Posi-joists on suspended ground floor with large air gap

[DanielShepherd]

Hi,

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!
 

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.

 

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.

[PeterW]

If it was me - and there was space - I would ignore the super quilt as it’s crap, and basically get under the floor. 200mm of sheep’s wool between the joists then hold it up with the PIR attached to the bottom of the lower chords with battens and keep working across. Will be warm but you may need to buy another sheet of PIR to complete it. Tape the joints if you like with foil tape but it will work without. 
 

Where is the UFH in that pic..?

[ProDave]

Peter's suggestion is the bees knees but means crawling under to fix the PIR.

 

A half way house might just be to fix battens across underneath the posi joists to support the existing PIR dropped down to the bottom, and use Earthwool as the infill above the PIR.

 

You want te whole floor up to do this properly.  Then an air tight membrane all joints taped over the top before putting the floor back down again.

 

P.S Posi joists would not have been my choice.  I would ave chosen Engineered I beams. They then are easy to fit a bottom panel of OSB that just drops in onto the bottom chord and you fill with insulation, no web section to have to fill.  (that paragraph for the benefit of anyone thinking of using posi joists in a similar situation)

[DanielShepherd]

Thanks guys - Posi-joists wouldn't have been my choice for this - although 10 years ago I was nothing to do with the project so. It appears the architecture specified them for some reason. I presume my Dad was sold of the fact they could span the 5 meters easily and would very rigid. But as there's no services to go into them etc it seems more trouble than it's worth.

No UFH installed there yet Peter - that's why i'm getting it up in prep.

 

Fortunately there's a good 500m-600m to work underneath so that's no problem, but I am planning on pulling it all up and doing it properly. 

My only real concern really as I mentioned is if I seal the PIR boards at the bottom - is it ok for the top chord to not have breathable access to the void below? Pushing the PIR board to the bottom would mean the top chord is sealed off from the fresh air flow by the PIR board and the bottom chord. Should I move the PIR up a bit past the bottom chord a little so that in theory the fresh air can make contact with the wool (whatever breathable kind I decide to use to fill the beams with) meaning the top chord could...even though it'd have to travel through 200mm or so of breathable insulation have access to the cold fresh air. Or should I just not worry and seal it all up at the bottom with the knowledge the top chord is insulated with 200mm of insulation anyway?