UFH upstairs with no insulation beneath it

sas

Hi,

We will have a 200mm hollowcore middle floor. In this case the slabs have EPS cast into them to form the voids and the ends of the slabs are formed closed.

Engineer has suggested (not mandated and has left it up to me) that we use a structural screed. It is a better job but not necessary according to him. I do like the idea of it too if I'm honest with myself.

We are building to passiv spec. i.e. full certified passiv.

I know typically it is not the done thing to use UFH without insulation beneath the pipes on hollowcore. Given however that the build spec. is so high I am wondering if we'd get away with it so to speak. House is externally insulated masonry (regular 4 inch solids) so quite a bit of mass to heat I realise.

To answer the "Why are you planning for UFH if it's a PH"

A company I am talking to about my HRV, solar and buffer tank combined system has suggested that I run a simple UFH system upstairs and downstairs as 2 zones only. This would draw low grade heat from a buffer tank being fed buy a stove and solar panels. The simple UFH system will apparently keep the price down and make it a viable possibility. Plus if we don't reach the PH standard then we already have the heat distribution system in place for our fallback. The gentleman advising this worked on 3 of the first 5 certified PHs in Ireland ( I know this for a fact ) and is worth listening to in my opinion.

To complicate things the engineer only suggested this at 4.00pm on Friday and I had intended getting block layers back monday 17th assuming the flooring arrives during this week. I was going to make the call on the UFH at a later stage. I have no spec agreed with plumber or electrician and as I understand it they would have to do some work prior to the pouring of the structural screed too.

All feedback appreciated.

SAS

heinbloed

Usually a PH certification is done according to the drawings, meaning the energy demand is calculated, not meassured.
So if the plans are ready and the builder equipped with them it would be difficult to change things.
It can be done however, but change of design demands a new PH calculation.

To the technical questions:
What you're suggesting can be done, the integration of heated surfaces into a structure is called " thermal activation" and not only done with PH buildings but in general construction as well.

If the building is large enough, it's energy demand accordingly, this activation makes sense. A proper job demands proper(expensive) input, be it the material or the workmanship. With small buildings/energy demands the combination of several heating systems makes little financial and ecological sense. The oposite could be true: maintenance costing more than the savings would generate.
Standard ST systems last for 20 years, if maintained and serviced. No matter how much or how little they contribute to the overall energy demand they need to be serviced. With a service charge of € 100-200 incl. materials per year or two most standard sized PH would already be supplied with their full energy demand, but sourced from the electricity grid. Take the chimney sweeping costs, capital costs for fuel storages and stoves, ST material, costs of space and insurance and you'll see that in most cases the multi-thermal energy suply for a PH is a waste of energy(and money), not a gain.
This calculation gets better the larger the energy demand is, multi storey apartment blocks in Frankfurt for example (PH certified) use combined thermal technologies similar to the one you're describing.
Energy-Plus Houses and Solar Houses as well, but they don't care much for PH certification.The certificate comes with the electricity/fuel bill.

Someone should do the energetic calculations for your project (as said above:these should be ready before the building works start) and then the material/equipment choices can be done according to their primary energy and capital costs.

The idea of the PH is to have a house which needs no permanent input of primary energy and the capital for it. Maintenance costs are part of the ongoing capital costs. Therefore most small PH showing best energetic performance use direct electric heating. Cheap to run due to low overall energy demand and free of maintenance, long lasting and cheap to replace.

sydthebeat

sas wrote: »

. Plus if we don't reach the PH standard then we already have the heat distribution system in place for our fallback.

i think this is the important point...

if budget allows, this makes sense at this stage. Id even go as far as to specify something 'cheap' such as 40mm EPS under the screed.

Poor Uncle Tom

sas wrote: »

I run a simple UFH system upstairs and downstairs as 2 zones only..

Will this be good enough to run economically as a primary heating system if your house does not reach the PH standard?

sas

sydthebeat wrote: »

i think this is the important point...

if budget allows, this makes sense at this stage. Id even go as far as to specify something 'cheap' such as 40mm EPS under the screed.

I should have be clearer on this one. It wasn't the cost of the insulation that was the concern. If I use the insulation then the screed is seperated from the slabs (by the insulation) and hence offers a considerably reduced structural advantage.

So if I decided not to go with the structural screed then I'd go with the original plan and build the upstairs walls straight off the slabs and make the decision on the UFH at a later stage before I pour the screed. This is the prefered option because I'd hate to pay for UFH and not have money left for a kitchen!

sas

Poor Uncle Tom wrote: »

Will this be good enough to run economically as a primary heating system if your house does not reach the PH standard?

Fair point. I have no info. on this because up to friday evening that decision was months away. I'll take a look at what could be a happy medium price wise if I decide to push ahead.

I believe that we will be close to the standard so either way the house should be relatively cheap to run. The PHPP calcs show that if we achieve an air change rate of 0.6 then we will have succeeded. This naturally assumes that we have built to the agreed details and nothing major crops up during the course of construction that throws a significant thermal bridge into the mix. All fun and games I'm sure you can imagine.

As I see it the only place we can actually fall short at this stage is at the air test.
I'm doing everything I've been asked to do to ensure we do get there. It really is a terribly high standard to hit however.

Complete aside but...

I worked with an approved installer for the main airtightness system available here on site yesterday fitting the membrane for around the perimeter of the flooring. We got all the corners made and fitted and about 1/3 of the "in betweens" fitted. Started at 10am, left site at 10:45pm. Having watched him construct the corners I now see why going the airtightness route yourself is not for the faint hearted.

heinbloed

If this uninsulated floor slab is to be heated up to be used as the prime source of space heating then the ventilation aspect resp. the energy waste caused by it has to be examined as well.
An uninsulated but heated floor slab will get warm at both sides. Whilest the radiating heat upwards is wanted the heat going down isn't wanted, a warm head would be uncomfortable. Worse, the ventilation system is usally fixed at and along the ceiling, plenty of warm air will be sucked or blown out.

This would eliminate such a floor slab as a primary heating source as Poor Uncle Tom has already questioned.

As a thermaly activated part of the building, where there slab is used only as a thermal storage at around ambient room temperature, as a back ground source of heat, this problem would be greatly reduced.
The higher temperature of the ceiling/slab the higher the energetic loss - if the ventilation is running from and along there. Which it usually does in Passive Houses.
Alternativly the ventilation could be run in the (partition) walls, a higher than standard velocity should be taken into account to make up for the less than ideal positioning of the flues. More energy losses though as well, but not as high as with a (heated!) ceiling.
Check the pros and cons with an energy advisor, a civil engineer,a ventilation specialist.

soldsold

My opinion is that using 40mm or 50mm EPS would be best, if the 200mm hollowcore doesnt NEED a structural screed, then there is no structural advantage to putting one in.

But the insulation will do two things:

1. make the upstaird UFH more responsive, although in a house to your spec this should not be an issue as you wont get wild temperature swings.

2. provide much better soundproof quality from upstairs to downstairs. Sound transfer prevention is either through mass or isolation. You have lots of mass in a 200mm floor with a screed, but little isolation.

40mm EPS or 50mm EPS will provide that. Id also use edge insulation upstairs for the same reason, 25mm would be plenty.

Steve

sinnerboy

SAS - keep the faith . You have done (or have had done ) the PHPP calcs and if going the certification route have commissioned a detailed set of construction drawings to eliminate thermal bridging .

Use wet plaster to all wall surfaces - including behind voids and ducts . Pay attention to elec back boxes by using intumescent sealant at the cable entries . Keep a close eye on piped services penetrations of your walls and roof ( how will the SVP exit the building ? ) . Tape seal your windows before plastering . Do a pre test (AT) before plastering proper after roof is on , windows are in and M+E first fix . Look forward with confidence to beating n50 0.6

Your building will have a high thermal mass so a heating slow response time . Not really suited to UFH . Your annual heat demand will be low but more critical is your heat load - i.e. what if we get another Jan like Jan '10 ?

Keep it simple - drive down to Argos and buy 2 - 3 max elec blow heaters . You will get away with sparse usage of them - 2 hours per day .

Have you looked at pre heating the supply air to your HRV ? A simple loop of your winter DHW heat source would suffice .

There is one reason , budget allowing , why I would bury UFH pipes in the ground and first floor constructions - in the screed ( structural / non structural ) without an insulation layer . That is to overcome the initial lag of the thermal mass . To overcome the initial kick-start of the internal environment without the discomfort people do talk about in the first heating season of a thermally massive PH . You will assist the drying out of all that masonry too .

You are unlikely to need to activate the UFH circuits for your second heating season .

sas

Thank you one and all for suggestions and feedback.