Sole plate condensation risk

[bissoejosh]

Apologies in advance for dragging up an old topic....

 

A large element of our build requires the use of a beam and block floor on strip footings. In an ideal world this wouldn't be the case but our site dictates it. As mentioned in other posts we are using an i-Stud frame and whilst not expecting passive standards, would like to do the best we can.

 

I've read numerous threads regarding the issue of sole plate decay through condensation when using a cold foundation detail such as ours, and if possible I would like to minimize the risk. I'm happy to consider the use of thermo blocks etc if they are genuinely required but wondered if a simpler solution exists. I've read several comments suggesting running external insulation above and below the sole plate however that isn't possible without significantly increasing our vented cavity.

 

The detail below uses a 215mm block with the 245mm frame overhanging by 30mm (approved by the frame engineer). This would allow space for 40mm of EPS directly below the sole plate and continuing down as far as necessary. I would finish this in a similar way to some of the MBC houses on EPS slabs - fascia board etc. Does anyone think this would be enough to minimize the condensation risk?

 

Another option seen on the touchwood site is a basic cavity wall concept filled with EPS on which the sole plate sits, would this be a better option? Both are viable for us in terms of footing width.

 

As always comments greatly appreciated.

 

DewPointDetail.pdf

[Jeremy Harris]

Adding insulation around the outside as a skirt is a reasonably effective way of mitigating the risk, provided you can also ensure that there is enough thermal resistance vertically under the supporting structure.

 

Sadly there are a lot of details out there that are not well thought through, including some from major and well-known manufacturers.  The tendency is to rely6 on well-known steady-state thermal models, and then just assume that they represent the real world.  Sadly they don't, and the methods that BRE suggest, or that are included in well-established standards for assessing interstitial condensation risk are far from being foolproof.

 

To be fair, most of these models and assessment methods were developed long before anyone dreamt of making ordinary houses with good thermal performance.  As the PassivHaus Institut found, when they first put together the PassivHaus standard, things get far more complex, and dynamic, when you both increase the overall insulation level and decrease the heat capacity of parts of the structure.  The dynamic problem that I've tried to explain here before is the major issue for the UK climate - we often get days when the overnight temperature may be well below freezing, where the relative humidity near ground level in the early morning, as the sun comes up, hits 100% (hence early morning mists) and where there is a high probability of water vapour moving inwards towards colder internal structures that may well be below dew point.  The problem then is that the heat energy required to cause condensed water in these areas to change phase to vapour, so it can migrate back out again, is pretty high, and there may well not be enough energy available during the day to cause this to happen.  After a few days it's possible to get to a situation where moisture will be present in those areas until the seasons change and the whole structure warms up, and it may well be that this is long enough to cause rot to start.

 

If you can increase the thermal resistance of the foundations under the wall support, and then add external insulation to decrease heat loss through the outer face, then you stand a reasonable chance of mitigating the risk.  I wouldn't wholly trust the well-established interstitial risk models for this, for the reasons given above, but if you can get the risk into the very low category using such a scheme then that may well be OK.

 

I have a horrible feeling that we are going to see a repeat of the disasters of the 1970's, when a certain well-known mass builder introduced timber frame construction to England without enough knowledge to understand that what works very well in parts of the USA and Canada was a disaster when used in South West England (I have personal knowledge - a work colleague bought a new house in Helston around  that time, where the timber frame rotted out within a few years............).

[bissoejosh]

8 minutes ago, JSHarris said:

The dynamic problem that I've tried to explain here before is the major issue for the UK climate - we often get days when the overnight temperature may be well below freezing, where the relative humidity near ground level in the early morning, as the sun comes up, hits 100% (hence early morning mists) and where there is a high probability of water vapour moving inwards towards colder internal structures that may well be below dew point.  The problem then is that the heat energy required to cause condensed water in these areas to change phase to vapour, so it can migrate back out again, is pretty high, and there may well not be enough energy available during the day to cause this to happen.  After a few days it's possible to get to a situation where moisture will be present in those areas until the seasons change and the whole structure warms up, and it may well be that this is long enough to cause rot to start.

This is by far the easiest to understand of all the explanations I've read and certainly represents conditions experienced here in Cornwall.

 

11 minutes ago, JSHarris said:

Adding insulation around the outside as a skirt is a reasonably effective way of mitigating the risk, provided you can also ensure that there is enough thermal resistance vertically under the supporting structure.

 

 

I guess the question I need to answer is whether there is enough thermal resistance with the detail I've drawn or if a cavity wall concept with both the core and external face insulated provides a more robust solution.

 

What method or software would you use to model this?

[Jeremy Harris]

The BRE method is WUFI, and is generally well-regarded as a reasonable 2D steady state method, taking account of the limitations above when trying to model something that's both 3D and subject to relatively rapid change, due to a lower heat capacity than the model was really designed to work with.

 

Bear in mind that a lot of the older models available on the web pre-date the use of internal vapour control layers (VCLs) and so assume that water vapour always moves from inside to outside.  That's not the case now, as with a VCL all the movement is in and out from the outside.

[bissoejosh]

32 minutes ago, JSHarris said:

The BRE method is WUFI, and is generally well-regarded as a reasonable 2D steady state method, taking account of the limitations above when trying to model something that's both 3D and subject to relatively rapid change, due to a lower heat capacity than the model was really designed to work with.

 

Bear in mind that a lot of the older models available on the web pre-date the use of internal vapour control layers (VCLs) and so assume that water vapour always moves from inside to outside.  That's not the case now, as with a VCL all the movement is in and out from the outside.

thanks, much appreciated. Will see what I can come up with...

[oranjeboom]

Have you considered one of the insulation block approaches beneath the sole plate (Foamglas, Marmox...)? This is my approach:

[TerryE]

I've got an outer stone skin sitting on Foamglas. The structural engineer was fine with this approach.  Download the date sheets and have a look.

[bissoejosh]

On 11/7/2017 at 23:19, oranjeboom said:

Have you considered one of the insulation block approaches beneath the sole plate (Foamglas, Marmox...)? This is my approach:

I have and may still use a something like marmox if I can't reduce the risk enough without. One of the issues is our sole plate is 245mm so much wider than the standard sizes I think.

[oranjeboom]

22 hours ago, bissoejosh said:

I have and may still use a something like marmox if I can't reduce the risk enough without. One of the issues is our sole plate is 245mm so much wider than the standard sizes I think.

Ahhhh sizes!!!! Don't get me started on that one. Okay, well you have now.... In Europe they have other sizes for Marmox. I was after a bigger size that the std ones you get here in UK. But I was speaking to the local Jewson's manager who assured me he could obtain the size I needed ("Yes, I can get hold of them, no problems, stop fussing you belligerent home-builder...now leave me to deal with some other orders" was what he was coming across like). So after 3 confirmations from Jewson, the truck arrived...with the standard UK size!!!  That really set me back a bit with a feeble excuse from Jewsons. They lost a lot of business from me after that. In the end, I ended up collecting the Marmox blocks from Belgium on the back of a trailer. Weight is no issue of course with these blocks!   So have a look at the Belgian/European sites for sizes. Many of the yards would probably ship them over to UK (I live in Kent so Belgium is next door).  

 

EDIT: I needed 190 breadth. I see they do 240 and 290 for you: http://marmox.com/products/61.pdf page 67.

Edited November 10, 2017 by oranjeboom
more details!

[jpinthehouse]

Hi

I'm thinking about changing to marmox, to sit the  140mm frame on with block outer skin traditionally built, is it that straight forward, marmox then block with frame on top 

Thanks 

John