Warmup insulation boards

[EllieP]

This is probably a daft question, but.... I'm laying 10mm warmup insulation boards on 18mm ply (no sub-floor below the ply). Warmup site says to use flexible adhesive, then screw the insulation boards using washers.

I am concerned that if I use a washer the screw/washer will not be flush with the top of the insulation board, so will make it difficult for the tiler. (My 'builder' mate told me that if the floor is not smooth as glass, the tiler will have problems...")

If the screw-head/washer is pround by a millimetre, will this make tiling difficult? I suppose I could drill out countersunk recesses for the washers, but this seems a lot of fanying around if not necessary...

Also, should the ply (structural spruce ply - EN636-2) be sealed with PVA before I lay the insulation boards?

Lastly, the warmup site calls for FG tape over the joins in the insulation board. I have some self adhesive plastering tape - will that do?

Many thanks

 

[Richard C]

Not familiar with Warmup insulation boards, why are you using them? are you installing an UFH system?

 

[EllieP]

Not familiar with Warmup insulation boards, why are you using them? are you installing an UFH system?

Yup - warmup UFH, and the peeps from warmup tell me that as I have a 18mm plywood floor straight onto joists I will need tile backer boards. Both warmup and my local tile supplier tell me that the warmup boards will suffice as backer boards.

 

[Richard C]

This is probably a daft question, but.... I'm laying 10mm warmup insulation boards on 18mm ply (no sub-floor below the ply).

25mm WBP is the BS standard; 18mm WBP is the minimum but can be used on it’s own in light load/use areas (bath/shower rooms) but it depends on the size/pitch/span of the floor joists.

Warm up site says to use flexible adhesive, then screw the insulation boards using washers. I am concerned that if I use a washer the screw/washer will not be flush with the top of the insulation board, so will make it difficult for the tiler. (My 'builder' mate told me that if the floor is not smooth as glass, the tiler will have problems...") If the screw-head/washer is pround by a millimetre, will this make tiling difficult? I suppose I could drill out countersunk recesses for the washers, but this seems a lot of fanying around if not necessary...

The floor needs to be flat (especially with large format tiles) but it hardly needs to be as smooth as glass; not many builders know how to tile anyway so I wouldn’t take too much notice. Ideally the heater mat should be set into a self level screed before tiling; you can tile straight onto it but you will never be able to repair it in the event of a problem. Can’t see what the boards are made of form the website but if cement based, the washer are probably specified to prevent the board cracking. Assuming the screw head will be below the level of the heater wire, it should not be an issue. Floor tiles should normally be laid on continuous thick bed adhesive, 3-4mm thick but this can be increased using the correct trowel so should not be a problem for your tiler.

Also, should the ply (structural spruce ply - EN636-2) be sealed with PVA before I lay the insulation boards?

You’re using the wrong ply it should be WBP (EN 314-3) for tiling applications. It should also be sealed with acrylic primer on the underside (not the top) & edges before it’s laid.

Lastly, the warmup site calls for FG tape over the joins in the insulation board. I have some self adhesive plastering tape - will that do?

Yes

Never use standard PVA as tiling primer, its water soluble; if you need a primer, use an SBR or acrylic primer

Select your tiler very carefully, surprisingly there are some that can’t actually lay tiles! Make sure he uses only quality trade materials of the correct type for your tiles; read the tiling sticky & forum archive posts, it could prevent you making potentially expensive mistakes.

Are you aware installing UFH or any electrical work in a bath/shower room is notifyable building work for which you will need a minor works certificate?

 

[EllieP]

Thanks for all the info Richard,

Ply is a big issue, cos it's already glued and screwed! Joists are not equi-distant - range from 360mm centres to 450mm, but I have put noggins in everywhere (glued and screwed). Also too late to seal the underside and edges of the ply.... But if I am laying more insulation board over the top I might get away with it????

My tiler seems to know his stuff... When I asked him about using PVA he told me "Deffo NO!" He will sort out priming/sealing, and provide addy and grout, all I'm doing is supplying the tiles.

Bathroom has been in use since the house was built 15 years ago and the old chipboard floor (beneath carpet) was bone-dry. This could indicate that we do not get the place too wet lol!

In respect of the UFH electrics, all I'm doing is laying the mat and will be checking the resistance at regular intervals as laying/tiling progresses. Part P sparks will be doing the easy bit!

 

[Richard C]

Ply is a big issue, cos it's already glued and screwed!

Ahh well, you’ll know next time; who recommended you use EN636-2? Hopefully it will be OK, it’s suitable for humid conditions or use externally when covered but the veneer bonding adhesive is not water proof; tile adhesive & grout contains – lots of water!

Joists are not equi-distant - range from 360mm centres to 450mm, but I have put noggins in everywhere (glued and screwed).

The size & span of the joists are also critical factors in floor rigidity; additional noggins are fine for supporting the edges of ply floor but do virtually nothing to improve the overall rigidity of the floor.

Also too late to seal the underside and edges of the ply.... But if I am laying more insulation board over the top I might get away with it????

Primer on the underside & edges is to prevent any possibility of ambient moisture, which varies summer to winter, from being absorbed into the underside/edges of the ply causing expansion/movement which, in turn, can cause tile failure if it’s beyond the range of the flexy addy your using. If they are also backer boards, they will stiffen everything up but that wont stop moisture entering the underside of the. You should be OK but no guarantees.

My tiler seems to know his stuff... When I asked him about using PVA he told me "Deffo NO!" He will sort out priming/sealing, and provide addy and grout, all I'm doing is supplying the tiles.

At least he knows that but be sure to ask what products he is using; post back if you want.

In respect of the UFH electrics, all I'm doing is laying the mat and will be checking the resistance at regular intervals as laying/tiling progresses. Part P sparks will be doing the easy bit!

Make sure you get a minor works certificate (BS7671) for it.

 

[EllieP]

Thanks again Richard.

As you say, the deed is done and I will know for next time! In the meantime, fingers crossed for this job...

I looked at the joists and spans again last night. Joists are not huge (approx. 190mm x 50mm) with a double-joist across the centre of the room. Span-wise, the maximum each joist can be is around 4 to 5 metres, and the house is relatively new (1996). When I take the shower tray footprint and pipe boxing out of the equation, this leaves me with only around 3 square metres of floor, so not the biggest disaster in the world if it does go pear-shaped, lol!

I also checked the Warmup website again, and the recommended rate for screws through the insulation boards is only 5 screws per square metre, min 30mm from the edge, so not a massive undertaking!

Will check with the tiler on sealant/primer, addy and grout, although other half bought a bag of flexible addy for the insulation boards - 50 quid! (BAL as far as I know). At that price it needs to be good...

Just have to wait and see now.....