Generally - Wall/Slab/Roof build-up advice for very large new build.

[ElliotP]

2 hours ago, MR10 said:

 

I'm assuming 'worked with before' was on a much smaller scale and detailing than required for this proposed build.

Thread carefully.

2 hours ago, saveasteading said:

 

Beware. Do you mind losing a friend?  Some things will go wrong. As long as you pay extra every time it is requested ( and happily so) then it could be OK.

 

Indeed, it was a 2 story extension, sizeable but not on the scale of this.  However he's experienced, smart and enthusiastic.  We dealt with plenty of curve balls together hidden sewers, build over agreement, deep footings etc.  We got through it. We're not that close anyway 😁

 

2 hours ago, gavztheouch said:

Good luck! Im builidng a 210m2 shell for hopefully a similar budget of 300k. It should be doable depeding on how much work you do yourself.

 

Edit, I just reread your post and noticed you have a swimming pool triple garage and annex are these all inlcuded in your 300k budget. Minus the swimming pool I would budget around £600k to build the weather tight shell.

 

That's interesting, and also slightly unnerving. When I say swimming pool - in the '£300k' (which admittedly i've pulled out of my ass), I'm nut budgeting the actual pool in that or even the hole in the ground - just the room to house it, which will be on the bottom of the 3 floors. So I'm hoping that given we're doing 3 stories will help.  The garage and annexe will be combined, with the bedrooms above the garage, again looking at savings there.  No idea on my m2 yet until I get some plans, but looking at yours, that's about £1400 / sqm just for the shell?  Is this 2 stories?  Ex VAT also assuming it's a new build? 

We'll need to get the plans and discuss with the builder before I have any idea where I am, there are certainly going to be tradeoffs.  

 

41 minutes ago, joe90 said:

Parse coat, cement render (even between floors) and plaster finish. Around windows and reveals was air proof foam and foaming glue (not a big fan of taped membranes).

The reveal deal is also what I was concerned about, especially on a wide cavity. I had a quick research.  Suggestion looks to be boarded with ply, and foamed for airtightness, and some more detailing I'm yet to understand.

 

2 hours ago, joe90 said:

I built brick and block with 200mm cavity, wall ties were not that expensive, I also used concrete lintels internally and brick arches externally so no cold bridging. You will read many threads here where attention to detail makes a big difference especially with airtightness (don’t let trades bore random holes in your membrane whatever it is).

What kind of ties did you use? Are basalt a requirement due to the bridging or is it not worth it? 

 

So overall judging by the responses here, there hasn't been a suggestion of going timber frame which I expected to be honest. So this has kind of validated my traditional wall build up a little more, even at a wider cavity.  I just need to nail down the slab, roof and junctions. 

[Adsibob]

11 hours ago, jack said:

But in general, GSHP setups are at least double to price of an ASHP of the same output. If properly done (and that isn't always the case), the GSHP will have slightly higher efficiency, but not enough to ever pay back the price difference.

GSHP does have a significant aesthetic and comfort advantage though: much of the ugly stuff is underground and there is less noise. Might not be an issue on a large plot where you can hide it away in the corder somewhere, but it’s a problem in densely populated areas with small plots.

[joe90]

57 minutes ago, ElliotP said:

What kind of ties did you use? Are basalt a requirement due to the bridging or is it not worth it? 

I decided the cold bridge from stainless ties was minimal (not everyone agrees) so I used them as basalt are so expensive  for (what I decided) was little return. 
 

 

59 minutes ago, ElliotP said:

need to nail down the slab,

I used strip foundations and copied this (nearly, we moved the DPC one block lower on the internal wall so as not to create a slip plane above the skirting board then added a second DPC in the outer wall above ground level, the whole house was surrounded in a French drain as the water table was high.

 

[Iceverge]

3 hours ago, joe90 said:

decided the cold bridge from stainless ties was minimal (not everyone agrees) so I used them as basalt are so expensive  for (what I decided) was little return. 

I calculated this. From memory all you need to do is widen the cavity by about 5mm to make up the difference. SS every time.

 

Here's my alternate version of the above detail. 

 

1. Full fill of EPS beads, right to the foundation. Allowance for mortar droppings so they won't build up to DPM level. 

2. Less thermal blocks, they don't really make a difference to the heat loss below the insulation in the floor. 

3. 2 x Layers of EPS 150mm insulation with the DPM in between for protection. 

4. Separate DPC on outer leaf. 

5. 50mm insulation at perimeter really helps the heat loss. 

6. Much easier to level this concrete floor than the other design. 

7. Allows for the use of cavity ties below the DPC increasing stability.

8. Simpler threshold detail for DPM. 

[ElliotP]

Thank you both @joe90 / @Iceverge, that's really helpful and exactly what I needed - some real world validation on the diagrams I've been staring at. 

[jack]

13 hours ago, Adsibob said:

GSHP does have a significant aesthetic and comfort advantage though: much of the ugly stuff is underground and there is less noise. Might not be an issue on a large plot where you can hide it away in the corder somewhere, but it’s a problem in densely populated areas with small plots.

 

Aesthetic, agreed. If you don't have an out-of-sight place for one, they are generally pretty ugly. We're lucky that we were able to site ours on the far side of our attached garage, where it's out of sight unless you walk along that route (and there's generally little reason to do so).

 

I'm less sure about comfort. There are lots of reports about how noisy GSHP pumps are, to the point where it's sometimes recommended they be sited in their own sound-insulated housing outside the main building envelope.

 

I can't hear my ASHP when it's running unless it's a very still day and I stand pretty close to the unit.

 

I'm not saying there are no use cases for GSHP, just that the economics rarely make sense if you can choose between one or the other (as appears to be the case with OP, since he mentioned both).