Wall insulation at intermediate floor void

22 Dec 2010
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United Kingdom
hi all - hope I have posted in the right area

Having just got off the phone to the Celotex technical line I would really welcome your comments on the best way to tackle this problem as I don't really think they understood my query!

We are renovating solid brick property and plan to internally insulate ourselves. We had planned to use Celotex 50mm board on 25mm battens and then counter batten to create a service void. However, we have since revised this after advice from our builders and are now thinking of using 77.5mm insulated plasterboard (PL4000) to save time and avoid issues with cables needing to be in conduits.

My query relates to what to do with the insulation board at intermediate floor level. We have solid timber joists that sit in the solid wall. With the original plan i wanted to bring the boards through the intermediate floor to avoid cold bridging and keep a good vapour control layer by putting sealant around joist ends. That way we can put recessed lights in the ground floor ceiling without worrying about vapour control etc. However, can this be done with pl4000 and is the plasterboard left on or removed? Celotex have advised me against this but couldn't really come up with a reason why not. They recommended using ga4000 between but why create more joints??

Look forward to your comments.


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Do you really expect your builders to have to trim the ends on each sheet of insulated board around each joist? What a pain in the back side. Much easier to just stick an infill piece of insulation in between each joist and seal any gaps with foam with some foil tape over.

Given the price of insulated plasterboard it may well still be cheaper to use individual insulation + plasterboard.
Thanks for your comments. I am doing the internal insulation myself so cutting whilst a pain is not out of the question if it is a better option.

I agree separate board and insulation will be cheaper but possibly more time consuming to install given the extra battening.

If I use insulated plaster board in the rooms and separate infill, what infill would you suggest? If I use standard ga4000 I will be trying to tape foil faced board to plasterboard which seems wrong.

If I use pl4000 combined board between the joists the skim coat seems to be part of the vcl and skimming n the floor void again doesn't seem the right thing to do and will not be possible if the ceilings go in!

I may given the kingspan helpline a call tomorrow to see if they have any better suggestions for k18.
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Commercially even when taking the extra labour into consideration insulated plasterboard still comes out more expensive.

Regarding taping over a joint of insulation and plasterboard frankly you're thinking too much! House renovation is fraught with areas here or there that you would not do in a new build, you just have to do the best you can.

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