Understand thermal implications of underfloor heating Your slab R-value requirements soar

15 August 2023 by Jason Quinn

Embedded under-floor heating is a luxurious way of keeping a house warm. You may encounter clients who hate heat pumps and who aren’t deterred by the cost of under-floor heating. But there’s some implications for a high-performance design that architects/designers must be aware of.

Passive House buildings by definition have a very low heating demand. Heating the slab is an efficient way of keeping the house warm, but the capital costs are very high. It’s a good idea to check your clients’ expectations also. If they have lived in a thermally-inefficient home with underfloor heating in the past, they’ll be used to nice toasty feet and a decidedly warm floor. That won’t be their experience in a Passive House, where the floor temperature will likely only be several degrees warmer than the air temperature—that’s all that would be required to keep the indoor air temperature in the Goldilocks zone.

If the home doesn’t need air-conditioning and if the clients loathe the experience of heat pumps blowing hot air on them and they are not deterred by the cost, then whole-of-house underfloor heating powered by a hydronic heat pump is a perfectly acceptable solution. (I don’t recommend running these systems using electric resistance heating, this is inefficient and wasteful.)

Under the new H1 provisions, specifying embedded underfloor heating will see the slab R-value requirements increase significantly. For Passive House designs, the slab is likely very well-insulated already. But designers need to understand the new requirements, because it is possible that a slab in a Passive House project in a warmer New Zealand climate would fail H1 on this count. 

It’s more of a headache for homes built to the Building Code legal minimum. The slab insulation requirements even in the new revised H1 are pretty minimal and they can likely get away with just slab-edge insulation: but that all changes if embedded heating is installed. And here’s the catch: under-tile floor heating is classed as embedded under-floor heating, even though it’s on top of the slab not within it. To be perfectly clear: if you heat under the tiles in a bathroom in one little corner of the house, then the WHOLE slab needs to be much better insulated. It will essentially require a Passive House style insulated slab. That will add material costs and also some time, as the designer does the calculations to demonstrate compliance.

I’ve been having some discussion around this lately, which started when I reviewed a recent BRANZ appraisal of different under-tile heating systems. This document clearly stated that under-tile heating needs to meet the embedded heated slab R-value requirements. I directed some questions to MBIE, which confirmed this. MBIE’s official response is that it is up to the individual Building Consent Authorities (BCAs) to interpret and enforce this provision. 

It would have been better if the new H1 requirements had clarified this rather than passing the burden to building consenting officials. Anyway, it sits with councils. I recommend that if a design specifies ANY under-floor heating in a slab-on-ground system (including under-tile heating), ensure it meets the special embedded floor heating R-values. Otherwise, the building consent officials could enforce their ruling at the consent stage. That means late stage redesign (but at least before any concrete was poured, it could get worse.)

If you need information about how to design a hydronic heating system, there is good advice in BRANZ Bulletin BU586 Embedded Floor Heating, which replaces BU491. Please note however that the slab edges detailed in this document are not particularly thermally efficient. They will likely fail to meet current H1 requirements for low A/P ratio slab-on-grade foundations.

The argument that under-tile heating is not embedded heating does make some sense. This is particularly so if the design specifies even a small amount of insulation below the under-tile heating system, as is common practice in Europe. That said, we consider it too risky to try to sneak around the special R-value requirements using this argument.

Detail: Stiebel Eltron

All the previous BRANZ bulletins specifically on in-floor electric heating are withdrawn. There were many. At first titled Electric Floor Heating, it started with BRANZ BU24 (1961), replaced by BU79 and then BU293. BU179 in 1975 became known as Embedded Electric Floor Heating, followed by BU344, BU491 and now the current BU586, all with the same name.

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