External door threshold detail

[maxdavie]

Hello, 

 

I was wondering whether anyone could offer any thoughts or advice about the following threshold detail. I'm actually an architect myself and am working solo, so I don't get the benefit of second opinions. I've always worked on larger schemes at the earlier 'conceptual' stage of projects so my practical knowledge of construction details is a little rusty now. 

 

This spring, I'm piloting a digitally-enabled kit-of-parts construction system assembled from cnc-cut plywood or OSB that I've been in the process of developing over the last year. Its just a small garden room, so I'm not too concerned about U-values, but I am trying to achieve airtightness to test out how all the details work before I self build our family home next year. I'll then have it tested to see how airtight it is. 

 

So, on to the detail - see image attached.

 

Here's the floor build up from top to bottom: 50mm brick pavers, 10mm mortar bed, 18mm T&G Wisa Ply floated floor, 60mm Rigid insulation, RC slab. I'm thinking I can just secure the concrete threshold piece into a bed of mortar over the floated ply substrate - just like the brick. The DPM coming up the slab edge can fold over and tuck under the threshold piece protecting the exposed edges of ply and insulation. An additional waterproof board will go over the DMP too. 

 

The only reason I'm questioning it is because I haven't found many examples of how it is done online and so am relying purely on my knowledge/research and common sense. 

 

Anyway, any thoughts greatly appreciated. 

 

Thanks

 

 

 

 

[Iceverge]

Hi @maxdavie and welcome.

 

A Couple of issues I can see. 

 

If you're expecting good airtightness results you really need compression seals on the door. This looks like an old fashioned brush seal. 

 

The concrete threshold will constitute a substantial thermal bridge here. 

 

I would ensure any thresholds overhung the door drain to drip clear. There's a chance water will run back under the threshold as is. 

 

Given it's a high traffic area I would question the integrity of the insulation under the ply. Over time it might compress. 

 

What is your planned airtight seal between the door and the floor? If it's the mortar I would revise your thinking a little. Any seals between different materials really need to be permanently flexible for durability. Special tapes, mastics, compression tapes, paints and membranes are most often used. 

 

The ply would most likely suffer decay over time in the current location under the threshold. Timber doesn't like being trapped between vapour closed materials , especially outside the heated envelope. 

 

I hope this helps. I'd love to see your system. It sounds very interesting. I think there was something similar on Grand Designs a couple of years ago. 

 

 

 

[maxdavie]

Hi Iceverge, 

 

Thanks so much for the advice and your keen eye - this is a massive help! 

 

Yes, in essence, my system is very similar to that one shown on Grand Designs.  I've attached a screenshot preview of the system as it will look behind the membranes and rainscreen in case you're curious. 

 

Okay, I've had a re-think of this detail based on your advice.  

 

I'll use some rigid insulation with greater compressive strength like Compacfoam which I can bond to the slab and raise the threshold cill piece and shuffle it over to drip into the slot drain channel. 

 

The ply and insulation edges can then slide back behind the Compacfoam into the warm interior. I'm not sure how to totally eliminate the cold bridge of the threshold cill piece. I haven't seen any thermally broken threshold cill pieces on the market. Maybe I can just find a thinner one. 

 

Re airtightness between door and floor - I'm planning on using the Proctor Wraptite Airtightness Barrier over the walls and behind the rainscreen in conjunction with the DPM (which is also airtight, though not vapour permeable) to complete the airtight seal under the threshold cill piece.

 

See images - I'll first fold the DPM (in dark blue) over the top of the Compacfoam (light blue) and fix down. Then, I'll bring the airtight membrane down to lap over the DPM by say 100mm and fold it back over the DPM at the threshold with tapes. Then, sit the threshold cill piece over. See final detail. Any further comments of course welcome and greatly appreciated. 

 

Thanks again for the advice - I imagine I might be returning to seek the sage advice from fellow builders/architects/self builders in the future! 

 

 

 

 

[Iceverge]

Sorry for the delayed reply. I'll have a closer look at your drawings tomorrow. 

 

 

 

Here is a cross section of a passivhaus door, note the sweep and the compression seals, The red part is a thermally broken threshold I think. The door is almost entirely insulation. 

 

For a quality install this should be sat on some suitable insulant, like compacfoam which in turn is continuous with the floor insulation, The external should drip clear, like in your example into a door drain.

 

For airtightness I would use something like compriband or FS500. 

 

https://www.illbruck.com/en_GB/solutions/window-installation/brick-block/

 

 

As to DPC's. The reality of site work is that they are rarely uncompromised. The need to be installed out of the way of foot traffic which is almost never done. A robust solution is to use DPC paint on the threshold but it is messy and time consuming. 

 

 

 

[maxdavie]

Thanks for the additional advice and comments Iceverge