MarkH

The Man came round today to check our sub-grade, waste pipes etc before we pour the concrete later this week. All was good with the plumbing (he didn't seem to pay it much attention for all the cogitation I did about bloody rocker pipes etc...) BUT he noticed the deep layer of aggregate in the smaller downhill section of our build and stated that as the aggregate was just greater than 600mm we needed to pour a slab that overlapped the walls by some amount (he thought 100mm but is confirming tomorrow) and is reinforced with heavier mesh than the a142 we're using in the main part of the house. This is going to mean removing the inside 100mm of the top course (our solid EWI walls are 215mm thick) or removing the whole course and relaying 100mm blocks cut down to slab thickness. I'm wondering about the cold-bridge we'll form by continuing the slab onto the walls like this. Will the EWI (250mm of EPS probably) make the lack of upstand irrelevant or should I look to do something about this new cold-bridge?

Thanks. I'm thinking - given I can't come up with a practical solution - I might live with it (the bridge). The ground beneath is well drained and there is over a meter of wall (the first 350mm of which are surrounded by the slab & insulation). I'm now contemplating 300mm of EWI so really the bulk of the sub-slab stuff is going to be well insulated for over a meter below slab level. I'm not that worried about small losses in overall performance, more problems from high temperature differentials from bridges - condensation and mould. It seems (I hope I'm right) that this cold bridge won't be significant to cause that.

I'm worried that an external/internal wall we have might be a cold-bridge problem. It separates the two sections of our house and whilst i can work out how to deal with the external insulation well enough I'm not so sure about the bottom of the wall and loss of heat down into the ground. In the image below you can see the EWI - which will extend down into the trenches about 750mm below DPC - and also the sub slab insulation. My concern is that the wall which devides the two sections of the house (whilst being insulated from the 'outside' by the ewi) is a direct path down through the floor insulation into the ground, a hole in the insulation envelope. The wall is a meter high relative to the foundation at that point so its not a short path, but still... Another query: our walls are currently built up to slab level and we're waiting for materials before continuing, when I lay the DPM, how do I deal with that internal wall? Do I lay it over the wall and then build up on it (except in the doorway, what there?)? Should I have terminated that wall a course lower, DPM'd and then built up the final course? As always - thanks for any and all input!

Ok thanks folks. We'll stick with the mesh - can't see a good reason not too - and probably attach the pipes to it.

What method of attachment did you use? 'Snappied'?

Our garage has a 150mm slab with reinforcement specced by planning but the house is just 100mm, no reinforcement (the architect mentioned having a couple of movement joints recently). Since getting prices for mesh and taking into account the convenience of mesh acting as a suitable spacing for the UFH pipes I can see your point - why not use it... Thanks! I'll order some steel on Monday.

Here's our floor construction. I'd appreciate any opinions on it and in particular on a couple of details currently in decision limbo: 1) I'd assumed the DPM would go as pictured in this diagram - under the insulation - as it seems to make sense to have it contained within the waterproof envelope. A couple of people have suggested that the membrane goes OVER the insulation though. 2) slab reinforcement & UFH... am I overkilling by adding reinforcement mesh mainly to give me something to attach the UFH pipes to? Would it be easier to clip the UFH to the insulation (I saw a handy clip gun thing) and leave out the steel? By the way, the EWI isn't shown on the diagram above but is (probably) 200mm of EPS extending down 200mm below the slab insulation.

They're easy to get the hang of, and fun. But you need to plan your work - and every move - well to minimise mudpocalypse.

I guess the dpm takes care of that.

Was yours reinforced Ian?

Our slab is devided into a 4.8m by 10.5m section and a smaller 3.8 by 6.3 part. The architect long ago suggested two transverse movement joints in the large section but my dad - the ex-cowboy builder - reckons there's no need. The slab is 100mm, steel reinforced. My research so far suggests the easiest route would by to use a 'groover' to cut a partial depth joint in the top 25% of the slab (before it is fully cured). Am I on the right track?

Ha! Maybe... Well we're going to install the pipes anyway - the cost isn't significant - blank them off and time will tell whether we ever progress from there.

Thanks!

One other thing... Is there any reason why I can't run the pipes up to the loft space above the bathroom (ours is a single storey building) and site the manifold etc there? I don't think that should be a problem... Hard to find info on simple slab cooling or circulation systems, I've contacted a few companies with no useful response: Thanks!

Thanks a lot. Good info.

Thanks everyone. It seems like putting the pipes in anyway might be sensible. Considering our 85m2 of floor area (mostly one open plan room) is a single circuit ok? I'll be attaching the pipes to the reinforcement mesh. I'm getting different answers to the question of whether a single loop is ok or multiple loops leading to a manifold are the way to go. Incidentally, we're also lumbering ourselves with loads of glass and the problems that entails. Besides having a nice view and being very lucky to have no-one else nearby we also like a house with a loose barrier between inside and out and all that (inefficient and expensive) glazing does give that. We're mitigating the problems with shading and overhangs and aren't worried all that much by achieving a constant 21 degrees so for us it's worth it. Probably. Hopefully.

Hello. We're soon to get cracking on our build again now the tourists have gone and I'm no longer employed to drive them in circles on a boat. We have arrived at the point where we must decide whether or not to install the pipes for slab heating/cooling and I'm right on the fence. On the one hand I think such a system might bring unnecessary complexity into a very simple building (and we like simple a lot). On the other hand maybe we're going to need to shift heat from the building on certain days and slab-cooling might be the best way to do this, we might also want to add heat... I'm thinking more straightforward cooling methods exist for cooling: opening a window, summer bypass on mvhr (which we'll have anyway). And for heating an inline air heater on the mvhr would be a simple addition. I'd appreciate any input from you all. Thanks! Mark

Everyone recommends and gets on with Sketchup, except me; Sketchup makes me want to smash my laptop over my own head. I'm quite computer literate - have used photoshop professionally, am not too bad with FCP - but for some reason Sketchup won't happen. We've used Live Interior 3D Pro, it's somewhat basic but has done the job.

I was told that wasn't allowed but the guy who said it had no idea why it wasn't allowed. Makes perfect sense to me though as wouldn't the steel more efficiently dissipate the heat?

Our local planning commitee made the process of getting any permission on our plot such a nightmare for the prior owners (our friends) that - even though they eventually won on appeal to the welsh assembly - they gave up on what had been their dream home and sold the plot to us. Some of the stipulations included moving the driveway and lowering the ridgeline to avoid offending the sensibilies of passers-by (we have, at most, maybe a half dozen walkers wander past PER MONTH), planting trees to hide the house from the neighbours (half a mile away)... the list goes on... What is interesting is that just one mile away as the crow flies the former vice-chairman of our planning commitee built TWO houses on a greenfield site without any planning application, let alone permission.

No reinforcement as specced by the architect, just a couple of movement joints, I have wondered about sticking steel in anyway... Ok, thanks for that info Peter. I assumed the pipes would have to be within the 100mm of concrete and wasn't sure how that was acheived. Our local concrete people do offer fibre filled but I'd heard it makes the mix less workable - steel seems sensible.

OK, I'm beginning to get my head around this! I don't need the screed and leaving it out will allow a more rapid progression once the slab is down. Our architect doesn't specify any steel in the 100mm slab (dimensions approx. 5 by 12m for the largest section). Do the UFH pipes need to be suspended within the slab concrete or attached to the underlying insulation as with screed-installed pipes?

I have been looking at PIR insulation but in all honesty I've not had much time to research fully (the downside of earning all your money in the summer...) The 'slab' isn't load bearing, no ring beam and no internal beams (we have no internal load bearing walls). I believe what we're building is a called ground bearing slab? I was under the impression that screed was a thing you did regardless of how the rest of the 'floor' was made up... it seems I was wrong there - we can get a 'screed' finish on the slab concrete.

The current proposed build up is for insulation (200mm) under the slab (100mm) with 50mm screed. The design we inherited called for over slab, under screed insulation but under slab seems to make more sense... I'm operating under the assumption that the screed layer is a strong, smooth layer that finishes the floors ready for final coverings? Am I right?

Is there a significant advantage to installing the UFH/cooling in the slab rather than in the screed (which I understand is the norm?). I can see the former would best utilise the whole body of concrete but for me sticking it in the screed would simplify things - we want to get our slab poured soon and have limited time to get the pipework and insulation done as it is without dealing with a stack of UFH pipings...