Should I add internal insulation to my cavity wall - please help

[Johnny Jekyll]

I'm driving myself mad, please help.

 

Our new build is up and watertight, currently in first fix.

As in the picture the walls are made up from out to in - traditional sand cement lime render with Emperor paint, 100mm medium dense outer leaf, full fill PIR, 100mm medium dense inner leaf. It's passed SAP under 2019 regs when we started (yes it's taking years to build this thing).

Air tightness is excellent through a mixture of intello, sand cement lime parge, tescon vana tape, and airtight paint.

A Zehnder MVHR system is going in.

The floor has 300mm of PIR insulation below the screed. The cold loft has 500mm of loft roll insulation as ceiling level.

I have created a service void and straightened up the walls with 25mm battens ready to plasterboard.

 

Here's what's driving me mad.

 

The main cavity wall insulation did not go in too well (I won't bore you with the reasons why). To help, I have purchased 25mm PIR boards to add between the battens internally. But now I'm worried will this affect dew points and cause any condensation / mould problems to or behind the plasterboard / plaster. Or will the MVHR solve all that for me? Am I worrying about nothing or making an error?

 

Any advice greatly appreciated as I have 5 days off work now and want to get cracking (the wife is going mad). Thanks.

[Redbeard]

3 hours ago, Johnny Jekyll said:

 

The main cavity wall insulation did not go in too well

I think we need to know more about this before we express any opinion as to whether 25mm will make any significant difference. What is the width of the cavity? (So that we can work out, at least roughly, what the U value might have been intended to be.) What went wrong? You say full fill, so there is no *intentional* ventilation to the cavity. What about 'unintentional'?

[Johnny Jekyll]

3 hours ago, Redbeard said:

I think we need to know more about this before we express any opinion as to whether 25mm will make any significant difference. What is the width of the cavity? (So that we can work out, at least roughly, what the U value might have been intended to be.) What went wrong? You say full fill, so there is no *intentional* ventilation to the cavity. What about 'unintentional'?

 

Thanks for replying @Redbeard . Sorry the cavity is supposed to be 100mm, although it's more like 110mm. The full fill PIR is Cavity Therm and there's a little gap behind in many places (see photo below). The PIR is rigid in place and the house is rendered and painted so no access to the cavity anymore. I simply worry that cold air may get behind the PIR in places so thinking 25mm of PIR between the battens can only help, if just a little. Considering I have an excellent MVHR system going in, and if you guys don't think there should be any mould or other condensation issues, why not add the PIR between the battens (I already have it on site)?

 

If adding the PIR:

1. Should it fit flush against the wall or flush to the front of the battens (I had to space the battens off the wall to get them vertical in places, so there would be a gap either in front or behind)?

2. Presume VCL and taping the battens isn't really needed considering it's a masonry construction?

 

Thanks again for any advice.

[Andehh]

Of all the things to get bent out of shape over. Yikes.

 

Your house is already hugely well Insulated, with running costs of tiny £ a month (mine is £150 a month and very inefficient in design, and only 'above average insulation' - 160mm in floor, 150mm warm roof, 125mm full full wool (which is poor) and we are heavy electricity users anyway). Air tightness of about 2.9.

 

That gap may help, as still air is an insulation, even slightly moving air in that cavity is probably better then cold brick pressed up against PIR. Coldest days are usually windless days anyway.

 

Give yourself a break, know when to stop.... You'll put yourself/your wife into an early grave if this is the level of 0.001% detail you're obsessed over. 

[IGP]

I’d agree with the above. Don’t sweat it, yes it’s not *the* best, but it’s not a big deal. 
 

Continue the focus on airtightness in the remaining build, and the design of heating system to counter it by being really efficient. 

[Johnny Jekyll]

Thanks guys, I agree I can assure you.

But I still need to make the decision, do I add PIR in between the battens? I have it on site and would like to, just don't want to create a problem by doing it?

P.S. The gap in the cavity is behind the insulation, between the insulation and the inner leaf, so will create a bit of a thermal loop. 

[Mike]

5 hours ago, Johnny Jekyll said:

do I add PIR in between the battens? I have it on site and would like to, just don't want to create a problem by doing it?

Aren't the wall ties pulling the insulation back against the internal leaf? Or can you add more so that they do? If so, and if the insulation board has an interlocking profile and is intended for full fill, then I'd not worry too much, though it's far from ideal.

 

Otherwise, I'd consider foaming the top of the gap using Illbruck FM330 applied with a foam gun - very gently, so that the foam doesn't push the insulation further out of place. It may help to use some blocks to restrain the boards temporarily while the foam hardens. That will more-or-less seal off the lower part of the wall and limit the convection between wall and insulation to below that level.

 

Consider adding additional wall ties if necessary for the rest of the wall - it's most important they they are snug against the inner leaf. At least you have render externally, which is a useful shield against problems of rain penetration.

[Andehh]

It'll take decades to recoup the savings made on the PIR. Save the money and look at where else that investment could be put for greater effect.

 

What's the U rating if the wall? (also roof and floor out of curiosity?)

[crispy_wafer]

If you already have the PIR on site, then as you say, it’s the condensation risk that’s the key.  However…. If you come to sell the PIR you may turn a good profit on FB marketplace, I did when I over ordered and sold off a couple of sheets individually, Joe Public don’t use merchants and are very used to wickes/B&Q pricing I found.

Edited August 11 by crispy_wafer

[SteamyTea]

As you have MVHR fitted (or will have), I really don't know hink that 25mm of insulation on the outside will make a difference.

Just fit it between the battens,hard against the wall, it will act like a rain screen.

OK the condensation point may move a bit, but probably only a few millimetres, and only in very rare weather circumstances.

You want a VCL on the inside, masonry is not immune to moisture (if it was then 'damp walls' would not happen in old houses).

[Johnny Jekyll]

54 minutes ago, SteamyTea said:

As you have MVHR fitted (or will have), I really don't know hink that 25mm of insulation on the outside will make a difference.

Just fit it between the battens,hard against the wall, it will act like a rain screen.

OK the condensation point may move a bit, but probably only a few millimetres, and only in very rare weather circumstances.

You want a VCL on the inside, masonry is not immune to moisture (if it was then 'damp walls' would not happen in old houses).

 

 

@SteamyTea Thanks for the advice. Let's say I don't fit the 25mm PIR between the battens, and just leave the service void between the battens empty. Considering it's masonry construction and the walls are airtight (combination of parge and airtight paint), am I correct in thinking I don't need a VCL?

I've already fitted Intello to the ceiling (cold loft) and the dormer area. I don't want to install a VCL to the masonry walls or mid floor area, I'm way too far into fist fix for that.

Your opinion is much appreciated thanks?

[SteamyTea]

20 minutes ago, Johnny Jekyll said:

Considering it's masonry construction and the walls are airtight (combination of parge and airtight paint), am I correct in thinking I don't need a VCL

Airtight and vapour tight are not always the same thing.

There is also the problem that buildings move over time, and any finish onto masonry will fail in time, so fit the internal VCL, it is only a polythene sheet fitted and taped carefully after all.

[Johnny Jekyll]

9 hours ago, SteamyTea said:

Airtight and vapour tight are not always the same thing.

There is also the problem that buildings move over time, and any finish onto masonry will fail in time, so fit the internal VCL, it is only a polythene sheet fitted and taped carefully after all.

 

@SteamyTea Ok so may I ask:

1. Would the PIR boards foil sides act as a VCL? If so, presume I'd only need to foil tape over the battens to make it continuous?

2. Do I need to foam seal the gaps to the mid floor area? I have plenty of Soudal Window Door SWS Airtight Expanding Foam on site.

3. Do I need to carry the VCL through the mid floor area where the posi joists are (they are installed using joist hangers to a 9by2 wall plate which is resin bolted to the wall as per structural engineer design)?

 

[SteamyTea]

6 hours ago, Johnny Jekyll said:

Would the PIR boards foil sides act as a VCL

It is meant to.

Remind me, is the PIR going on the outside or inside (warmside)?

 

Always best to foam/adhere the boards to reduce air movement.

A VLC has to cover everything ideally.  Think of it as a bag that stops air and moisture getting to outside, the MVHR does that job.

[ruggers]

Theres a lot of mention that your doing this to the outside but your OP states battens to the inside with PIR between. I considered adding insulated plasterboard instead of regular board to the inside of my self build walls which were the same construction as yours plus a 50mm cavity air gap and these are 0.17W/m2K. The whole property heat loss for 200m2 was only going to improve by 0.3W/m2K when I tried it on my heat loss survey which didn't seem enough for a 2 grand+ outlay on the boards.

 

I was also unsure whether it was worth adding 37.5mm insulated plasterboards because it affects the thermal mass of the building. Some say more insulation trumps thermal mass, but others say having thermal mass is good with weather compensated controls in an unstable weather area so I'm  likely to leave the insulation & not to risk it.

 

You need to think how you will heat your home, will it be on all the time weather comp, or will you have on off controls and the house be fluctuate from really cold to full temperature and how fast you want it to respond. Masonry build will take a lot longer to warm from the heating being off than timber frame or if you always keep a minimum set back temperature of no less than 3 degrees of your desired temperature.

 

[Johnny Jekyll]

@ruggers thanks for the reply.

 

Yes I've decided to ditch the PIR between the battens. I've since found out that the Soudal Soudatight LQ (airtight paint) that I have on my walls is also vapour retardant so I'm not going to add insulation behind that. I'm painting some more of that on, then I'm leaving the service void between my battens empty. I've started plaster boarding and just need to move onto the next part of the project. I'll monitor the humidity in the house and make sure the Zehnder MVHR does its job once moved in. Overall it's now more than four years, and still a long way to go. 😩

[Andehh]

Good decision! No way will you regret it with the amount of insulation you've already got!