screed in rear extension - cavity bridging, insulation, DPM

[johnhenstock83]

afternoon,

 

I had several questions on the next step of my extension project, I'm hoping you can help me out with some advice. we have a rear extension and at some point next year we'd like to resume the works. screed is one of the things that need doing and I'm confused about several things:

 

1. the original house has the DPC 2 courses above ground level, but the internal screed is 3 courses above ground (as are the doors). basically I have the DPC, another course of bricks and that's where the floor level is. the extension outside has been built with the same specs in mind, so the top of my new screed in the extension will be above DPC. the question I have is around the DPM. we have left around 1ft of skirting in the extension, tucked nicely under the DPC. if I create a vapour barrier and attach it to that all round using a DPM, will that be enough? won't I have any problems with damp or humidity? is there any good practice regarding this or am I worrying for nothing?

 

2. we're looking to use at least 100mm thick celotex type insulation under the screed, potentially even thicker. suggestions around thickness are welcome, there is some flexibility. my question was more around upstands. I understand the concept and I'm happy to use them, but upstands are around 20mm thick. won't they compress when the screed is poured? also, what's best: celotex upstands (20mm thick) or the blue, squashy foam I see used in every screed job? I'm rather insulate the edges with an upstand, but if the blue foam is enough, it's cheaper. any thoughts on this?

 

3. at some point, we will knock down the wall between the house and the extension. as this is an external wall (cavity, 300mm, brick/brick or brick/block), I expect a cavity will be revealed when the wall is removed. one or two courses of bricks will have to go, to allow the new screed to connect to the existing one. what can I expect to find in the cavity and what's the best way to bridge the gap? can I fill the cavity with concrete and not worry about damp? what I fail to understand is how to create a damp proof barrier underneath the screed, between the two screeds.

 

4. we are going for a 3.6m wide bifold. the opening is nice and square, but there is a cavity directly underneath it. brick on the outside (2 to DPC, 1 to bifold, to match existing house) and breeze block inside (1 course). I don't understand how to bridge the cavity under the bifold without filling the cavity or compromising on structural strength. someone suggested removing the blocks on the inside along the width of the bifold and screeding all the way to the brick course. is that do-able? if so, can I just run the DPM 1 course above DPC? it's too late to tuck it nicely under DPC.

 

worth mentioning that the oversite is block and beam with about 4 inches gap underneath.

 

any help is welcome and I'm happy to provide more details.

 

thanks!

[johnhenstock83]

no answers? would it help if I posted some diagrams? I could really use some advice, though I'll admit it's not urgent, we're not resuming works until the second week of january.

[Jilly]

Hi, people tend not to read through long posts, it puts them off as it involves a lot of cross referencing and cut and pasting.  You’d be better off breaking it down to the first most important conceptual question, to check your understanding and build it from there and you’ll get to know the posters and their interests and specialisms. 
 

For starters try to get more insulation under the screed. 

Edited December 20, 2022 by Jilly

[johnhenstock83]

thanks jilly, that makes sense. I'll create separate posts for each question, not all at once.

[Samuelmusic101]

Hi @johnhenstock83 I am currently building an extension and have the same issues you have mentioned in points 1,3 & 4. I was just wondering how you went about it in the end…

 

any info would be greatly appreciated. Thanks.

[johnhenstock83]

1. we simply used a damp proof membrane and connected it to the skirting, which tucks nicely under the DPC (on the internal skin). essentially, you have a barrier between ground level and floor, absolutely no issue with humidity at all, with the extension sitting nicely at 50%, except for in the summer, when the doors are open a lot and the humidity inside reflects what's outside.

 

2. internal wall between existing kitchen and extension was taken down to screed level, so we didn't remove any bricks from the "footings". we then used liquid DPM to close any tiny gaps, but that was completely unnecessary, as the underside of the extension is bone dry anyway. we then used self-levelling compound to combine everything nicely, again, all nice and dry despite a lot of rain and humidity.

 

3. we removed the internal skin along the width of the bifold and used the same damp proof membrane to create a "tray" underneath the screed and insulation. again, absolutely no problems with humidity, but I have to admit that area of the floor is a bit colder that the rest, despite using celotex. not a big deal and there's no condensation, which for me is good enough.

 

from the outside, you can see the ground level at 3 bricks under the internal floor, with a DPC 2 bricks above ground floor, so it's quite difficult for anything to get damp/wet, even in heavy rain.