Detailing - external skin below DPC

[Hilldes]

Looking for some input on the specific products to use and the detailing for the concrete block wall below my timber frame. There have been number of posts in this area, but mine is looking at some of the details - e.g. what specific types of membrane to use and and where.  Here is my draft detailing drawings:

 

 

A few questions please on the specific products:

 

The 50 mm EPS is OK?

EPS is glued to blockwork with Titebond? Or screwed with SS screws and plastic insulation washers?

The geo textile membrane could be this: Wrekin Multitrack. Should it just cover the outer face of the EPS? Wrap the EPS entirely? What is the purpose of the geo membrane anyway - as EPS does not absorb water???

The covering (e.g. ubiflex) is glued to the geo membrane with Titebond???

Pebbles - between paving and external skin - are these only required by those with passive slabs that need land/french drains?

 

 

 

 

[Hilldes]

Should the external EPS bridge the DPC?

[Iceverge]

I have attached a photo of a more typical detail, with a wider internal upstand. Why only 140mm timber frame? Wider is better.

 

In any case I would have the render board overhang the eps externally or omit the eps entirely. Your breather membrane should always direct any moisture drips down and out. It will direct them behind the EPS here. 

 

EPS will absorb moisture if the ground becomes saturated. A french drain topped by pebbles is your best plan in any case. I put one in our new cavity wall house. Remember just because you can’t see water on the surface it may still saturate the ground just below. Don’t give it a chance to get into your foundations or floor if the DPC ever fails. It must have an easy option to drain away from the house and footings.

 

A geotextile membrane is permeable to allow water to pass through but retain the fines in soil. This prevents mud and silt from clogging any drainage systems or mixing of permeable aggregate and soil making them unstable. What is your intention for it here? 

[tonyshouse]

I like eps below dpc - breathable 

 

thermal bridge at soleplate can end up with condensation forming on top of dpc?

 

I like the chamfer but it is likely not necessary or fit offcut back in with membrane on the chamfer 

[Russell griffiths]

I don’t like that the timberframe membrane exits behind the xps, I believe a tray type should be used with any moisture build up behind your cladding being shed to the front face not behind it. 

 

You also need more more detail on that drawing to show floor buildup, as it’s not clear as to if the 50mm xps will be enough to protect against cold bridges. 

[Hilldes]

2 hours ago, Iceverge said:

Why only 140mm timber frame? Wider is better.

 

The frame and rendering is complete...

 

2 hours ago, Iceverge said:

Your breather membrane should always direct any moisture drips down and out. It will direct them behind the EPS here. 

I first drew the timber frame breather membrane overlapping the EPS, but then I thought if the EPS gets saturated (although not sure it physically can), that moisture will sit against the sole plate.

 

2 hours ago, Iceverge said:

EPS will absorb moisture if the ground becomes saturated. A french drain topped by pebbles is your best plan in any case. I put one in our new cavity wall house. Remember just because you can’t see water on the surface it may still saturate the ground just below. Don’t give it a chance to get into your foundations or floor if the DPC ever fails. It must have an easy option to drain away from the house and footings.

 

I was thinking that with a conventional two skins of masonry foundation wall (cavity filled with concrete), then the whole section of wall will be wet below DPC and DPM. Why is my blockwork wall any different? Why would I need a french drain? P.S. We do have clay in the soil so it doe not drain too well.

 

2 hours ago, Iceverge said:

A geotextile membrane is permeable to allow water to pass through but retain the fines in soil. This prevents mud and silt from clogging any drainage systems or mixing of permeable aggregate and soil making them unstable. What is your intention for it here? 

 

That was my understanding too of the purpose of a geo membrane - e.g. to wrap soakaway crates. I only drew it on my foundation detailing because I had seen it specified there before. Not sure it is fulfilling any purpose here.

 

[Hilldes]

1 hour ago, tonyshouse said:

thermal bridge at soleplate can end up with condensation forming on top of dpc?

 

Are you happy as drawn or would propose something different? EPS going higher up the frame above DPC?

 

1 hour ago, tonyshouse said:

I like the chamfer but it is likely not necessary or fit offcut back in with membrane on the chamfer 

 

I drew the chamfer because I thought if and water drips on it it will run off rather than settle. Also I need to maintain an air gap below the render board for cavity ventilation. Would need to use thinner say 25mm EPS if I cut it square and still overlapped the sole plate.

[Hilldes]

23 minutes ago, Russell griffiths said:

I don’t like that the timberframe membrane exits behind the xps, I believe a tray type should be used with any moisture build up behind your cladding being shed to the front face not behind it. 

 

Thanks, will lap the breather membrane over the EPS.

 

23 minutes ago, Russell griffiths said:

You also need more more detail on that drawing to show floor buildup, as it’s not clear as to if the 50mm xps will be enough to protect against cold bridges. 

 

More detail shown here - the floor is an insulated beam and block suspended floor...

[Hilldes]

more information on floor build up here...

 

One reason for applying insulation externally is this from the insulated floor system BBA cert:

 

"8.3 To minimise the risk of interstitial condensation at junctions with external walls, specifiers should ensure that wall insulation extends to at least 150 mm below the bottom of the EPS infill panel." - which means extending >= 150mm below the bottom of the pre-stressed concrete beams.

Edited October 19, 2021 by Hilldes

[Iceverge]

40 minutes ago, Hilldes said:

 

The frame and rendering is complete...

 This changes things a bit. What is the width of the ventilated cavity and the render board?

 

40 minutes ago, Hilldes said:

I was thinking that with a conventional two skins of masonry foundation wall (cavity filled with concrete), then the whole section of wall will be wet below DPC and DPM. Why is my blockwork wall any different? Why would I need a french drain? P.S. We do have clay in the soil so it doe not drain too well.

 

Yes, and you rely entirely on a good DPM/DPC to prevent your floor + insulation from becoming wet. As you've witnessed no doubt, building is a very tough environment and guaranteeing no holes in a membrane is all but impossible. A french drain (assuming it can always drain to a lower plain) guarantees that the saturation level of the soil under you house is never higher than the bottom of the french drain. It means that the DPM in effect doesn't need to be prefect. In older buildings with a working French drain and no DPM the floors stayed dry. 

 

In your case with a suspended floor (block and beam) it is less of a issue than a ground bearing slab but will still help to keep your rising blockwork + insulation keep dry. 

 

Below is our as built foundation detail. (There's a couple of things I'd change) 

 

 

Below I have attached a couple of picture taken just now of our arrangement. It's been raining a lot today and last night but you can see (maybe) where I've scraped away the stones in the second picture the wall is dry. It's difficult to make out as some silt in the gravel has discoloured the wall.   I am inferring from this that the wall inside must be dry also although it is below DPC. 

 

Note there's no splashing of the wall either.

 

The final picture is of the porch area (porch yet to be built). There is nasty splash marks on the wall and the blocks adjoining the footpath are wet. This is probably the case for most walls with adjoining footpaths. 

 

 

I've been told that it looks nice too but that was never my aim. (forgive the messy painting, I'll see that I give myself a flogging!) 

 

It does give the footpaths a nice extra bit of elbow room with no extra concrete although they were trickier to lay. 

 

 

 

[Iceverge]

Here I what I would do next time. 

 

[Hilldes]

Many thanks for the drawing and pics @Iceverge - I can see the benefits of good drainage around the foundations.  I'll do some reading up on french drain construction.

 

P.S. how wide is your gravel section - looks about 30cm?

Edited October 19, 2021 by Hilldes
added a P.S.

[Iceverge]

400mm. Can't remember why I choose this. I think it was the width of my rake. 

[Hilldes]

Has anyone experience of these products?...

 

XPS INSULATION Finnfoam

 

AQUABASE – EWI-226 as an adhesive for the above

 

@Mr Punter, on another thread, you mentioned using natural slate tiles, with adhesive and mesh on the insulation, then the tiles.  What adhesive and mesh products did you use please? Was it EPS/XPS insulation or PIR?

 

 

 

Edited October 20, 2021 by Hilldes
typo

[Russell griffiths]

Why not render it, I’m using the ewi226 as a render below my dpc. 

[Hilldes]

2 minutes ago, Russell griffiths said:

Why not render it, I’m using the ewi226 as a render below my dpc. 

 Thanks @Russell griffiths - I might render it. You are rendering with EWI226 onto EPS? And you are using one coat of EWI226 as a render base coat, then fibreglass mesh, then EWI Mosaic Render for the top coat?

[Mr Punter]

2 hours ago, Hilldes said:

@Mr Punter, on another thread, you mentioned using natural slate tiles, with adhesive and mesh on the insulation, then the tiles.  What adhesive and mesh products did you use please? Was it EPS/XPS insulation or PIR?

 

 

I think we just used Weber LAC and the standard pink fabric mesh and then tile adhesive.  It was on XPS.

 

Edited October 20, 2021 by Mr Punter
added pic

[Hilldes]

Thanks @Mr Punter! So apply render base coat and mesh to the XPS and let it dry.  Then apply slates with external tile adhesive?

 

[Stupid question alert] what is the purpose of render base coat and mesh - why not stick slate tiles direct to XPS with tile adhesive? Assuming the XPS is well fixed and the joints are flush? 

[Russell griffiths]

1 hour ago, Hilldes said:

 Thanks @Russell griffiths - I might render it. You are rendering with EWI226 onto EPS? And you are using one coat of EWI226 as a render base coat, then fibreglass mesh, then EWI Mosaic Render for the top coat?

Yes, exactly that. 

[Mr Punter]

The render base coat and mesh is really sticky and forms a really good solid substrate to tile onto.  We tried tiling onto the XPS and it just did not work.

[Hilldes]

Thanks @Russell griffiths - I'll look at the cost of these products.

 

2 minutes ago, Mr Punter said:

The render base coat and mesh is really sticky and forms a really good solid substrate to tile onto.  We tried tiling onto the XPS and it just did not work.

 

Thanks @Mr Punter - I knew there would be a good reason for it ?

[Hilldes]

Any ideas on to to handle dropped thresholds?

 

The drawing shows the foundation blackwork structural inner skin (orange), with the proposed external EPS/XPS (dark grey), the window set 39mm below DPC and a cast in situ concrete cill (white) below the window - the cill cantilevers about 20mm over the face of the blockwork.

 

 

Don't want to extend patio slabs through the exterior EPS/XPS insulation to meet the cast in situ sill. But then how to cap the top exposed edge of the external insulation?

 

 

[Hilldes]

Interesting point of view here on use of EPS vs XPS below ground. I would have thought XPS would perform better, but maybe not?

[Iceverge]

I did an experiment with PIR and Graphite EPS 70 by weighing them and submerging them both is water for a month. Towel drying them both and weighting them. I can't find the details now but the EPS had absorbed some water and the PIR almost nothing.  I think this comes from the EPS being more porous . However this is an extreme case as if your insulation should never be saturated for long periods of time. I would always look at EPS first as it's the cheapest and "dumbest" material. However in this case I would consider PIR as you have very little thickness to play with to prevent quite a bad cold bridge. 

 

More important is how you'll deal with rain and possible pooling of water by the window. What level do you hope to put the paving slabs? 

[Hilldes]

11 minutes ago, Iceverge said:

More important is how you'll deal with rain and possible pooling of water by the window. What level do you hope to put the paving slabs? 

 

Certainly not looking for a level threshold - most of the dropped thresholds are bifold. I have not planned the patio area in detail, current thinking is the paving surface would be around 100mm below the base of the window frame. Not so much that will require a step according to the regs.