MBC passive timberframe internal window reveal insulation - required and if so best material and fitting?

[markharro]

As the heading enquires do I need to bother with this? The architect has raised it with me and there would be room to put in maybe 20mm or 30mm of insulation board but a few questions....

 

1 I hate PIR and would much prefer to use something like this - https://www.ecomerchant.co.uk/interior/insulation/wood-fibre-rigid/steico-therm-wood-fibre-insulation-board.html

 

I guess the downside is inferior thermal performance but its breathable (not sure if relevant) and as I say I hate PIR.

 

2 fitting I guess we would glue it to the reveal? What type of glue?

 

3 Then how would we put the final board finish over it? This for some windows will be 12mm ply, others 12mm fermacell; others 12mm plasterboard? the latter two would probably be flat enough to glue but the ply would need screwing I think to keep it flat. Is there any issue screwing into the airtight ply around the window?

 

thanks

[Redbeard]

Without a dwg of the structure it is difficult to be certain whether or not it is required, but if the option is there I would take it. If you are using WF why not screw it to the ply and lime-plaster it? How are the adjacent walls finished? I assume the surrounding walls do not have the finish boards on yet. With plasterboard it's easy: Use a plastic bead and simply skim the pl'bd up to the bead's 'nose' (or 'notional nose' - users of WF may know what I mean) with gypsum then 'all change' for the reveals and plaster in lime with a thin-coat reinforced basecoat and smooth lime finish. The lime plaster (at around 8-10mm) could be your airtightness layer here.

[PNAmble]

My understanding is that MBC standard details don’t include insulation in the reveals (MBC details attached)

 

We didn’t go MBC in the end due to their lack of flexibility for a difficult site.  
 

we are insulating our reveals 20mm insulation; we are doing this post our first air test (result was 0.42) just to avoid cold bridges and to create a consistent finish around the windows.  This allowed the plaster board 9mm around the top and edges and a 12mm cill.  Can provide our details if unclear 
 

 

 

 

 

Edited March 9 by PNAmble

[Russell griffiths]

You could use, jackoboard on the reveal as you can skim straight over it. 
however I’m unsure if it’s needed on an mbc frame as they are very well detailed, without a drawing of your window placement it’s all a bit guesswork. 

[PNAmble]

Just to follow. Here is an example of our window detail. PN-D010.pdf

[Mike]

10 hours ago, markharro said:

I hate PIR and would much prefer to use something like this [wood fibre board]

I'll soon be ordering 30 or 40mm cork board for my current refurb - it's waterproof, breathable and rigid enough to glue plasterboard to.

[markharro]

Thanks for the various ideas - plaster over wood fibre may work in some locations but in one room I am fitting ply to match the ply on the wall so need a solution on how to fix that.

 

Be good if any other MBC customers could comment.

 

Where are you sourcing your cork from @Mike ?

[Nickfromwales]

42 minutes ago, markharro said:

Thanks for the various ideas - plaster over wood fibre may work in some locations but in one room I am fitting ply to match the ply on the wall so need a solution on how to fix that.

 

Be good if any other MBC customers could comment.

 

Where are you sourcing your cork from @Mike ?

Is the ply the final finish?

 

I've worked on a number of MBC PH frames, from only being the M&E consultant and contractor right through to finishing complete homes (coordination of boarding & skimming etc), and can say with confidence that this is a non-issue.

 

Self-builders are notorious for over-thinking and micro-managing, largely because they are face to face with every single tiny detail, plus they don't want to have to redo anything from un-investigated / non-mitigated misadventure (having often zero prior experience to fall back onto). One of the reasons this forum is so highly valued.

 

On a SIP's TF dwelling that I am currently working on, I have recommended that the client use (x)mm Jackoboard / other generic insulation tile backerboard to suit each reveal. Also, to use this in the openings for the skylights / lanterns. You affix with multiples of screws (prob 100mm o/c max) or bond on with something like CT1, screws if over a membrane, or screws plus CT1 if any other (such as the airtight green OSB etc). 

 

2 sets of skim over that and you're 2 birds one stone, and any worries long gone away. You can recess the back of those boards to accommodate the window fixings, as tbh these are the only significant cold bridges on an MBC TF (those and the skylights which they need to rethink their design on IMHO (sorry!)), remembering the TF is pretty much kosher and the issue is caused by the windows going in after MBC have done their bit.

 

Using packers behind the PB fixings to create an air gap is probably more than suffice on an MBC PH TF, and is 100-fold better than what is in most homes at this moment, without having life-changing losses / cold bridges around windows!!

[markharro]

Thanks @Nickfromwales yes you will see from the photo what I am doing. Ply below and fermacell above and I want to bring both into the reveal.

 

You mention tile backerboard. I wouldnt have thought that this would have much insulative value!? Thats why I wondered about something like wood fibre?

[markharro]

Ah I see where you are coming from now - Jackoboard has a polystyrene core! The only problem with this is that I hate polystyrene almost as much as I hate PIR and I can see potential for lots of horrible waste getting everywhere when routing the back to go over the window brackets.

[Mike]

2 hours ago, markharro said:

Where are you sourcing your cork from @Mike ?

It's for my French renovation, so the supplier won't help much! However this looks very similar: https://www.corkstore24.co.uk/shop/11-thermal-and-sound-insulation-expanded-cork-boards/

[Nickfromwales]

7 hours ago, markharro said:

Ah I see where you are coming from now - Jackoboard has a polystyrene core! The only problem with this is that I hate polystyrene almost as much as I hate PIR and I can see potential for lots of horrible waste getting everywhere when routing the back to go over the window brackets.

Get over it mate   

[Nickfromwales]

And have you started filling / sanding the FST for the Fermacell yet? If you gave that job to inmates, the crime rate in the UK would drop by 50% overnight. 
Seriously consider a plaster skim over those boards, it’s punishing work with very questionable results.

[Nickfromwales]

7 hours ago, markharro said:

You mention tile backerboard

 

7 hours ago, markharro said:

Ah I see where you are coming from now - Jackoboard has a polystyrene core!

👍

[Iceverge]

@markharro

 

Can you use rigid woodfiber boards and render directly onto them?

 

It's an important detail as when you get to really low U values the weak spots stick out like a sore thumb. 

[Nickfromwales]

On 10/03/2024 at 18:24, Nickfromwales said:

Get over it mate   

Also, don’t use a router! Stanley and a chisel is fine to break out small chunks, then pop a 10mm hole in. Once fitted over the window brackets you inject foam through the 10mm hole to fill the void you left from rebating. Don’t forget to mask the window frame whilst the foam expands. 

[Dan F]

We have a MBC twin-wall build in the process of being certified.  We uses Gaulhofer windows from @craig.

 

At one point in the process someone insisted that we should calculate psi values for windows junctions:

- Window cills without any additional insulation (but with MDF cill internally) were 0.023 W/(m·K).

- Window jambs  without any additional insulation were 0.040 W/(m·K)

 

Can these numbers be improved? Yes of couse they can, but these psi values are already fairly good and you're not going to improve them by a massive amount with the additional reveal insulation. 

 

Lets say you did half the jamb psi value to 0.020W/(m·K) In our model that only saves 0.25kWh/m2.yr.  (about £1.50/yr given 300m2 and ASHP heating off-peak @ COP 4).

 

So, if it's easy/cheap do it, yes.  But otherwise don't worry about it.  Also, why internal only, can you not add insulation externally in reveals too?   I'd use whatever material you are using for rest of internal/external walls, and then add insulation behind, rather than use a different material for reveals.

Edited March 15 by Dan F

[markharro]

All of £1.50 a year @Dan F! Ok if thats the case then it would seem that there would be no point at all. It seems to me then that the easiest way to deal with the reveals would simply be to pack between the metal window fasteners with some scrap 12mm Ply of OSB to take out the bumps and then glue or screw the final finish board to this. That would completely remove the need to buy more material to rout or cut out or spray foam or any of that hassle?

[Dan F]

2 minutes ago, markharro said:

All of £1.50 a year @Dan F! Ok if thats the case then it would seem that there would be no point at all. It seems to me then that the easiest way to deal with the reveals would simply be to pack between the metal window fasteners with some scrap 12mm Ply of OSB to take out the bumps and then glue or screw the final finish board to this. That would completely remove the need to buy more material to rout or cut out or spray foam or any of that hassle?

 

Ensuring window are propertly installed and foam/compriband has no gaps at all and that the airtighess is all good, are all more important.   If you do have void behind reveal board internally or externally then yes or course it makes sense to stuff it with insulationn or spray foam, but I wouldn't go out of your way to do much more than this.

[Kelvin]

I overthought all this too. I concluded that the most important thing was making sure the windows were sealed correctly with no gaps and good airtightness. My reveals are packed with insulation  but I formed the reveal wall with ply largely because I had loads of it handy. We then fixed the plasterboard to that.  My windows are fitted flush with the kit. I used jackoboard at the bottom of all the full height windows onto the insulated foundation block. This was as much about giving me a good surface to tile onto as anything else as my screed didn’t flow across the reveal to the back of the window. I considered doing all of the reveals in jackoboard but didn’t as I figured the cost benefit would be pounds vs using materials I already had available to me. 

Edited March 15 by Kelvin

[Iceverge]

On 15/03/2024 at 09:51, Dan F said:

We have a MBC twin-wall build in the process of being certified.  We uses Gaulhofer windows from @craig.

 

At one point in the process someone insisted that we should calculate psi values for windows junctions:

- Window cills without any additional insulation (but with MDF cill internally) were 0.023 W/(m·K).

- Window jambs  without any additional insulation were 0.040 W/(m·K)

 

Can these numbers be improved? Yes of couse they can, but these psi values are already fairly good and you're not going to improve them by a massive amount with the additional reveal insulation. 

 

Lets say you did half the jamb psi value to 0.020W/(m·K) In our model that only saves 0.25kWh/m2.yr.  (about £1.50/yr given 300m2 and ASHP heating off-peak @ COP 4).

 

So, if it's easy/cheap do it, yes.  But otherwise don't worry about it.  Also, why internal only, can you not add insulation externally in reveals too?   I'd use whatever material you are using for rest of internal/external walls, and then add insulation behind, rather than use a different material for reveals.

 

You must have very good windows. What profile did you use?

 

 

[Dan F]

25 minutes ago, Iceverge said:

 

You must have very good windows. What profile did you use?

 

 

Gaulhofer Inline Aluclad.  Greenbuildingstore did the psi calcs, assume they are accurate.

 

0.040 W/(m·K) is pretty standard and is used as a default by PH consultants if they don't have calculation.  The cill number 0.023 W/(m·K) is very good though, not sure why this is significnanlty better.

Edited March 18 by Dan F

[JamesP]

I used a Steico reveal board https://www.mikewye.co.uk/product/steico-reveal-board/ but these were fitted before the windows against the timber frame and not used

as a final layer for plastering.