Airtightness (0.6ach target) - does screwing into Pro Passive Board create a leak?

[Great_scot_selfbuild]

Hopefully I’m overthinking this, but having read about the impact of small gaps in mortar on airtightness I just want to check…

 

Situation:

Timber frame build.

Internal face of stud wall is Pro Passive Board.

Intello airtight membrane used to lap joints and maintain airtight layer where PPB won’t fit.

Timber frame company has a proven reputation for achieving better than 0.6ACH.

They will install PPB & service battens, that’s where we pick it up from.

 

Plant room:

This is the first area where I will be installing 18mm ply throughout to give us a solid fixing for all the equipment that goes in here. I’m thinking that we can do away with the need for a service void here because all the wiring, piping, ducting etc is going to be on show anyway. I’m very keen to see how others have done their plant room fit-out.

 

My questions are:

1. Should I still keep a service void in the plant room? (Why?)

2. When screwing through and into the PPB (which would be the case with/without a service void), this will pierce the airtight green coating on the PPB - will this affect the ACH performance?

3. There will inevitably be cases where screws are put in and then taken out, having gone through into the PPB layer. Will this affect the PPB? How significant can these piercings impact the air tightness performance?

 

Photo for interest. This is ‘the big one’.

[G and J]

Ummmm, isn’t that a but too big for a domestic plant room?   😉 

 

Being serious I am sort of facing a similar set of worries.  Our airtight layer is our VCL,  and we are stapling that up.  OK, where we can we are putting the staples on the edge outside the tape so on the non airtight side but still, there are a good few places where the staples pierce the membrane.   Where a staple is removed or pulled out we should probably put a bit of tape over it, and that’s ok as none of our membrane will be visible.   But your walls will look like they’ve got measles lol

 

Presumably you’ll be painting your the PPB,  so can’t you simply fill and paint any screw holes from removed screws?

 

I suspect comments about air gaps in mortar are more relevant as there will be thousands of ‘em and they add up.

 

But I’m sure a grown up will be along shortly to confirm.  
 

As for plant room and a service void isn’t it just your preferred aesthetic?

[JohnMo]

When I was screwing into stuff I put a blob of hybrid silicone stuff on the screw. Whether it made any difference who knows? Once a screw is in the wood the wood contracts around the thread, so I doubt there will be any leaks anyway.

33 minutes ago, G and J said:

there are a good few places where the staples pierce the membrane

Ours aluminium faced so used aluminium tape to cover all the staples.

[saveasteading]

7 minutes ago, JohnMo said:

screwing into stuff 

Depends on the "STUFF".   If it's a 1mm sheet of metal then water and air will get through so special screws have special washers that bind to the thread.

A screw into timber might allow damp but not air through...so perhaps a smear of mastic is worthwhile. Yes through wood has been compressed so is tight, but could get damp. If you look at screws into cedar you can see that damp has got in.

 

[Great_scot_selfbuild]

The intention for plant room is that the 12mm green PPB has 18mm hardwood ply board nailed directly to it, then items will be installed into it, with screws going through both. My main concern is if screws are put in and then later removed (items moved), then how do I maintain the airtightness.

 

All of this is, of course, easy to keep an eye on when I’m doing it myself - my concern is once I have the electrician, plumber and others involved who don’t get the significance of ‘its just a screw’.

 

So, how important is it to fuss over this level of detail - am I right to be so picky? (Possibly my ‘default’ approach 🫣)

[MikeSharp01]

58 minutes ago, JohnMo said:

When I was screwing into stuff I put a blob of hybrid silicone stuff on the screw. Whether it made any difference who knows? Once a screw is in the wood the wood contracts around the thread, so I doubt there will be any leaks anyway.

If you use a wood screw it punctures the membrane and fills the hole with the shaft so provided the thing it is attaching is down tight there won't be any leaks. So we didn't bother with anything other than screwing down tight generally, we got a 0.2 on the PH scale for our air tightness and there must be 1000s of screw holes through the VCL all clamping down wood, but only wood. As @saveasteading

 points out anything else will need special treatment. I tended to mount them with self sealing screws, such as the ones used to screw down roof sheets, or mounted them on a strip / square of mastic tape to fill any holes.

30 minutes ago, Great_scot_selfbuild said:

The intention for plant room is that the 12mm green PPB has 18mm hardwood ply board nailed directly to it, then items will be installed into it, with screws going through both.

Your worry here is jacking as your screw might push the two layers apart slightly but in the end the screw will fill the hole makes won't it. 

 

30 minutes ago, Great_scot_selfbuild said:

My main concern is if screws are put in and then later removed (items moved), then how do I maintain the airtightness.

You have to plug the holes! We did cover every staple with a tape square and fastidiously marked damage for repair if we made any.

[MikeSharp01]

Just another thought, we kept the service void in the utility room and had we not done so we would have had problems with mounting things like threaded inserts through the VCL used for things like munsen clips and the like.

 

[G and J]

3 hours ago, Great_scot_selfbuild said:

18mm hardwood ply board nailed directly to it, then items will be installed into it, with screws going through both

Why not use smaller screws so they don’t go through both layers? i.e. 25mm plus payload. 
 

I’m trying to get my head round airtight.  To check my thinking:

 

We test at 50pascals.   I think that’s equivalent to less than 6mm difference in a water manometer.  
 

So it’s sod all in pressure terms.  
 

So unless an ex screw hole is huge and ragged, even if there’s a few dozen of them, would that make a difference?

 

Have I got my sums right?

[JohnMo]

45 minutes ago, G and J said:

Why not use smaller screws so they don’t go through both layers? i.e. 25mm plus payload. 

That's the smart solution.

 

46 minutes ago, G and J said:

trying to get my head round airtight.

If you put it in a bag you will soon learn all about it.

 

47 minutes ago, G and J said:

We test at 50pascals.   I think that’s equivalent to less than 6mm difference in a water manometer.  
 

So it’s sod all in pressure terms

True, about the same as 20mph wind, hitting all sides of the building at the same time.

[Russell griffiths]

Plant room

18mm ply direct to the walls then 2.4x1.2 sheets of fermacel jointed using their joint stick, then two good coats of eggshell paint. 
all the plumbers handprints and pencil marks wash off easily or a quick touch up and there gone. 
 

this is what I did. 

I cannot believe the amount of unpainted walls in plant rooms I see on here that then have ufh manifold fitted and tanks put in place before the walls are painted

 

looks totally rubbish. 

[crispy_wafer]

21 minutes ago, Russell griffiths said:

Plant room

18mm ply direct to the walls then 2.4x1.2 sheets of fermacel jointed using their joint stick, then two good coats of eggshell paint. 
all the plumbers handprints and pencil marks wash off easily or a quick touch up and there gone. 
 

this is what I did. 

 

Simple and effective, Needs putting in a memo and stickying at the top of this site TBH, then email it out to all architects and architectural designers to put in there schedule of works!  I’m well behind on my build, partly because I haven’t yet had my plant room skimmed, and I refuse to crack on with all that on unfinished walls, this has had a knock on effect with plumbing, electrics, etc etc.  Get the plant room done and ticked off first, then everything else can follow.

[Iceverge]

7 hours ago, Great_scot_selfbuild said:

. Should I still keep a service void in the plant room? (Why?)

 

Yes, keep the gap behind the ply facing. To stop screws penetrating the airtight layer. Also it may allow you to run wires more tidily to plant even if you've still got everything else surface mounted. 

 

7 hours ago, Great_scot_selfbuild said:

When screwing through and into the PPB (which would be the case with/without a service void), this will pierce the airtight green coating on the PPB - will this affect the ACH performance?

 

No. A screw in a hole is airtight from the purposes of house building. We didn't make 0.2 as above but 0.31 and we didn't tape any staples or screws.

 

7 hours ago, Great_scot_selfbuild said:

There will inevitably be cases where screws are put in and then taken out, having gone through into the PPB layer. Will this affect the PPB? How significant can these piercings impact the air tightness performance?

 

 

Keep your Ply facing spaced off the airtight layer for this reason with a service cavity. 

 

Have a tube of sealant and caulk the holes on blower door test day. 

[Great_scot_selfbuild]

5 hours ago, Russell griffiths said:

Plant room

18mm ply direct to the walls then 2.4x1.2 sheets of fermacel jointed using their joint stick, then two good coats of eggshell paint. 
all the plumbers handprints and pencil marks wash off easily or a quick touch up and there gone. 
 

this is what I did. 

I cannot believe the amount of unpainted walls in plant rooms I see on here that then have ufh manifold fitted and tanks put in place before the walls are painted

 

looks totally rubbish. 

@Russell griffiths This has given me pause for thought - although I would like it a nice painted, clean room, the practicalities of slowing everything down for this room (when I actually like the natural finish of the hardwood ply) is something I'm weighing up. I also already realise I've built the garage before the house and if I tell my wife I'm going to board and paint the plant room, before we've guaranteed the kitchen and other family bits of the house, I may not survive the build! 🤣

 

Do you have any photos of your plant room for inspiration please?

[Great_scot_selfbuild]

5 hours ago, Iceverge said:

Keep your Ply facing spaced off the airtight layer for this reason with a service cavity. 

@Iceverge This is the space I'm in atm, it will remove an inch of useable space from each wall, which on the end walls would bring it down to 888mm wide.

 

My main specific challenge from a sequencing perspective is the following:

 

Timber frame is a Larsen-truss wall, from the outside:

Breather membrane

12mm medite vent board

350mm cellulose insulation

12mm Pro passive board

(assuming I keep the service void) 25mm batten

18mm hardwood ply

(fermacel/similar?)

 

@crispy_wafer @JohnMo @G and J @MikeSharp01 @Russell griffiths @saveasteading So, I need to drill/core holes for the MVHR supply & extract ducts (~210mm dia hole). I've been pondering the challenge of drilling all the layers - I don't trust doing this by measuring, so I'd want the ply in place first, then I figure I would:

Drill the ply and pro passive board

Remove the ply so I have a (little closer access to reach through to drill to the outer medite-vent board.

Once the duct is installed, I can then seal around the pro passive board, then re-fit the ply over the top and then think about painting / finishing.

 

QUESTION: How do I drill through the propulsive board and then bridge the 350mm gap to the outer board? (for info, it is highly likely the insulation will have been installed already, though I'm going to see if I can delay this)

 

[Russdl]

12 hours ago, Great_scot_selfbuild said:

am I right to be so picky?

Yes. Every little helps. 

[JohnMo]

7 hours ago, Great_scot_selfbuild said:

How do I drill through

I would buy a long small dia drill, that can go through the whole lot, drill to give the fall you need in the duct. From there you have a centre for the duct. Mark up the diameter you need. Then drill a pilot hole and use a jigsaw to take out the hole, repeat on the other face.

[Great_scot_selfbuild]

51 minutes ago, JohnMo said:

I would buy a long small dia drill, that can go through the whole lot, drill to give the fall you need in the duct. From there you have a centre for the duct. Mark up the diameter you need. Then drill a pilot hole and use a jigsaw to take out the hole, repeat on the other face.

This is an excellent suggestion (just the idea I was missing!) - to add to it, I'm thinking that rather than jigsawing, the pilot hole could be used with the pilot drill on a hole saw.

[Russell griffiths]

16 hours ago, crispy_wafer said:

 

Simple and effective, Needs putting in a memo and stickying at the top of this site TBH, then email it out to all architects and architectural designers to put in there schedule of works!  I’m well behind on my build, partly because I haven’t yet had my plant room skimmed, and I refuse to crack on with all that on unfinished walls, this has had a knock on effect with plumbing, electrics, etc etc.  Get the plant room done and ticked off first, then everything else can follow.

That’s the good thing about the fermacel boards, no skimming needed, just fill the screw holes and paint. 

[Russell griffiths]

9 hours ago, Great_scot_selfbuild said:

@Russell griffiths This has given me pause for thought - although I would like it a nice painted, clean room, the practicalities of slowing everything down for this room (when I actually like the natural finish of the hardwood ply) is something I'm weighing up. I also already realise I've built the garage before the house and if I tell my wife I'm going to board and paint the plant room, before we've guaranteed the kitchen and other family bits of the house, I may not survive the build! 🤣

 

Do you have any photos of your plant room for inspiration please?

 

[Russell griffiths]

Your not going to drill a 210 mm hole with a hole saw 

definitely a jigsaw job as John said

fit ply

mark centre point for duct

drill centre hole with a slight fall

remove ply and trace outline of duct

cut out hole in ply

draw around off cut of ply on the pro passive

cut out pro passive

draw around cut out of ply on outer layer of truss

cut out outer layer 

Fit duct  and seal up well

fit ply and seal up well. 
 

or leave the duct loose until you are ready for the mvhr. 
 

I would always cut the holes larger as I like a good squirt of illbruck foam around everything 

[Dunc]

6 hours ago, JohnMo said:

I would buy a long small dia drill, that can go through the whole lot, drill to give the fall you need in the duct. From there you have a centre for the duct. Mark up the diameter you need. Then drill a pilot hole and use a jigsaw to take out the hole, repeat on the other face.

+1.

I'm about to do this on my MBC larsen truss frame, though I don't have any inner lining (ply or plasterboard to worry about) and it's before the cellulose fill. I  sourced some very cheap 600mm long auger bits from Amazon and intend to use a jigsaw. Just need to buy a couple of airtight grommet/patch things to slide over the duct to provide a seal against the propassive board.

 

I'm curious how easy (or otherwise) it will be to push the duct through, or dig through, the insulation if its installed before you put the ducts in. There is an old thread where @Jeremy Harris described putting a small (~30mm) duct through his (similar construction) wall after insulation. My recollection is he described it as quite tight, but I can't find the thread now.

[Iceverge]

1 hour ago, Dunc said:

Just need to buy a couple of airtight grommet/patch things to slide over the duct to provide a seal against the propassive board.

 

You can use airtight tape.

 

Just cut it into lots of little strips.