Correct height difference between DPC overhang and drains

Lumen

1980s house, am currently relandscaping the back.

In the attached pics, you can see that the drop from the DPC "overhang" (what's the proper term?) and the drains varies between about 200-240cm. The total drop down to to the bottom of the plastering is about 400mm.

It's clear that the gravel has been moved, sunk or migrated such that the drains are exposed. That'll get sorted with a new patio.

Question is, given that I'll be digging all this up anyway, is there a particular finished ground level to aim for with the respect to that DPC overhang? The set of steps shown on the edge of one pic are probably going to be changed anyway so I'm not concerned about step rises.

I'd like to get it right and if that means changing the level of the pipes I can get that done, but obvs dropping them would have consequences for the fall of the drains so that's probably a terrible idea.

Advice please!

…will add photos from phone shortly…

Lumen

photos

The Continental Op

Having been a landscaper my guess is that has never had any topsoil on it, job was never finished. Good thing is it looks like you have plenty of room under the DPC. Not sure of Ireland but UK was always min of 150mm between dpc and surface level.

Penn

Typically you need a minimum of 150mm from DPC to external ground level. You can go more than that, but you shouldn't go less than 150mm, particularly for an existing house.

Lumen

https://www.boards.ie/discussion/comment/122785688#Comment_122785688

Having been a landscaper my guess is that has never had any topsoil on it, job was never finished.

It was "finished" in gravel, maintenance of which has been the bane of my life since I bought the house!

The Continental Op

https://www.boards.ie/discussion/comment/122785958#Comment_122785958

I bet there was hardly enough gravel to do the job properly.

Penn

https://www.boards.ie/discussion/comment/122785958#Comment_122785958

Given the height of the gulleys, I'd say it was proposed to be finished in concrete on the gravel but it was never finished.

Lumen

Ah, the fog is clearing.

Turns out I don't really have two steps. The lower one is the same level as the grate.

Lumen

Next question: if I'm fitting external insulation and a patio, and bearing in mind the inevitability of screwups, what should the correct order be:

1. Install patio first, and then screw it up with bond/render/paint/whatever splattering around during the insulation install.
2. Install insulation first, and then screw it up with cement splattering during the patio install.
3. Some incredible plan I haven't thought of.

I'm inclined to do insulation first, because surely patio installers are used to working around a finished house. Right…?

Penn

https://www.boards.ie/discussion/comment/122786366#Comment_122786366

Your external insulation probably needs to go even deeper than the current ground level. I'd be inclined to have it done first before the patio, there'd be very little issue with cement/concrete splattering on the plaster. You would have to confirm with the external insulation installer/supplier what depth the insulation needs to go to.

sydthebeat

https://www.boards.ie/discussion/comment/122786366#Comment_122786366

EWI has to finish a minimum of 300mm below the finished floor level.

in your case it looks easy enough to clear around the walls to reach the top of the foundation, which is where i would start the EWI from.

The Continental Op

Patio first then cover it with sheet corriboard

https://williaamcox.com/products/plastics/corriboard/

The Continental Op

https://www.boards.ie/discussion/comment/122786409#Comment_122786409

Makes sense I was wrong.

Lumen

https://www.boards.ie/discussion/comment/122786409#Comment_122786409

The top of the gully is about 200mm below FFL, so I could get to 300mm without any digging, but since the gully pipe is only about 40mm from the wall I guess I'll need to dig a bit to move the gullys away from the house.

standardg60

https://www.boards.ie/discussion/comment/122786409#Comment_122786409

Can you explain or link to that because I've yet to see EWI finished to that depth?

What is the point of insulating below dpc/ external ground level?

Penn

https://www.boards.ie/discussion/comment/122791774#Comment_122791774

Acceptable Construction Details, link below. If anything, given it's an 80's house and so would have no thermal blocks, it might even need to be 430mm below floor level, though those diagrams show solid block walls rather than cavity wall. The depth is required to minimise thermal bridging through the substructure.

https://assets.gov.ie/201048/8a35795a-0876-4877-b5d6-2166238ce84b.pdf

standardg60

https://www.boards.ie/discussion/comment/122791896#Comment_122791896

Cheers, the problem there is they are construction details, not retrofit. There is a continuous dpc along block and insulation at dpc level. Given that's not possible with a retrofit extending it below dpc level would be more of a thermal bridge?

standardg60

https://www.boards.ie/discussion/2058378769/correct-height-difference-between-dpc-overhang-and-drains

Anyway to answer you, don't dig up any of the existing hardcore, which has naturally settled over the years and isn't going to move any further. All you have to do is lay the patio to the level of the drains, which are correctly levelled at least 150mm below dpc.

Penn

https://www.boards.ie/discussion/comment/122791993#Comment_122791993

Why would fitting EWI below the existing DPC be more of a thermal bridge? The DPC has nothing to do with thermal bridging. There are other ways to fit the EWI which incorporates DPC to prevent moisture ingress, while still extending the EWI low enough to reduce thermal bridging through the structure.

All this depends on the system/EWI being used, and manufacturer's/installer's instructions.

standardg60

https://www.boards.ie/discussion/comment/122792730#Comment_122792730

To be frank if you believe a dpc plays no role in thermal bridging you shouldn't be posting on the subject.

Your own link provides for a dpc between EWI at that level. The first insulating aspect in any build is moisture prevention, because wet equals cold.

Lumen

https://www.boards.ie/discussion/comment/122792854#Comment_122792854

I think the details under discussion are:

Wall insulation installed below the wall DPC must be fit for purpose with regards to water absorption

All of the EWI systems I've looked at specify specific materials for the "facade" and "plinth" components, shown below in a spreadsheet I did in 2022 covering all the certified EWI systems I could find, presumably because of how they deal with moisture below ground level.

What I don't understand is whether/how in a retrofit scenario I'm expected to get that DPC continuous. I'd guess it's more practical to bond a "downstand" (?!!?) to the face of the plinth, and wrap it over the top, although I somehow doubt any of the EWI installers are doing this.

w.r.t. moisture more generally, I have big damp issues to the front of the house (where the ground rises to half-way up a basement wall) so I was expecting to have to dig down to the foundations there and backfill with drainage stone to create a deep French drain, or tank the front below ground level before applying the EWI.

Penn

https://www.boards.ie/discussion/comment/122792854#Comment_122792854

Moisture ingress through the structure (which DPC helps prevent) is not a thermal bridging issue. Thermal bridging relates to areas in the building envelope where heat loss can occur due to a lack of or gaps in sufficient insulating materials, which you would have by not extending the external insulation low enough (below DPC level). While moisture ingress can cause cold spots at those points, it's not specifically a thermal bridging issue, it's a different issue in which one of the consequences is the effect on thermal performance.

That's why I say manufacturer's instructions based on the EWI system being proposed would be required to incorporate their detailing for any DPC or other measures to prevent a moisture path through the EWI and over the DPC. But it's likely you would simply cut out pointing at the existing DPC and lay new DPC on top which also extends out to the width of the EWI.

Calahonda52

https://www.boards.ie/discussion/comment/122792854#Comment_122792854

So what is your definition of a thermal bridge and how it works for a dpc?

I am genuinely curious as I have just qualified as certified PH designer, and as you will not doubt be acutely aware, TBs are a key design issue in PHs.

Your second paragraph points to something else

Calahonda52

Lumen, as others have said, 150 is the "going" rate so that step will need to be dropped a bit or ….

The ewi below the DPC is of a different grade due to the wet.

You sometimes see an angle fixed to the wall at DPC level and the upper EWI starts there and the lower ewi, which is generally thinner, butts up to it. gives a clean line and creates a drip for the water from the upper ewi

muffler

https://www.boards.ie/discussion/comment/122792854#Comment_122792854

First of all as you aren't a mod of this forum you shouldn't be dictating as to who can post here. Secondly I think you seriously need to learn a bit more about "thermal bridging" if you think a dpc is an integral component of same.

Mellor

https://www.boards.ie/discussion/comment/122791993#Comment_122791993

Extending insulation does not create more of a thermal bridge. It's simple not possible for insulation to be a thermal bridge.

https://www.boards.ie/discussion/comment/122792854#Comment_122792854

The DPC is to prevent moisture. It doesn't make any difference to whether there is a thermal bridge or not.

standardg60

https://www.boards.ie/discussion/comment/122793928#Comment_122793928

Apologies that was poor on my part.

standardg60

I suppose my question is does insulating below the existing DPC without a DPC in the insulation itself lead to more of a thermal bridge than finishing the insulation at the existing DPC?

Surely a DPC in it's role as a moisture blocker also prevents thermal bridging?

Penn

https://www.boards.ie/discussion/comment/122799978#Comment_122799978

I think some form of DPC would still be required, though if the existing wall is a cavity wall, it's likely any moisture which rises up through the external insulation and over the existing DPC would come back down through the cavity, so it's unlikely to ever travel to the interior and cause an issue. But even then, the role of the DPC is nothing to do with thermal bridging, and the bigger issue would be having no external insulation at plinth level (under the DPC), because that gives too much of a path for actual thermal bridging (which in turn can cause lower surface temperature at the base of the wall, which leads to condensation forming, which leads to mould and degradation of finishes).

It's not an either/or situation, there are still ways damp-proofing can be incorporated into the EWI to minimise or fully negate the risk of moisture travelling through the structure.

The term Thermal Bridging is just a very particular thing. What you're talking about with damp getting through the structure due to gaps in damp-proofing, while part of the issue it causes is thermal related (though I would say it's a minor part of a much bigger issue), it just doesn't constitute thermal bridging, which is a term related to the excess passing of heat through part of the structure due to inadequate insulating materials (not the actual definition, just the phrasing that came into my head as I typed).

Calahonda52

https://www.boards.ie/discussion/comment/122799978#Comment_122799978

NO, in the same way as xeruD do not create a thermal bridge.

Please google thermal images and select images and feast your eyes on the topic.

standardg60

https://www.boards.ie/discussion/comment/122800137#Comment_122800137

Thanks, so thermal bridging is solely concerned with outgress of heat from a building rather than ingress from outside?

In relation to cavity walls, and I've seen it posted in other threads, is that it's ok for it to be wet, as the cavity deals with moisture ingress. That's all fine, but in terms of thermal transfer the dryer it is the better.

Have there been any studies done in relation to heat transfer from extending below DPC versus not without extending DPC?