherd
Are you sure that heat loss through the floor is the issue?
Have you checked insulation at ceiling level and insulation in the walls?
Regards S
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