Thermal Bridge / Condensation - ???

[Ryan Bazeley]

Can anyone who has built with SIPs help me please?!

 

I have seen and read everything on here and the green building forum about this topic I have even written posts asking about it. Even with this my SIP is about to be erected (groundwork complete) and the picture below is what I have, the SIP company and Kingspan have all said when I asked them that there is no issue and it’s over the top installing Marmox blocks etc.

 

My head is about to explode because I feel I have made a big error here, I am not going for passive standard I am just worried about this so called condensation issue and my sole plate rotting.

 

Has anyone here built a SIP house on a standard block upstand with DPC under the sole plate and clad it in render board? Will what I am building be okay?

 

 

[Bitpipe]

Is the yellow area a timber soleplate or some kind of insulated block?

 

What error do you think you have made?

 

Is your SIP company giving you an insurance backed warranty against any soleplate degradation based on an approved (BBA) design they have signed off?

 

Do you have a latent defect warranty and have they done similar?

 

[Ryan Bazeley]

Hi Bitpipe,

 

sorry, yes the highlighted section is the sole plate on top of the DPC.
 

Not sure on I will find out, I spoke with Kingspan and my installer at length about it and they said this is a standard way to do it and there will no problems as long as sole plate is kept dry which it will as it will be covered by cladding.

 

searching on here for other things keeps coming up with the same issue of rotting sole plate?! It’s got me worried.

[Ralph]

I don't see any issue with this. It looks very similar to what I have. Instead of SIPs I have a Scotframe kit which i s really a SIP hybrid. Looks a bit like this except polyurethane instead of rockwool. 

 

[Ryan Bazeley]

6 minutes ago, Ralph said:

I don't see any issue with this. It looks very similar to what I have. Instead of SIPs I have a Scotframe kit which i s really a SIP hybrid. Looks a bit like this except polyurethane instead of rockwool. 

 

I see you have 50mm perimeter insulation, I also see this a lot, what is the purpose of this. Do you finish it beneath the sole plate?

[Ralph]

12 minutes ago, Ryan Bazeley said:

I see you have 50mm perimeter insulation, I also see this a lot, what is the purpose of this. Do you finish it beneath the sole plate?

All it does is prevent cold bridging between the screed and the blockwork. I think It's slightly lower than the sole plate, hard to tell now that the screed has been poured. On the outside it comes up past the sole plate.

Edited May 21, 2021 by Ralph

[Ryan Bazeley]

Thanks for your comments, so being rational my sole plate isn’t going to rot out and house fall down? ?

[Phaedrus]

I've been wrestling with the same issue and still don't have a definitive solution so many thanks for posting the wall/floor junction detail.  I was advised that 100mm insulation (presumably PIR?) was woefully inadequate and that 300mm EPS under the slab was the way to go.

 

Now I'm thinking that a course of Marmox block (or similar) as shown in purple would reduce the cold bridging.  Would appreciate any of the experts wading in with their views.

 

[Ralph]

28 minutes ago, Phaedrus said:

I was advised that 100mm insulation (presumably PIR?) was woefully inadequate and that 300mm EPS under the slab was the way to go.

100 seems to be the minimum recommended,  we ended up with 200. There's a point where the cost benefit is so reduced you're better spending money elsewhere.

[tonyshouse]

I have been here a few years ago and still have concerns about condensation risk on top of the dpc at the outside underside edge of the soleplate.

 

i would like to see the 50mm eps carried a few hundred millimetres above the soleplate to mitigate the thermal bridge 

 

you can have the detail modelled but for me the coldest place is under the soleplate 

[Ryan Bazeley]

Surely in the UK the temperature swing would not cause it to condensate enough to cause a real problem as it would dry out??

[LA3222]

All I will say is I went round the mill with this on here when I was planning my build. Ignore it, all the talk of rot is a load of bollocks that people on here and GBF have used to argue against SIP (the same conversations are generally shilling the larson truss!). The soleplate setup in SIP is exactly the same as a standard 140mm stud build - its not different, yet I have never seen anyone mention the risk of soleplates rotting with that type of build! 

 

I have fixed 20mm of insulation to the outside of my soleplate, but I don't see it really doing much tbh and I don't care - I just don't rot this being an issue.

[Ryan Bazeley]

14 minutes ago, LA3222 said:

All I will say is I went round the mill with this on here when I was planning my build. Ignore it, all the talk of rot is a load of bollocks that people on here and GBF have used to argue against SIP (the same conversations are generally shilling the larson truss!). The soleplate setup in SIP is exactly the same as a standard 140mm stud build - its not different, yet I have never seen anyone mention the risk of soleplates rotting with that type of build! 

 

I have fixed 20mm of insulation to the outside of my soleplate, but I don't see it really doing much tbh and I don't care - I just don't rot this being an issue.

I have seen you comment on this before on older posts, thanks for your comment that is exactly what my installer has said!

[IanR]

9 hours ago, Ryan Bazeley said:

 

Why don't you ask the Kingspan to provide you a condensation risk analysis of your sole plate detail?

Without any external insulation much of the sole plate will be at the outside temperature. If the soleplate is below the dewpoint for the internal house air and that air comes in contact with it there is a risk of condensation.

20°C air with 65% RH has a dewpoint of 13.2°C

(https://www.calculator.net/dew-point-calculator.html?airtemperature=20&airtemperatureunit=celsius&humidity=65&dewpoint=&dewpointunit=celsius&x=54&y=16)

At anything below around 13.2°C, should air from inside the property contact the soleplate there is a risk of condensation (that doesn't mean a risk of rot).

 

To stop interstitial condensation on the solepate from the internal air, you need a vapour control layer on the inside face of the SIP to block/reduce the amount of moisture that can get to the soleplate. Overlapping and sealing the VCL to the DPC I would think is important, as is general detailing of the VCL.

 

I'm not sure what you could do about surface condensation though on the internal face of the wall at the sole plate. My feeling is that when it is cold outside, I can't see how the inner part of the soleplate could be above 13°C, as it has no insulation. In high moisture rooms, where the RH can easily be 70% - 80%, I would think there is a high risk of condensation for prolonged periods. But whether that is long enough without the ability to dry out,  to cause mould growth, I don't know.

It would certainly be improved with EWI over the block upstand and sole plate, but whether there is a risk without them you need a CRA. The higher you could take the EWI the better, but this would also need to be checked with the SIP Supplier since with the internal VCL you need anything outside that to be vapour open so that any moisture that does get in to the SIP can get back out.

 

Edited to add:

This is not a weakness of SIP, it's an issue of a cold bridge at the soleplate.

Edited May 21, 2021 by IanR

[Ryan Bazeley]

Thanks for your comments IanR, very well explained.

 

I take it from this then that the most important think to do is to ensure the VCL is installed correctly so the inside is as airtight as possible.

 

Will email Kingspan to ask about EWI.

[SimonD]

12 hours ago, IanR said:

This is not a weakness of SIP, it's an issue of a cold bridge at the soleplate.

 

I'd agree with that. The unfortunate thing is that this is an accepted aspect of "standard" timber frame details, whether SIPS or not. See, for example the Scottish timber frame standard details here. This detail is a thermal bridge, but it is 'okay' (not my words btw). Better would be to add a block with improved thermal performance as a starter - this is actually mentioned in the Scottish standard details.

 

3 hours ago, Ryan Bazeley said:

Thanks for your comments IanR, very well explained.

 

I take it from this then that the most important think to do is to ensure the VCL is installed correctly so the inside is as airtight as possible.

 

Will email Kingspan to ask about EWI.

 

It's absolutely essential. This is where using a service void to the inside of a timber frame is a blessing as it will reduce risk of perforating the vcl.

 

Usually, the vcl in this buildup should extend beyond the bottom of the internal face of the wall and a distance along the floor over the floor insulation, under the screed which should then prevent excess moisture ingress to the sole plate.

 

As long as the sole plate remains dry enough, then the rot risks are significantly diminished, which does mean you need to ensure moisture can get out if it gets in too.

 

Kingspan do have a detail where there is an additional layer of insulation outside the timber frame, but I don't know if that extends to sips, or render board as I've only seen the details for this in brick clad timber frame.

 

It still bemuses me why building practise hasn't moved to standardise wrapping the timber frame with an external layer of insulation. I actually did this with my project. Initially BC were a weary of it, but having given them some research paperwork to back up my design, they've been fine with it.

 

21 hours ago, Ryan Bazeley said:

I have seen and read everything on here and the green building forum about this topic I have even written posts asking about it. Even with this my SIP is about to be erected (groundwork complete) and the picture below is what I have, the SIP company and Kingspan have all said when I asked them that there is no issue and it’s over the top installing Marmox blocks etc.

 

My head is about to explode because I feel I have made a big error here, I am not going for passive standard I am just worried about this so called condensation issue and my sole plate rotting.

 

The problem you're experiencing here is so typical. You're probably being squeezed between purists who will detail the hind legs of a design, and try to cover every worldly eventuality, whether it's overkill or not, and the commonly lazy, this will do construction company attitude. But also the answer to your question isn't entirely black and white which means you're going to get variations in opinion.

 

As this is causing you to worry, I would personally look at it slightly differently:

 

A material with better thermal performance under the sole plate would help things out from a thermal bridging perspective and this may help reduce risks of condensation. With that in mind, what is the opportunity/cost ratio if you will? Is it marginal, or significant? Do you have both the time, opportunity and budget to add this detail into the build? If so, would it significantly ease your concerns? If it would, then I'd think it's a sensible thing to do.

 

From a decision making perspective, one of the things I've found with my build is that when I've decided to do something that improves the performance of the fabric of my house, once I've made the decision and done it, I feel good about it, even it if came with cost and inconvenience. When I've decided not to go with a potential enhancement, I've had a lingering question nagging away in the back of my mind of what if (unless that 'what if' was totally out of budget or reality of course! ?).

 

[Ryan Bazeley]

SimonD….

 

Thanks for your comments, I have been looking into your points the last few days. I am at the point now where it is too late to add a material with better thermal performance such as a Marmox Thermoblock under the sole plate.

When I look at the make up of my build at soleplate (below) the inside insulation goes up over the sole plate so surely this is going to stop the thermal bridge / transfer of air? Most details I see have the floor level with the bottom of the sole plate.
 

Thanks for everyone’s comments on this I feel more relaxed about it, footings being poured this week so becoming very real, on to the next thing to worry about!