Warmboard dot & dab or mushroom?

DamoKen

Hi, looking for a bit of clarity on conflicting advice I've been getting re fixing warm boards to an external solid block wall. From a couple of places I've been advised to just fix directly to the wall with mushroom fixings. Wall seems to be old solid cement blocks judging from a vent hole & where some plaster came off taking off the skirting.

Most sites however including the board manufacturer don't mention mushroom fixings, instead they suggest dot & dab if not going with battens.

I've also discovered there's a significant bow in the wall, the middle of the wall is about 10-15mm in from the high points above and below the window frame.

So a bit concerned attaching mushroom fixings with a cavity in the middle, i.e. possibly crack it when tapping in. To counter this it was suggested to fix the top and bottom with the mushroom fixings and then drill the middle holes in the board and fill any cavities with expandable foam. Once set do the last fixings.

However just went to pick up the expandable foam and explained this in a local builder providers who is of the opinion that I should go dot and dab the whole way to align the board, once the adhesive is set I then add a few mushroom fixings.

Another builder provider where I bought the boards recommended mushroom fixings as did the guy who did an inspection of the house, specialises in old house renovations.

Bit confused so any advice appreciated. Boards by the way are Quinntherm, 60mm including plasterboard with foil backed insulation that is then covered in a paper finish suitable for adhesive.

Thanks

Calahonda52

I believe there are fire reg issues with the dot and dab which will require you to use some mechanical fixing per sheet anyway so unless you are skilled with dot and dab, then go mechanical.
The mechanical fixing also makes finishing off around the wall vents easier
Is the plan to skim the wall afterwards, internally?
What thickness is the external wall and what is the external finish?

DamoKen

Hi Calahonda, wall is about a foot or so in thickness, externally it's a pebbledash render, think a concrete render followed by lime but not 100% sure. Very old house that's had a lot of work done by different people over the years so no two walls alike.

Internally yeah I'd say I'll need to reskim. Some of the cut boards aren't going to line up on the taping edges, i.e. additional pieces required at top as wall is a non standard height. Tape will I think stand proud of the boards so a skim would probably be the best option.

So do you reckon just some expandable foam for cavities?

Cheers

Calahonda52

I would mech fix all the way and get the plasterer to fill out the hollows with bonding before the skim

DamoKen

Hi Calahonda, do you mean fill out the cavities before putting up the warmboards?

I'll be doing the work myself to keep costs down. When you say bonding do you mean gypsum bonding coat with maybe thistlebond applied to the wall first? And once set put up the boards with mushroom fixings?

Cheers

Calahonda52

No, I would fix to wall and then put bonding on to fill the hollows, which I assume are gradual.

Can you skim?

DamoKen

Sorry not sure if I understand. The hollow is at the midpoint of the wall, i.e. top & bottom of the board will be in contact with the wall but middle of board won't due to the dip in the current finish. Measuring with a 2.4m straight edge at the deepest point there'll be a gap of approx 15mm behind the board so I'd be concerned that the plaster facing on the board may crack if I try and just fix mushrooms to the midpoint of the board as is. Hence it was suggested to drill the holes at midpoint and fill behind with expandable foam to support it.

Would there be that much bend in the board?

Have only skimmed smallish areas to date, largest was the side of a substantial chimney breast. Awkward enough spot but seemed to go quite well, looked ok although main concern was just getting it flat for tiling afterwards, not a paintable finish.

Almost all the walls are in bad shape so overall plasterer cost would be prohibitive. I have the time in evenings and at weekends so want to learn to do it properly. If I make a balls of it first time, learn from it and do it again. Hopefully will have perfected my technique by the time I get to the more important rooms.

Cheers

Calahonda52

Well, I would not be messing around with expanded foam after fixing the board.
If you are quick about it put a few dabs of bonding on the wall before you offer up the sheet and fix it top and bottom, and then fix the next day where you have the dabs.

DamoKen

Calahonda52 wrote: »

Well, I would not be messing around with expanded foam after fixing the board.
If you are quick about it put a few dabs of bonding on the wall before you offer up the sheet and fix it top and bottom, and then fix the next day where you have the dabs.

Actually that's a good point. Didn't take outward pressure from the expandable foam into account once the board is fixed in place. With only a few drill holes to allow run off could be a bit dodgy trying to judge when to stop.

So how does this sound?

Start by holding the board in place, drill the top and bottom holes as I would if I was fixing straight away, take it down and apply dabs across a pre-marked area with the most pronounced bow, roughly to a depth of just above 15mm. Put the board back up, level it into place and insert the mushrooms into the pre-drilled holes top & bottom. Leave it to set a day or two and then as you say fix the rest of the the mushrooms.

dathi

http://www.xtratherm.com/wp-content/themes/xtra/downloads/publications/Xtratherm_UK_Drylining_Guide_11_3.pdf
have a read here might give you a few pointers

DamoKen

Cheers Daithi, yeah I had a read of that and it does reference fixing directly to the wall on page 16. Bit light on detail unfortunately, doesn't mention flatness of walls with this method. Battening seems to be the methods they suggest for uneven walls but not a route I wanted to go down due to room size plus the additional beading required on the original sash, just about got away with what I've already added.

Think filling out the hollows with adhesive as mentioned above might be the best route.

Will update with the result.

Thanks