Both the EST and Celotex are recommending that internal insulation with rigid foam boards be done by fixing the insulation to the walls and then having battens and air gaps with plasterboard over. This surprises me, as surely that approach will both trap interstitial condensation and run the risk of convective loss if the air gaps aren't 100% sealed, negating the insulation? Interestingly, all the related text on both websites talks in terms of moisture from inside trying to get out - but fails to acknowledge the possibility of driving rain etc entering from outside.

By way of an experiment, in late 2010 I fixed pieces of 75mm Celotex internally to a solid brick external wall in my house. Looking behind them recently, the paintwork has badly blistered and the walls get Protimeter readings in the 200s-300s (i.e. in the red area = unacceptably damp). Moisture that would previously have been driven back by heat loss is staying in the wall.

Can someone explain to me the rationale behind the EST/Celotex advice, as I would have expected to do the reverse, i.e. air gap between insulation and masonry? The latter would allow condensation from outside to run down the internal masonry surface (or better still ventilate the air gap??) and keep the thermal envelope airtight.

Of course the sensible approach might simply be to forget all about Celotex-type insulants and go for a breathable material instead, but can we focus on the Celotex-type approach for now. Before discounting their approach I'd like to understand the thinking behind it.

How do you ventilate behind the insulation and still keep airtight?

Regarding the different Celotex details I'm not sure the dates are relevant, rather the different types of insulation they're using.

<blockquote><cite>Posted By: andykent</cite>Both the EST and Celotex are recommending that internal insulation with rigid foam boards be done by fixing the insulation to the walls and then having battens and air gaps with plasterboard over. This surprises me, as surely that approach will both trap interstitial condensation and run the risk of convective loss if the air gaps aren't 100% sealed, negating the insulation? Interestingly, all the related text on both websites talks in terms of moisture from inside trying to get out - but fails to acknowledge the possibility of driving rain etc entering from outside.

By way of an experiment, in late 2010 I fixed pieces of 75mm Celotex internally to a solid brick external wall in my house. Looking behind them recently, the paintwork has badly blistered and the walls get Protimeter readings in the 200s-300s (i.e. in the red area = unacceptably damp). Moisture that would previously have been driven back by heat loss is staying in the wall.

Can someone explain to me the rationale behind the EST/Celotex advice, as I would have expected to do the reverse, i.e. air gap between insulation and masonry? The latter would allow condensation from outside to run down the internal masonry surface (or better still ventilate the air gap??) and keep the thermal envelope airtight.

Of course the sensible approach might simply be to forget all about Celotex-type insulants and go for a breathable material instead, but can we focus on the Celotex-type approach for now. Before discounting their approach I'd like to understand the thinking behind it.</blockquote>

Hi Andy, I'm also very interested in this. Have you read the Joe Little articles linked to here? http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6573&page=1

I've been looking for evidence which supports the theroretical modelling done by Little as most seem to believe that the risk of interstitial condensation is low.

His approach isn't the same as the EST details. How close did you follow their detailing?

I'm all ears

My thoughts until now are that the only way to be sure is to find something upgraded at least a few years ago to a u-value better than say 0.5W/m2K. Even better if its lower than say 0.3W/m2K.

Then all we have to do is open up an area and see whats behind it

It is the whole of one wall, approx 3.5m long x 2.3m high.

Protimeter readings from other parts of the house suggest I'm going to have even more fun once I try to do the front wall, where in patches the readings go over 900. I think the stone here might itself be moisture permeable, as most of the pointing has been renewed.

In any event, not something I would wish to leave an impermeable insulant in contact with for any length of time.

If you need Internal Wall Insulation we are the company that supply the product, we can also get grants to lower the cost of fitment and product. It is ofgem approved and part of the Green Deal so where it cost full money to fit other brands of insulation we can lower your costs



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Posted By: wookeyOf course there may not be enough cold nights left this year to get any numbers.

There probably will be enough days, but anyway it will show the trend and you can easily extrapolate from that. Your in the East where the RH is lower than the West so your dewpoint temperature will be lower than mine for any given day.

(Ambient RH/ Ambient Temperature) / (Wall RH/ Wall Temperature) should show the trend nicely.

May try the same thing in my loft.

As in Dense

Jeremy
Don't mince your words, say what you really think

Hi Andy.
Nice to see that Celotex have caught up with the method I have been using since 2004. I adopted it because it means that if there is condensation between the insulation and the wall at least the battens are warm and dry. Incidental benefits are that the separate plasterboard is easily screwed onto the battens, and any fittings such as curtain rails or kitchen units can be screwed on to battens suitably placed, larger battens if nec; I then put 25mm of further insulation between the battens, or where desired run services between them.
I seal the insualtion as well as poss, and any moisture which does get from room to behind the insulation is supposed to evaporate out thru the brick. An air gap achieves nothing as the route out for moisture is through the brick. I would worry if I had non permeable render. In areas of normal exposure this should be fine; in very exposed areas evaporation out through the brick might be too slow, or negative, at times.
The idea is that the rate of air/moisture escape from behind the insualtion should be several times the rate of flow of warm moist air from room to behind insualtion, so i attempt a complete seal from room to behind insulation, but if a modest amount of moist air gets through it can easily evaporate to outside.
To make the seal I use aluminium tape to attach insulation boards to each other, and all around the perimeter I leave a gap enough to get in the nozzle of an expanding foam gun and fill the full depth of the gap. this cos the tape does not adhere that well to old surfaces, tho it really grips the new foil surface of the boards.
Moisture getting in from outside is not usually a problem as it evaporates back out again in areas of normal exposure. I have no experience in areas of very high exposure, but I like the idea of ventilated slate or timber cladding.

Yes, but beware that you generally get a 'pass' on a condensation risk analysis *because you have a 100% effective VCL*. Very few installed VCLs, in refurb particularly, are anywhere near 100% effective.