We are self building a low impact home with a suspended timber floor. We are on heavy clay and no bedrock and are building a 400mm high plinth wall around the perimeter with air bricks set every 1800mm to provide sufficient airflow to and from the void underneath the home. We are based in west Wales and it rains a lot here especially in the winter. The void underneath the house will be exposed to the elements for a month or so and with the blockwork I've realised that we may have a swimming pool on the leveled ground in the interim. I wonder how others cope with this situation? And It's too late and not ideal to use piers as an alternative. Also this given, when should the plastic DPM layer go on underneath? Presumably this will only compound matters.

FWIW, if this is going to be a crawlspace, then IMHO 400mm seems very minimal for future access, in which case you might consider digging access galleries or refuges or even a sump while there is still time...

However, your building code is certainly different from mine, which demands 600mm height minimum, and 0.6 sq. metres of access.

If on the other hand it is a construction void, you will not have future access for checking your joists or adding extra insulation.

(without wanting to appear overtly intrusive, what is your floor insulation plan ?).

I inherited an 800mm unfinished concrete CS, that is twice the height of yours... When I discovered the actual heat-losses through the floor-above and decided to insulate it, I discovered it was a lot of work... even with years of caving (and cave-digging) experience not to mention a (once) slender frame... (?)


You might try running a Topic Search for "suspended timber floor"...

Otherwise, here is a good read:
https://www.sciencedirect.com/science/article/pii/S0378778817311350

Good luck

gg

Posted By: gyrogearFWIW, if this is going to be a crawlspace, then IMHO 400mm seems very minimal for future access, in which case you might consider digging access galleries or refuges or even a sump while there is still time...

However, your building code is certainly different from mine, which demands 600mm height minimum, and 0.6 sq. metres of access.

If on the other hand it is a construction void, you will not have future access for checking your joists or adding extra insulation.

(without wanting to appear overtly intrusive, what is your floor insulation plan ?).

I inherited an 800mm unfinished concrete CS, that is twice the height of yours... When I discovered the actual heat-losses through the floor-above and decided to insulate it, I discovered it was a lot of work... even with years of caving (and cave-digging) experience not to mention a (once) slender frame... (?)
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You might try running a Topic Search for "suspended timber floor"...

Otherwise, here is a good read:
https://www.sciencedirect.com/science/article/pii/S0378778817311350

Good luck

gg

The perimeter wall goes all the way round so no crawl space.

Posted By: bardo

Posted By: tonyHow much insulation will there be in the floor? And how will this join to insulation in the walls, will partition walls bridge your thermal envelope?

Dpc is neither here nor there under concrete obersite

What do sleeperbwalls sit on?

Trench is 900mm deep with 300 mm of concrete poured in base then twin skin blockwork on top.
300 mm of treated sheepswool insulation laid within steico i joists. These are fixed to an insulated timber box beam which sits on top of the plinth wall (DPC laid in between).

There is no concrete oversite.

I would put steel mesh in front of a fine insect mesh to keep out critters of all kinds. It may be there are pre-made components that are suitable. I know very little about bricks and blocks.

Posted By: Peter_in_Hungarygo for a well insulated solid floor, either concrete or limecrete

+1 for PIH !

Although OP *did* say "a low-impact home"...

Guess the "impact" bit is summarized by need to compare the grey-energy balance of cutting down trees to make joists, thus removing CO2-absorption possibilities, versus use of concrete that creates C02, versus the energy balance of the future heating system, modulated by the benefits of mass versus the grey-energy balance of plastic insulation etc. etc.

gg

Posted By: gyrogearenergy balance of cutting down trees to make joists, thus removing CO2-absorption possibilities

Actually cutting down trees absorbs *more* CO2 rather than less, provided it is managed properly and replacements are planted.

Posted By: bardoIf we didn't use underfloor heating would that leave us with a cold floor?

Not if you insulate the floor.
The floor will be warmer with UFH but the walls will be colder without radiators on them. In other words where you put the heating will always be warmer than an adjacent place that doesn't have a heating source. The escence is that if you have sufficent insulation the heating demands are low and it is easier to acheive an overall even temperature without cold spots either on the floors or the walls.

Other considerations - radiators respond quicker than UFH and some people find that their feet/ankles swell up with UFH, but this could be because a high(er) temperature is needed due to insufficient overall insulation.

Glapor vs. EPS - I would go with price and availability. Galpor is significantly worse at insulating than EPS so more will be needed.

Posted By: Peter_in_Hungarybardo - Is it too late to go for a well insulated solid floor, either concrete or limecrete. Insulation is easy, no worries about ventilation or rot in the future and works better with UFH if you are going that way.

Peter, those who work with natural materials suggest using limecrete for a floor. It is apparently more responsive than concrete to heat and moisture buffering. Could limecrete be used with EPS or would that defeat the object of using a more open structured material? When I last looked at the cost of glapor it was relatively expensive.

Posted By: djhTo some extent the relative merits of the various heating systems depend on how well insulated your building will be and how you plan to use it. You mentioned 300 mm of sheepswool in a timber floor, which is reasonably well insulated. What are you proposing for the walls and roof, and what standard of glazing and airtightness are you aiming for? Will you want to keep it warm pretty much constantly, or what?

If it's well insulated then it makes more sense to keep it warm constantly, and if you do that then a heated floor slab would be a useful contribution as long as it is well controlled.

Lime is certainly better than concrete for moisture management. I'm not sure there's much difference as regards heat. There's no reason you couldn't put a limecrete slab on top of EPS that I know of. Perhaps worth readinghttp://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5983" rel="nofollow" >http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5983and other threads referenced.

Walls are straw bale infil inside timber frame. Clay plaster inside, lime skin outside with larch cladding as rain screen. Triple glazing on all sides except south which is double. Wood fibre board insulation in roof. So well insulated with hygroscopic breathable walls. We are off grid and venting via extractors and trickle vents.

Posted By: djh

Posted By: bardoWalls are straw bale infil inside timber frame. Clay plaster inside, lime skin outside with larch cladding as rain screen. Triple glazing on all sides except south which is double. Wood fibre board insulation in roof. So well insulated with hygroscopic breathable walls. We are off grid and venting via extractors and trickle vents.

So you'll be close to passive but not quite, I suspect. If I were you I would upgrade to continuous ventilation - probably MVHR. I think it will make a big difference to air quality and comfort. Pay close attention to airtightness as well.

If it were me, I would use a slab with low-temperature UFH since I think the time constant of the building will be long enough to make continuous heating the best policy. I believe a UFH system can also usefully even out the effects of solar gain throughout the building.

Hi DJH, in the end we opted for a suspended timber floor to sit on the perimiter plinth wall. We are off grid with a sustainable supply of woodfuel so will use this for our cooking hot water and heating when the solar system low. On another note, I saw that you also used straw bale walls. Did you use a machine to render the walls? We intend to put a coat of lime on and clad for extra rain screening in our wet part of Wales.

Posted By: djh

Posted By: bardoOn another note, I saw that you also used straw bale walls. Did you use a machine to render the walls? We intend to put a coat of lime on and clad for extra rain screening in our wet part of Wales.

Yes, we employed a chap called Arthur Philip to do all the lime plastering on the bales. He's an expert plasterer who's extremely enthusiastic about bales. He uses a Putzmeister SP11 to spray most of the material on, plus loads of elbow grease to compact it and get a result. I expect you'll need an initial scratch coat to cover most of the bales and perhaps to fill out hollows and then probably a second coat just to get enough thickness. I think you can skip the third, finish coat if you're cladding over the top.

A pump needs good quality lime to avoid blockages; Arthur used to collect every stone and take them back to the supplier! The mix is everything to avoid cracking. Lots of fibres and some secret additives. One lime plasterer I know says that traditional mixes sometimes have up to 30% hair in them.

Thank you. Where is Arthur based and how long did he take to complete the job? Did you also clay plaster the interior?

We like the lime finish both inside and outside. Inside is a pretty smooth finish; outside has a slightly rough surface texture. Both the lime and the gypsum inside are painted with clay paint so they look similar. One of the main reasons for using lime inside was that I had already found a tradesman I trusted to do it. Clay would have meant finding another team and there are even less clay plasterers than there are lime plasterers. Lime is also stronger and I trusted it (and Arthur) to have fewer problems with cracks, which matters since it is our airtightness barrier. There are only a few cracks, all in areas where there is a change of materials and geometrical stress but fortunately also a membrane behind all those places.

Posted By: djhWe like the lime finish both inside and outside. Inside is a pretty smooth finish; outside has a slightly rough surface texture. Both the lime and the gypsum inside are painted with clay paint so they look similar. One of the main reasons for using lime inside was that I had already found a tradesman I trusted to do it. Clay would have meant finding another team and there are even less clay plasterers than there are lime plasterers. Lime is also stronger and I trusted it (and Arthur) to have fewer problems with cracks, which matters since it is our airtightness barrier. There are only a few cracks, all in areas where there is a change of materials and geometrical stress but fortunately also a membrane behind all those places.

I hear your reasoning. Thanks for sharing that DJH.