Back or forward?

[ggc]

Hi again Hive…..

I think we are now pretty much ready to start the arduous task of installing our wall insulation.

What I was wanting to ask is regarding the placement of the insulation between the timbers please….

Our timbers are 140mm wide, and we have to fit 100mm Kingspan

Should our insulation be right at the back, almost touching the boards?

Or further out, sitting just behind the plasterboard (when fitted) ?

Many thanks

 

[Redbeard]

What does the condensation risk assessment assume?

[ProDave]

I would be completely filling the 140mm gap, then fitting an air tight membrane, battens following each stud and then plasterboard.  This makes it easy to seal the building and make it air tight, then gives a service void inside the air tight layer.  More insulation, better air tightness, easier for the trades, what is not to like?

[ggc]

48 minutes ago, Redbeard said:

What does the condensation risk assessment assume?

It’s our first time build, where would we get that information please?

 

We are following the SE’s submitted information, which was to use 100mm 

I guessed that it should be at the back, allowing space for the electrics, then the membrane over, followed by the plasterboard (which again is the insulted type)

TIA 👍

[ProDave]

14 minutes ago, ggc said:

 

We are following the SE’s submitted information, which was to use 100mm 

I guessed that it should be at the back, allowing space for the electrics, then the membrane over, followed by the plasterboard (which again is the insulted type)

TIA 👍

What do you hope to achieve from this building?  It seems like you are on course to get a building that just about scrapes through the minimum standard required of building regs.  But for just a little bit more effort and not a great deal more cost you could have a much better building with much lower heating bills forever.

 

I have not seen a 140m frame building done without at least another layer of insulation over the top of the frame members for some time now, surprised it still meets building regs with just 100mm of insulation.

[Temp]

I think fill the frame and line the inside with another layer. That inner layer will be easier to get air tight. 

[Iceverge]

3 hours ago, ggc said:

What I was wanting to ask is regarding the placement of the insulation between the timbers please….

Our timbers are 140mm wide, and we have to fit 100mm Kingspan

Should our insulation be right at the back, almost touching the boards?

Or further out, sitting just behind the plasterboard (when fitted) ?

Many thanks

 

Any further details please. 

 

Do you have a service cavity. Where is your air control layer? 

 

The kooltherm is an unusual choice as it is very very expensive. Normally it would only make sense where you had absolutely no extra space to work with and it was vital you hit a specific U value.  

 

In this case you have a U Value of about 0.27 w/m2K. 

 

 

 

I would take the Kooltherm back to the shop . Fit mineral wool batts between the studs. Then a vapour barrier. Then 50x50 battens perpendicularly with mineral wool infill as a service cavity and then some 15mm plasterboard. 

 

 

 

 

 

 

 

 

Edited April 29, 2023 by Iceverge

[TerryE]

The alternative is to counter-batten the 50×50s.  Somewhat like this Swedish guy does here on YouTube, but this does mean that you need to lay your PB in landscape mode.  At least this way the studs are supported on 600mm centres.  I like this continental idea of running all cabling in ducting, though note that they use multicore, rather than the UK practice of single core.

Edited April 29, 2023 by TerryE

[ggc]

Thanks everyone for your input, makes for interesting reading, but my brain is well and truly hoovered out!

 

For clarity, this is what we are working to………

 

 

Our SE and Architect have specified:

100mm Kooltherm

Between 140mm studs/timbers 

 

In my layman’s mind, the electrics would run on the inside face of the insulation? (Reason for the 40mm cavity before the 37.5mm insulated plasterboard goes on) 

Edited April 29, 2023 by ggc

[ggc]

Clearer image

 

[Iceverge]

Your Architect has been taken out for lunch by the Kingspan salesman I think. 

 

 

To get a sense of what I mean lets look at some £/R Values or how much insulation costs for a given heat blocking ability.  All prices from insulation4less. Please feel free to check my sums but I think it's accurate. 

 

 

 

Lets take the k112 and something like the Flexirock slabs. 

So what I hear...... its only £5. What s that between friends!? 

 

 

For your walls of a u value 0.15W/m2K or R 6.6. However that rises to an R value required of 8.25 by the time you account for 10% timber framing. Its a £5.46 hike from the Rocksilk to the Kooltherm per R per m2.

 

So £5.46 X 8.25 x say 100m2 of wall area = £4005 more expensive. 

 

Now there are less costs with reduced thickness but nothing like this. We haven't even considered the substantial waste with fitting boards between studs. I think the salesman might have had a week in the Algarve too. 

 

Find out if you can return/exchange the K112 would be my first port of call. 

 

 

 

 

 

Edited April 29, 2023 by Iceverge

[Iceverge]

 

 

There's no reference to a sealed vapour control layer here. 

 

There's a danger that you will get interstitial condensation. That means that water vapour (like steam, exhaled moisture from your breathing/cooking etc) will be able to pass through small gaps in your wall. If it meets any cold solid surface ( which it will outside the insulation) the vapour will condensed into liquid water. Because it's now cold and a liquid it's stuck there, it can't evaporate. Rot and mould are the result. 

 

The Kooltherm would work fine if it was a continuous sheet inside the house but it's not. There'll be holes at every plug socket and  gaps between the boards where they're butt jointed as insulated plasterboard.  

 

 

 

Given you have a U value target of 0.15 I'd do something like the following. 

 

Rocksilk 140mm £16.78/m2

50mm PIR with taped joints as VCL £9.05/m2

50mm Rocksilk £5.50/m2

With 50x50 battens £1/m2

15mm plasterboard £5.25

 

Total £37.58/m2

 

Or use a layer of 11mm OSB £4.71/m2 and 12.5mm plasterboard instead of 15mm plasterboard. Nice and solid to fix to vs PB alone. 

 

Total £40.76/m2

 

 

 

 

Compare that to the current buildup. 

 

100mm Kooltherm k112 £58.40/m2

37.5mm Kooltherm Insulated plasterboard K118. £ 29.58

 

Total £87.98. 

 

Less labour but twice the price. 

 

 

 

 

The free german website Ubakus makes Kooltherma poorer K value of 0.021 than the UK kingspan declaration . I don't know why, but I trust them about as far as I could throw them. Even with a K value of 0.018 you'd be optimistic to hit 0.15 and that assumes perfectly still air in the cavity which would be almost impossible to achieve. 

 

 

 

 

 

Edited April 29, 2023 by Iceverge

[TerryE]

IMO, this whole issue of where to place Air Tightness Layer (ATL) and Vapour Barriers (VB) is not settled in terms of proper scientific investigation informing building advice and guidelines, so at the moment there is no right answer.  Note ATLs and VBs are not quite the same in that a VB is breathable but generally moisture resistant.  

• As I said in an earlier post, our MBC house adopts the approach of having a minimally breached ATL as a racking plasticised OSB layer on the inside, allowing to frame and insulation to breath outwards.
• @ggc's seems to flip this.

As I said , this isn't settled so we can't say one is right and the other wrong. As I see it, the MBC approach has the advantage that the temperature gradient from inside to out goes uniformly from house to ambient temperature to the relative humidity gradient goes from low to high helping to keep moisture out of the frame itself. But I might be wrong here.  However I can make some observations.

• If you want an airtight (+MVHR) build then you need to have a robust and simple strategy / design for ensuring airtightness from design through to build completion.   Tradesmen will not understand this and can easily compromise it.  So you need a simple rule to enforce.  In our case there is a 45mm service void in front of the ATL, so all wiring, cabling, plumbing, etc. was run in this void; flush to plasterboard pattresses sat in the void, and we personally fitted any through viod ducting.  Tradesmen were not allowed to break it.  If you don't do something like this, then your as-built house will end up leaking like a sieve.
• Any twin wall void needs to be breathable on one side.  Double ATLs are an absolute no-no, because if there is any water ingress then it has nowhere to go; it can't dry out and rot will set in.
• Someone on the build must fully understand this strategy and police it.  This could be you, a PM or an architect, but this needs to be done preferably on a daily basis, because even with the best intentions some tradesman will make mistakes, and these need to be picked up and remedied before the mistake is hidden and buried.

All of the above also applies to the thermal insulation design, thermal bridge prevention, etc.  Same arguments apply: Maybe trust, but always verify.