epsilonGreedy

Floor screeding, why so late in the build process.

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I am trying to understand why screeding of a ground floor is delayed and often left to plasterers, maybe I am confusing floor leveling screeding with structural screeding over a block & beam floor.

 

This question is important to me at this point because I am considering either a normal block & beam suspended floor or something like Jetfloor with large inter beam blocks of insulation. In the latter case I assume the structural screed is applied before the wall elevations reach 2 feet above dpc in order to protect the floor insulation as the build continues.

 

If I do go for Jetfloor is there a some downside to having a finished screeded floor exposed for longer during the main build phase above dpc?

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Normally you would screed the floors as soon as watertight 

Though some do plaster down to damp course and screed after the plastering  is finished

I can never understand this as you are losing months of natural drying time

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9 minutes ago, nod said:

Normally you would screed the floors as soon as watertight 

 

 

Is there a concern over rainwater ingress into an insulated screeded floor? I can imagine that a combination of a floor membrane under insulation sheets with screed over the top could trap significant rainwater ingress, then introduce frost freeze/thaw cycles = ouch.

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27 minutes ago, epsilonGreedy said:

 

Is there a concern over rainwater ingress into an insulated screeded floor? I can imagine that a combination of a floor membrane under insulation sheets with screed over the top could trap significant rainwater ingress, then introduce frost freeze/thaw cycles = ouch.

No I’ve worked on sites were they have screeded without being watertight 

But ideally you need to be watertight if only to keep the screed from getting damage in the first 48 hours

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14 minutes ago, nod said:

No I’ve worked on sites were they have screeded without being watertight 

But ideally you need to be watertight if only to keep the screed from getting damage in the first 48 hours

 

 

Hmm I am beginning to understand why beam and block is the default choice, this is not just a material, technology and insulation performance decision. The attraction of beam and block is that it permits a flexible build workflow that can tolerate adverse weather and/or a slow build hit by unscheduled delays.

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1 minute ago, Dreadnaught said:

Out of interest @epsilonGreedy, are you aiming for Passive House standard?

 

 

I think 15 Georgian wood windows including 10 sash windows makes a Passiv House an aspiration too far in my case. My motivation for interbeam insulation is that some variations have insulation blocks that wrap around and lock together under the concrete beams. I assume this saves a few inches in finished floor height compared to having 130mm of insulation laid above the beams.

 

I am juggling with a planning limit imposed on the overall roof ridge height plus an already low upper floor eve height plus an enviro flood survey report recommendation to raise the dpc 300mm higher than is typical.

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36 minutes ago, epsilonGreedy said:

 

A new build on a green field site.

 

So why use a suspended floor at all? 

 

It needs more insulation (because the air underneath it in winter will often be colder than the ground temperature) and I doubt it's more cost effective than a simple insulated slab floor, where you can just lay EPS (or PIR) then pour a concrete floor slab on top.  Power float the slab whilst you still have good access and you can get it flat enough to tile etc with no further work.  You can easily and cheaply include UFH pipes in it too.

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22 minutes ago, JSHarris said:

So why use a suspended floor at all? 

 

 

There is a moderate concern about a 1 in 30 year surface water flash flood risk and a recommendation to raise the dpc. Would this require many more cubic meters of concrete using a slab design?

 

25 minutes ago, JSHarris said:

It needs more insulation (because the air underneath it in winter will often be colder than the ground temperature) and I doubt it's more cost effective than a simple insulated slab floor, where you can just lay EPS (or PIR) then pour a concrete floor slab on top.

 

 

The science here seems to contradict the prevailing view that ground source heat pumps are far more effective than their air sourced cousins. Surely air has much less thermal capacity to draw away heat than solid ground hence this concern about a foundation air void soaking away a home's heat feels overstated.

 

30 minutes ago, JSHarris said:

You can easily and cheaply include UFH pipes in it too.

 

 

I do not share the current Selfbuild vogue for UFH, at least not from a financial performance perspective. Why charge up the humongous heat capacity of a concrete floor slab at 10 pm or 9am when the occupants of a house will typically be away at work or kicking off the duvet cover because the house is too warm to sleep comfortably.

 

In my view UFH only works if a home's occupants are retired and the heat source is lower than 30 degrees i.e. not a gas boiler and the insulation level is so high 1Kw of heat will maintain a 20 degree heat differential and mechanical ventilation distributes heat around the home. I will not tick three of those boxes.

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Posted (edited)
42 minutes ago, epsilonGreedy said:

There is a moderate concern about a 1 in 30 year surface water flash flood risk and a recommendation to raise the dpc. Would this require many more cubic meters of concrete using a slab design?

 

 

This is very interesting for me. My target plot has a similar flooding risk. I am wondering if I can use an insulated floating raft and wondering if the height can be raised just with more EPS. Haven't asked someone like IsoQuick yet but tempted to do so.

 

42 minutes ago, epsilonGreedy said:

The science here seems to contradict the prevailing view that ground source heat pumps are far more effective than their air sourced cousins. Surely air has much less thermal capacity to draw away heat than solid ground hence this concern about a foundation air void soaking away a home's heat feels overstated.

 

 

The point here is the temperature gradient. The ground maintains a more consistent temperature across the year than does air. In the winter it is comparatively warmer than air and in the summer the reverse. There is a second concern over wind cooling. This is distinct from just cold stationary air. Wind penetration of insulation materials can impair their insulative functioning, possibly dramatically. Best to shield insulation from the wind wherever possible.

 

42 minutes ago, epsilonGreedy said:

I do not share the current Selfbuild vogue for UFH, at least not from a financial performance perspective. Why charge up the humongous heat capacity of a concrete floor slab at 10 pm or 9am when the occupants of a house will typically be away at work or kicking off the duvet cover because the house is too warm to sleep comfortably.

 

 

UFH is more relevant for a Passive House where the internal temperature variation over 24h is relatively small. Then it does not matter very much when you impart energy into the slab (or the house) in the 24h cycle. The occupants wont notice anything other than a constant temperature being maintained.

 

ADDED: in a Passive House, UFH has three useful roles: (1) use of the slab as a heat store (as @TerryE described); (2) UFH can redistribute heat around the house (like @JSHarris's example), and (3) UFH loops, if linked to a heat pump, can be used to cool the house (again like @JSHarris) but it should be noted the this delivers cooling in a less-than-ideal location (i.e. the floor, not ideal for convection) and can cause a condensation if taken too far. 

 

Thus for a Passive House, UFH is your flexible friend and pretty cheap if installed when pouring the slab.

 

Edited by Dreadnaught

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Posted (edited)
2 hours ago, Dreadnaught said:

The occupants wont notice anything other than a constant temperature being maintained.

 

 

I'm not convinced this is a positive. We are just highly evolved mammals conditioned by millions of years of evolution to snuggle down at night on a planet with diurnal rhythms, I am waiting for the NHS to discover "Passive House sleep deprivation syndrome" :/

 

2 hours ago, Dreadnaught said:

ADDED: in a Passive House, UFH has three useful roles: (1) use of the slab as a heat store (as @TerryE described);

 

 

I can see this as a benefit in Spring and Autumn when coupled with solar driven energy capture. The biggie is coupling UFH to a lower temp heat source such as ASHP.

Edited by epsilonGreedy

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14 minutes ago, epsilonGreedy said:

I not convinced this is a positive. We are just highly evolved mammals conditioned by millions of years of evolution to snuggle down at night on a planet with diurnal rhythms, I am waiting for the NHS to discover "Passive House sleep deprivation syndrome" :/

 

Thought provoking comment ?

 

If your diurnal rhythm requires it, just open a window. Voila! Even in a Passive House. 

 

Joking apart, I did think about this in relation to living in a Passive House. I wonder if anyone currently sleeping soundly or otherwise in a Passive House can comment?

 

By the way, there is indeed some evidence I have seen that temperature does play a role in the diurnal rhythm. For example a dose of cold before sleeping is soporific. However I suspect it’s effect is dwarfed by the effect of light. 

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4 hours ago, epsilonGreedy said:

 

There is a moderate concern about a 1 in 30 year surface water flash flood risk and a recommendation to raise the dpc. Would this require many more cubic meters of concrete using a slab design?

 

 

No. the finished floor height will be the same, and that is the only concern as far as flood risk is concerned (we're also notionally in  a flood risk zone and had to do a flood risk analysis, plus were given a set of recommended flood mitigation measures to consider).  A suspended floor carries the additional flood risk problem of how to drain the water out from the ventilated cavity under the house as well.

 

 

4 hours ago, epsilonGreedy said:

The science here seems to contradict the prevailing view that ground source heat pumps are far more effective than their air sourced cousins. Surely air has much less thermal capacity to draw away heat than solid ground hence this concern about a foundation air void soaking away a home's heat feels overstated.

 

The science does no such thing, I'm afraid.  If it is colder under a suspended floor than under a ground bearing floor then more heat will flow out from the house.  Air does have a low heat capacity but the airflow under the house in any breeze will be more than enough to ensure the void under the floor stays pretty close to the outside air temperature.  The average UK ground temperature is between 6 and 8 deg C, whereas on a cold day the air under the floor could be well below freezing.   On a very cold day you could double the heat loss through a suspended floor when compared to a slab.

 

 

 

4 hours ago, epsilonGreedy said:

 

I do not share the current Selfbuild vogue for UFH, at least not from a financial performance perspective. Why charge up the humongous heat capacity of a concrete floor slab at 10 pm or 9am when the occupants of a house will typically be away at work or kicking off the duvet cover because the house is too warm to sleep comfortably.

 

In my view UFH only works if a home's occupants are retired and the heat source is lower than 30 degrees i.e. not a gas boiler and the insulation level is so high 1Kw of heat will maintain a 20 degree heat differential and mechanical ventilation distributes heat around the home. I will not tick three of those boxes.

 

UFH doesn't have to have slow response times, it can respond as fast as having radiators.  It's all down to the design.  Bearing in mind that even a house built to "just" meet current building regs won't need a massive amount of heating, and one built like ours needs so little that frankly leaving the heating on 24/7 only costs a few pence a week, if that, over having it timed, so the whole working/retired heating pattern just doesn't apply. 

 

Why on earth can't a gas boiler run  UFH at below 30 deg C?  It's easy to get a gas boiler to run UFH at lower temperatures than this, as the boiler temperature is independent of the UFH temperature - it doesn't matter at all what the boiler temperature is, as the thermostatic mixer on the UFH will set the floor flow temperature, not the boiler.  Using a gas boiler with UFH is a great idea if you have mains gas - it's clean, efficient and cheap to run.

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4 hours ago, Dreadnaught said:

Joking apart, I did think about this in relation to living in a Passive House. I wonder if anyone currently sleeping soundly or otherwise in a Passive House can comment? 

 

I sleep very well in ours - it's very quiet, for a start!

 

In winter our bedrooms are cooler than downstairs by typically a couple of degrees. It's enough to be noticeable as you change levels. That makes it perfectly cool enough to sleep in.

 

Also, rather than cooling the room, you can just use a thinner duvet or a blanket to reduce your sleeping temperature. I'm struggling a bit at the moment, as we have a duvet that's a bit too warm now that the house is warming up a bit with this milder weather.

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Ours is much the same, the bedrooms are typically a degree or two cooler than the living rooms downstairs, plus the fresh air fed into them all the time tends to make the bedroom air quality a great deal better than homes without MVHR.

 

I did some measurements on our old house tht has no MVHR and found that the CO2 level in the early hours of each morning (with just a fanlight ventilation window open in the bedroom) were regularly reaching around 1600ppm, whereas with MVHR it's rare to see the level get above that outside, around 450ppm is typical.

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This is also the first house I've ever lived in where I don't wake up every morning with a stuffy nose.

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Posted (edited)

I support everything that the Js say.  Our MVHR stays at 30% except for the occasional boost, and doing the airflow calcs  this does a complete air change of the house roughly every 4 hrs, so the house smells airy and fresh all of the time.  When we do our 3-monthly clean of the MVHR filters, we still get a covering of fine white dust on the outbound filter which I suspect is a result mostly of the rubbing down as we finish off our P&D.  What horrifies me is the black particulate matter on the inbound filters.  Our house fronts onto the street through our village, but this is quiet except for the ½hr rat-run period morning and evening; and the inlet and outlet are 5m up and ~12m from the road, but we still clearly get diesel particulates (and perhaps wood smoke) being taken out by the MVHR filters.

 

As the slab, our ground floor is slated, but Jan prefers to walk around bare foot in the house because it is a nice steady ~23°C.  We haven't put in the upper control on our UFH, so we still do fixed heat chunking over night.  The conversation that I've just had with Jan went along the lines: "the average temp has risen by 0.8°C over the last 7 days because the average temp outside is starting to climb at last -- OK, I'll trim the overnight heating from 5½hrs to 4½."  Life is tough in a passive house. :P

 

We used to live in a traditional farmhouse: wavy floors and walls, draughty windows, wood-stoves, classic CH, walk from one room to the next and the temp drops by 5-10°C; within 1hr of the CH going off overnight, ditto.  Do I miss it at all: not one jot.

Edited by TerryE
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