Sorry all. I'm running word 2003. Though I would have thought 2010 should open it. I'ts nothing special by the way.

Don't get me started on that. Excel is even worse

Yep, I agree with all of that.

Hey! join the queue

You would think so wouldn't you. Trouble is it's not very palatable for either the legislators or the manufacturers. Rather like asking for money to argue against AGW

Ah, now there's a question. I missed that bit. I dont know whether it will rot, but standard OSB certainly goes like weetabix when its had milk poured on it. I don't know if the eco battens are the same material but if they are I wouldn't use them.

The point about the XPS+OSB 'ecostud' is that the OSB is on the warm side of the XPS, so only the XPS in the 'danger of sog' zone. The OSB will be right next to the room so will be fine. pics here: http://www.knaufinsulation.co.uk/solutions/residential/30_residential_refurbishment/internal_wall_insulation_iwi.aspx

Did you get too much sun yesterday

Surely the whole interstitial condensation thing is to make sure that any vapour getting into the structure from the warm side can get out easily at the cold side. So vapourproof the interior surface as much as poss, and you will have no problem in the typical case where the outside is porous (not rendered unless with a breathing material) and the location is reasonably sheltered. If the location is very exposed or otherwise wet (e.g. a basement) then just accept that there will be interstitial condensation and avoid any material which mught possibly rot.
So SIMMO, if you are still looking at this, go for the thickest celotex you can fit in without losing too much space, and seal well, especially the join wall to floor. With upstairs floors best to insulate between floor and ceiling below as well; I use bits of Kingspan or cellotex bedded in expanding foam and seal them to the main insulation with expanding foam. then cut floorboard down to allow for insulation thinckness.

A bit of encouragement: our bathroom was renovated and insulated by a builder in 1984, using treated timber battens under plasterboard backed with 18mm of XPS, with massive gaps at bottom. When we redid bathroom in 2009 I ripped the insulated p'bd and battens out, expecting rot and to my surprise there was none. There won't be any in the new insulation as there is no wood on the cold side and the whole is properly sealed. We do have unrendered brick and a sheltered location.

I don't know what has been suggested by e-mail but I would consider:

Posted By: martin.nIf the location is very exposed or otherwise wet (e.g. a basement) then just accept that there will be interstitial condensation and avoid any material which mught possibly rot.

How will you do this? Most houses have joists built into the masonry. Many also have original softwood lintols. The thicker the insulation is internally, the lower the temperature of the masonry will be. This increases the risk of rot, it does not reduce it!

Posted By: martin.nSo SIMMO, if you are still looking at this, go for the thickest celotex you can fit in without losing too much space, and seal well, especially the join wall to floor. With upstairs floors best to insulate between floor and ceiling below as well; I use bits of Kingspan or cellotex bedded in expanding foam and seal them to the main insulation with expanding foam. then cut floorboard down to allow for insulation thinckness.

No! or at least if you do get a WUFI simulation done first. It is impossible to seal all interfaces against vapour transmission from the inside to the outside. Also vapour from the inside is not your only problem, it is also water ingress from the outside which is exacrbated by the colder temperature of the masonry.

Posted By: martin.nA bit of encouragement: our bathroom was renovated and insulated by a builder in 1984, using treated timber battens under plasterboard backed with 18mm of XPS, with massive gaps at bottom. When we redid bathroom in 2009 I ripped the insulated p'bd and battens out, expecting rot and to my surprise there was none. There won't be any in the new insulation as there is no wood on the cold side and the whole is properly sealed. We do have unrendered brick and a sheltered location.

I don't see any reason to be surprised. This is exactly the point made in the Joe Little articles. A wall insulated with 18mm of XPS insulation will not achieve a u-value in excess of 0.5W/m2K. The flow of heat to the outside will be greater, warming the masonry and reducing any transfer of moisture from the outside. Less risk not more!

Posted By: wookey

It is impossible to seal all interfaces against vapour transmission from the inside to the outside.

Really? OK, it's impossible to get it to zero, but surely it's possibly to get it very low indeed. Low enough that there is no danger of it getting in faster on the inside than it's leaving again on the outside? You can argue about whether PUR foam alone is sufficient, or if foil/plastic is needed too, even on the little edge-gaps, but I don;t accept that it's 'impossible'.

Hi Wookey, I'll give you an examples of why its impossible: How do you seal the flow of vapour through timber joists.

Posted By: wookey

Also vapour from the inside is not your only problem, it is also water ingress from the outside

As Martin said 'if its very exposed' then I might agree, but I'm really not convinced that most walls collect water faster than it evaporates again, and the only thing keeping them dryish is the heat-transfer fro the inside. I guess the procatical problem comes if one accepts that there are _some_ walls like this, then how does one determine which class your particular wall is in (or more usefully exactly what 'ultimate expected dampness' the wall has).

Its about more than exposure, many many walls have been rendered with inappropriate cement based mixes, which crack over time and 'funnel' rainwater into the porus brick/block beneath. This moisture becomes 'trapped' as the render is [in the main] too impermiable to allow it to diffuse to the outside and evaporate. It is beyond doubt that internal insulation makes the masonry colder. This in turn allows the moisture levels to migrate further to towards the inside and ultimately cause timber degredation in joist ends and timber lintols.

Regarding the evaporation of such moisture build up, a general question to the forum: Who has ever hacked off a rendered wall and found the masonry to be bone dry? I cant recall any, and being a plasterer in another life, i've done a few

Posted By: wookeyBut the flow of vapour _through_ joists is not enough to do any harm.

Yes it is, and it does. Though the contribution of water ingress from the outside is more significant. By pure coincidence, I've been taking a floor up today

And this one shows the joist end in the bay which had no vent. There used to be a concrete step behind the joists. External floor level was approximately 150mm lower

Partially exposed. Note the worst rot is closer to the inside than the outside

Edited : Now imagine the consequences of internally insulating this wall. In the winter months, the temperature in the masonry will be colder; and the timber may become saturated through acombination of interstitial condensation and water transfer from the outside. As the seasons change to Spring and then summer, the temperature of the wet timber may well reach a level which will could promote dry rot.