Hi all. An easy one for many of you I'm sure but I find this whole interstitial condensation thing confusing...

I want to insulate the tiled part of the bay between our grnd & 1st floor windows (typical 30s house). I took some tiles off to investigate and it seems to be just concrete tiles hung on battens, on a 3x2" studwork timber frame, plus plasterboard.

Should the construction be like a warm roof - ie celotex or similar between the studs, and if so can I only add 25mm of insulation to leave the outer 50mm free of insulation for ventilation of the timbers? Do I REALLY need to leave this gap?

Are there any better ways of doing this? I wondered about adding a further 25mm on the outside of the studwork (I've been advised that the outer edge is more sensible than inside), or maybe more if I extended the cill, but wouldn't that prevent these timbers being ventilated. In this respect it seems like loss of airflow/ventilation vs greater scope for interstitial condensation. Confusing!

I could also add studwork on the inside, or add insulated plasterboard (and foil?).

I apologise that this might be a bit 'back to basics' for this forum, but I've had lots of conflicting advice. Oh, and the other thing to add is that we already have a condensation/mould problem at the head of the window (which I hope to resolve by adding better insulation in roof above), so don't want to do anything that could make this worse (eg could a vapour barrier to bay do so?)

Any advice appreciated.
Cheers

Probably the joists let cold wind in under the floor, fully foll the void, may mean all coverings off one side or the other, thermally line inside, make sure you are airtight from under the floor to outside.

Done this job lots of times, amazes me that there is never any insulation in there.

or to the walls and the window frames, do you have cavity walls, filled?

Posted By: smokeyjonShould the construction be like a warm roof - ie celotex or similar between the studs, and if so can I only add 25mm of insulation to leave the outer 50mm free of insulation for ventilation of the timbers? Do I REALLY need to leave this gap?

Are there any better ways of doing this? I wondered about adding a further 25mm on the outside of the studwork (I've been advised that the outer edge is more sensible than inside), or maybe more if I extended the cill, but wouldn't that prevent these timbers being ventilated. In this respect it seems like loss of airflow/ventilation vs greater scope for interstitial condensation. Confusing!

There are basically two ways to avoid condensation damage to timber:

1. The warm roof approach: keep the timber warm so that there's no condensation. Put at least as much insulation outside the timbers as there is inside or between the timbers.

2. The cold roof approach: ventilate cold timbers so that any condensation is rapidly dissipated. Put all insulation inside or between the timbers & ensure good air flow over their outer face.

I don't think you've got enough room for the warm roof approach without new sills & soffits. The first 50-60mm of insulation makes the most difference & 15mm ventilation gap should be sufficient given the additional 25mm batten space & the vapour open covering. So I would fill the voids with 60mm rigid polyurethane insulation pushed tight against plasterboard & sealed to timbers with expanding polyurethane foam.

David

Posted By: smokeyjonPaul - I've seen that spray insulation on t'internet (or are you just talking about the stuff in cans?). I presume it's a N American thing (I haven't heard much about it over here - except warmcell that is)? You have LARGE houses over there. My 4 sq m bay probably doesn't warrant the costs of getting the kit to my site! Thanks though ;-)

Most of the foam is made by BASF I think! There's many companies here who will come and sprayfoam an area for you - they don't use cans of course. It's very effective. Warmcell is blown cellulose - we're lucky that we can rent the machines for that from the hardware store. I think foam would be better for your application though. Over here, we also have Icynene which is a water-blown flexible foam - I think there's installers in the UK too, but it's more expensive than the regular closed cell foam.

Paul in Montreal.

''I'll give the spray foam more thought for our (unfelted) roof though! ''

Please include in your thoughts de-lamination of slates (if you have slates) and potential damage to timber. This use of spray-foam concerns me.

Thanks both
Nick - My starting point will be (already is) one of concern, and I will review all options (including, for the meantime taking the easiest of options - doing nothing!). I haven't heard of sprayfoam used like this very much, and I suspect that there are reasons for this. Tiles are concrete interlocking (and will be due for replacement sometime in the next 10-20yrs I reckon)

David - thanks. I'll make sure a breather membrane is tightly fitted if I fit one at all. I haven't yet seen any evidence of water ingress so you're probably right. Woodworm, however, has clearly ingressed (the whole house was treated 10yrs ago and I intend to treat these timbers whilst the tiles are off), and I thought a breather membrane may help prevent future access by any other similar undesirables!!

tony - My description of the tiles was about the rooftiles (marley RD807051s), not these vertical, plain ones (it was a separate discussion!)

I'll be interested to see how widespread the woodworm is when I get all tiles off. The whole house was treated in 1999 and the floorboards behind this bay were reinspected recently (under warranty) and confirmed not to be active. I'm hoping the infestation was historic, but it sounds like it could have been faciliated by dampness, possibly caused by condensation - good theory. Yet another reason to crack on with this then!

Hi Bampton, Do you mind if I ask a few questions? Are you concerned with any subsequent water penetration from external at all? If a tile or slate slips and you get water ingress how would that be dealt with as i would imagine it would totally soak the icynene before you would become aware of the failure? I note this type of insulation is often used directly onto tiles or slates or sarking boards and i have always thought it looks like a problem waiting to happen. As it is open cell does it breathe? Reading some of the literature it would seem to suggest it forms an effective air barrier and require ventilation when the space is to be used as a room. Does this mean it is sufficent enough to allow the passage of water vapour but not moving air? Over what thicknesses is this still effective? Does it form a vapour barrier? Or would you if using this in a sarking situation fit a vapour barrier internally before boarding out in a loft conversion scenario. I note the K value is only 0.039 so worse than glass fibre and unliklely to give a decent 'U; Value when used in the usual thicknesses possible with most retrofit insulation to rafters of the sizes normally found in loft conversions etc.

Posted By: tonywoodworm cant digest dry wood

I don't think they need it to be really damp though - I've seen a lot of apparently dry furniture riddled with live woodworm.