Just finished Celotexing a freezing cold old chimney breast in my dining room - 90mm Celotex + 12.5mm plasterboard screwed omto 25mm deep battens (suggested on insulation mfrs' websites to avoid trapping moisture).

It was a much more fiddly job than I imagined, cutting and fixing a grid of battens, strategically positioned so as to line up with the plasterboard edges and with the insulation and plasterboard secured with 125mm and 150mm drywall screws respectively (though with hindsight I could have used hammer fixings and dodged the battens). Long drywall screws are expensive, as well as creating 15-20 small thermal bridges per square metre.

For the next job I'm tempted to consider dot and dab instead, but I'm wary of asking the Celotex's vapour control layer to take all that weight, and I've read in old threads here that you have to be careful of creating pervasive draughts, especially if you plan to install electrical sockets in the insulated area.

What do the panel think?

Also can anyone put a rough figure on the percentage by which I'll have increased my heat loss with all those small thermal bridges?

Andy

How about big head plastic or nylon insulation fixings, six per sheet? That is what I would do.

See my whisper for an example of fixing

@Nick, as I understood it the reasoning behind putting insulation over battens is to avoid trapping moisture between it and the masonry. I think that's why the mfrs stopped recommending putting the battens over. Unless I'm mistaken ... ?

This only works if you ventilate the gap, and if you do, you have thermally disengaged the wall (225 brick, say, with R=c0.48) from the sandwich. It means you live in a Pu/pl'bd tent.

If the gap is not ventilated (and that's proper cross-ventilation) ten the method you have used is no better, or potentially worse, than that shown in the attachment. Imagine if vapour does get past the VCL. In 'your' method it has timber battens to 'munch'. In 'mine' it has only any other timbers (not that that's good....), not any new ones.

The attachment dates from 2010. Whether Celotex have withdrawn it I do not know, but I still agree with and use it.

Dont disagree with Nick, but I did leave ventilated gap behind insulation, as I previous ly had issue with penetrating rain. I did keep timber on warm side. The insulation gives R = a
bout 5 so don't regret loss of insulation value of masonry.

Happy camping!

Will in Aberdeen, I agree wholeheatedly with you:

''Total faff to make it fit tight, gaps all over the place, kingspan shavings everywhere.''

However I don't wholly agree with:

''Never again... '' (although I often feel I'd like to do it never again)

OK, if you do it the way I do it you don't have battens to trim between (and you do have v long fixings), but the irritating faff you describe is effectively the only way for more-or-less every sloping soffit (skeiling) job in every attic bedroom. It really is irritating, but I think it's a good indicator of the dgree of attention to detail (faff) which really is required if the building industry is to be part of the well-insulated low-carbon future (but *please* don't let us stop working out easier ways!!). Nick

Posted By: wookeyGlue boards to masonry. Glue plasterboard/multipro on top. Polyurethane foam glue, continuous bead and cross pattern to check any airflow.

you're using PU foam glue to attach the insulation to the wall / pb to the insulation? I would have thought this expanded and pushed the things off the wall...

Posted By: wookeyDot and dab is a bad plan too (thermal bypass). You can use that glue instead of polyurethane if you like mixing, but still need to make sure perimeter is continuous

are people putting the d&d glue into a gun to get thin beads?

Posted By: andykentand there is clear evidence of paint blistering where external moisture has tried to get through.

maybe that's internal moisture condensing? If the celotex's edges weren't sealed then internal air would get behind the board and water would condense. Is the wall rendered?

I have been racking my brains over IWI a concrete block/cavity wall (filled) gable end wall and am still doing so. Previous postings of mine are relevant.

I had a long talk with the technical guy at one phenolic insulation manufacturer and it is not on their radar to glue the boards to the wall owing to the foil facing. (OK though for insulation backed dry lining which has no foil facing). I wanted to do that to make a sealed 'unit', with the 50mm phenolic sitting between vertical battens which I would then cover with foil tape to link up with the foil facings on the insulation. The insulation boards would have to be cut down a bit as the worse alternative would be to cut down the plasterboard. The latter would be fixed to cross battens themselves fixed to the vertical ones. Another concern I had was the difficulty in using fixings directly into concrete block, so I want to use the rawplug fixings through the vertical battens but they would have made large holes through the insulation. The technical guy at R did a rudimentary condensation risk assessment and found no problem with the build up. However, I know that outside wall (inside face) will be cold. I am sorting out a condensation problem the house had under its previous owners with 1 x single room HRV in the kitchen and a standard ventilation fan in the bathroom (too expensive to do it there).

There will be vertical wooden battens between the insulation and behind the vapour membrane (i.e. foil tape) but hopefully these won't be at risk. The foil tape will obviously lose some of its integrity when the cross battens are fixed through into the vertical ones. Any improvements on this build up - comments welcome. Materials need to be ordered this week!

Suegreenbuilding, why do you not do it as per the Celotex sheet I attached above? That way you have no vulnerable timber on the cold side.

Re holes in the insulation, I gun the holes full of either Pu foam (if really big) or silicone, then tap in a screw and plug.

Since you are not proposing an air gap as per the OP, I cannot see the reason for *not* using the Celotex (no advert!!)/PinM method. It seems far better than including timbers on the cold side and hoping that ''... hopefully these won't be at risk''. If you cannot spare the space taken up by the batten/void, either insulate between tha battens (on the warm side) or fix loose boards over insulation, all mechanically fixed.

Nick

Posted By: SaintPaul, since reading your thread I've been recommending the same, seems eminently sensible and practical.....but not, for me at least though, with phenolic, other plastic foams yes.

What's the problem with plastic foams? Too much danger of condensation forming on the cold side? In my case, as I have solid brick and/or stone walls, I kept the thickness down to a minimum - between 15mm and 30mm, depending on the wall. This tiny amount of insulation actually made a huge difference in raising the surface temperature of the interior walls to almost room temp, compared to, say 11C when it was -11C outside. The "cold radiation" feeling is completely gone and the house feels warm. These thicknesses are also what is recommended by a local architect who specializes in renovating old solid wall buildings without having to resort to EWI. Usually removing plaster and lath and replacing it with the thicknesses I'm talking about ends up with the new wall in pretty much the same place as the old one, so, therefore, not destroying the profiles of moldings etc.

Paul in Montreal.

Posted By: suegreenbuildingI had a long talk with the technical guy at one phenolic insulation manufacturer and it is not on their radar to glue the boards to the wall owing to the foil facing.

When I talked to a well-known phenolic board manufacturer about gluing, they simply referred me to the glue manufacturer to select an appropriate one. i.e they took a 'not our business' view. You could try the glue manufacturers.

Posted By: wookeyGlue boards to masonry. Glue plasterboard/multipro on top. Polyurethane foam glue, continuous bead and cross pattern to check any airflow. Quick, easy, effective.

Is pu foam glue commonly used to glue sheet insulation to masonry for IWI?

Posted By: wookeyThe glue foam is low-expansion, unlike the normal foam. It does still have some tendency to push the board away from the wall a few mm so you have to lean on it for ~5 mins until it's set (or put in extra fixings)

hmm, that seems like it would take a long time if you're doing that for both the foam layer and the pb layer? Do you stagger the joints of the two layers?

I think the whole thread highlights that PIR/PUR/Phenolic foam isn't a fit for purpose solution for IWI.
When it comes to retrofit, on solid brick walls. The best method is to parge/level the wall and directly put insulation to the wall. (Nick is spot on about convection space, and that includes dot and dab)
If a lime render/plaster is used then the wall maintains its breathability/vapour openess.

The UCL (University College London) has many test rigs of different insulants, and they all FAIL when out onto a batten!

Saint raises good points about what happens when water does get into the foam - and it does because you penetrate the foil with the fixings. I've also heard about acidic leakage as part of this chemical process - anyone have any update/news on that... only research I found on that was to do with pipe insulation and PIR was deemed better and glass fibre deemed best (rock fibre also gave a chemical/corrosive reaction when wet and interacting steel surface). I believe Sheffield University are doing studies on this as I type this

When you speak to Scottish Heritage, English Heritage, SPAB, GHA, AECB etc (Particularly SH) they recommend wood fibre insulation, how comes this hasn't been mentioned on this thread? If you research products like PavaDentro, you will see the SYSTEM (so you wont get told to speak to a glue manufacturer Sue) tackles condensation in a robust way. Also, they recommend using dynamic moisture modeling software such as WUFI, which uses EN number 15026 for calculating moisture, rather than the Glazer Method (BSEN13788) which doesn't take into account wind driven rain, location of building, orientation of building.

Sorry for the long posts, some good stuff above and interested to hear your thoughts...