Why smart thermostats don't always save you money

[Oldgreybeard]

I think it's down to lifestyle. If you're at home all day then a long decrement delay costs you nothing (and keeps the place more comfortable with less temperature swing), if you're out at work all day then a cooler house is losing less heat.

Interesting that your 1980's block & brick house had a short decrement delay - guess that's down to poor insulation. Mine is timber frame so won't store much heat but also has 1980's insulation (apart from DG and more loft insulation) so loses it relatively quickly. I guess with passive house levels of insulation the heat loss is very slow regardless of the construction material.

Long decrement delay is definitely a bonus for summer heatwaves.

In terms of simple physics, I find the common statement that timber frame = short decrement delay a bit misleading, although accept it does apply to a lot of typical developer built homes. Our house is timber frame, yet has a pretty long decrement delay, simply because it was designed that way. The structural construction material has very little to do with decrement delay, the dominant factor is the combination of the specific heat capacity of the insulation together with the insulation performance.

Bricks are poor, for example. Their specific heat capacity is around 800J/kg.K, compared to softwoods that are around 2,300J/kg.K. Concrete isn't much better than brick, at around 1,050J/kg/K.

I opted to use a Canadian idea, a duplex timber frame (an outer frame and an inner frame spaced 300mm apart) and completely fill the void between the frames, and between the frame timbers, with tightly packed cellulose (very similar specific heat capacity to softwood). The result is a structure that has a pretty high specific heat capacity, as well as fairly good insulating properties (the walls have a U value of about 0.12W/m².K). The roof is similar, although uses I beam rafters that are a bit over 400mm deep, again with tightly packed cellulose between them, giving a U value of about 0.09W/m².K.

The net result is that our timber frame house significantly outperforms the vast majority of block or brick homes, and with an annual heating cost of around £56 at current prices it's not worth worrying about not heating the house when we're not at home. Another bonus was that it was cheap to build and was erected very quickly (house took about a week to build to the weathertight stage). Overall the build cost was around 15% or so lower than a brick and block house that had "just meets building regulations" level of performance.

The worst sort of construction, in terms of the decrement delay time, is where the insulation used has a low specific heat capacity. All foam insulation products fall into this category, and this was the main reason I chose not to use SIPS panels. They may have good insulating properties, but the decrement delay is very short when compared to the same insulating properties provided by insulation with a higher specific heat capacity. I did get quotes for a SIPS panel build, using the thickest panels available to get similar U values, but the cost was prohibitive and the decrement delay time was a great deal shorter, around 5 to 6 hours.

[richbee]

I like having my Tado room & radiator thermostats - so we can heat whichever room or rooms we are using. I'm working form home so my office is the only room which is being heated.
It detects when we go out and reduces the temperature to whatever I've got it set to.

There is a daily program for heating the bedroom up as we get up and go to bed, and kitchen at dinner time, etc, but it's quite often tweaked if we're doing something different - it's pretty obvious that you have to modify to suit, unless you have a very fixed schedule, and they are so much easier to control than the old fashioned central heating programmers - we can even ask 'she who must not be named' (Alexa!) to change the temperature if we want to

[Stig]

Bricks are poor, for example. Their specific heat capacity is around 800J/kg.K, compared to softwoods that are around 2,300J/kg.K. Concrete isn't much better than brick, at around 1,050J/kg/K.

Brick is much denser than wood though!

Interesting to hear about the heat capacity of cellulose insulation, that's going to be dominant factor as you've got so much of it. I'm guessing the maybe 50mm of mineral wool insulation in my walls (assumed, I've not taken one apart to look) isn't going to contribute very much -I have wondered about upgrading it but that would have to be foam or foam boards to have a good enough R-value so wouldn't increase the decrement delay. I do have an uninsulated concrete floor so that's probably my biggest factor!

[Oldgreybeard]

Bricks are poor, for example. Their specific heat capacity is around 800J/kg.K, compared to softwoods that are around 2,300J/kg.K. Concrete isn't much better than brick, at around 1,050J/kg/K.

Brick is much denser than wood though!

Interesting to hear about the heat capacity of cellulose insulation, that's going to be dominant factor as you've got so much of it. I'm guessing the maybe 50mm of mineral wool insulation in my walls (assumed, I've not taken one apart to look) isn't going to contribute very much -I have wondered about upgrading it but that would have to be foam or foam boards to have a good enough R-value so wouldn't increase the decrement delay. I do have an uninsulated concrete floor so that's probably my biggest factor!

The key difference is making the structural components out of softwood that has similar insulating properties as the insulation, so the entire wall thickness is effectively insulation. With a brick or block wall the structural material is wasted in terms of providing insulation, so if you want to get the same sort of decrement delay and insulation performance you need a much thicker wall, which wastes internal volume for a given footprint.

The same sort of performance can be achieved by using a fairly thick block, brick or concrete single skin inner structural wall, with a thick layer of insulation on the outside, but this costs a fair bit more to build, and is harder to make 100% airtight (unless using cast concrete). The thing that surprised me was just how little it cost to use the duplex timber frame method. The cost of the frames themselves was around 30% more than a conventional timber frame, but the cost saving from not having bricks or blocks, together with the very short build time (which was a another big cost saving) meant that the whole build came out cheaper per m² than a budget house on a big development.

[marshman]

__

[Oldgreybeard]

OGB, What is the outer skin of you house if not brick, is rendered?

The outside of the duplex frame is skinned with vapour permeable OSB as an anti-racking layer, then covered with a breathable membrane to keep insects from getting at it, then it's vertically battened along the lines of the frame timbers with 50mm x 25mm roofing battens, then there is larch cladding fixed to that. Pretty much identical to the way many houses in North America are clad.

The inside of the frame has another anti-racking layer, but of a vapour impermeable board, coated so that neither air nor water vapour can pass through it. This inner layer has all the joints sealed with a special (and gobsmackingly expensive) tape, made in Switzerland. The inner boards are also taped to the edges of the door and window frames, to ensure the house is airtight. There are vertical 50mm x 50mm battens on the inside board, to form a void for cables and plumbing, then plasterboard was nailed to them.

[AE-NMidlands]

OGB, What is the outer skin of you house if not brick, is rendered?

The outside of the duplex frame is skinned with vapour permeable OSB as an anti-racking layer,

What is "an anti-racking layer?"
something to provide diagonal stiffness?

[Oldgreybeard]

What is "an anti-racking layer?"
something to provide diagonal stiffness?

Yes, it stops the vertical frames from moving sideways. With these boards on both sides the whole frame becomes very stiff and rigid.

[Swwils]

I like having my Tado room & radiator thermostats - so we can heat whichever room or rooms we are using. I'm working form home so my office is the only room which is being heated.
It detects when we go out and reduces the temperature to whatever I've got it set to.

There is a daily program for heating the bedroom up as we get up and go to bed, and kitchen at dinner time, etc, but it's quite often tweaked if we're doing something different - it's pretty obvious that you have to modify to suit, unless you have a very fixed schedule, and they are so much easier to control than the old fashioned central heating programmers - we can even ask 'she who must not be named' (Alexa!) to change the temperature if we want to

In most homes this isn't a good idea. The heat just migrates to the unheated rooms and all you have effectively done is reduce your heating systems emitter surface area and is less efficient.

[Oldgreybeard]

Found a bit more about the double frame, Larsen Truss construction system for those interested: https://www.ecohome.net/guides/3418/lar ... stud-wall/

Ours are slightly different, in that the inner and outer frames are connected with four short noggins in each vertical stud pair, rather than the OSB web I joists shown on that web page, although our roof is built with 405mm deep I joists, skinned with OSB outside as sarking and filled with cellulose, like the walls. John Larsen, the chap that invented this system, came up with the idea to tackle the cold temperatures in Alberta, Canada, where he lived. Since then it's become a popular way to build a passive house. It's a design that's been around for about 40 years now, I believe, but as usual we've been a bit slow to pick up on it. I found that the building industry is very reluctant to do anything different and had a hell of a problem finding builders, most just didn't want to know and weren't at all interested in things like insulation and airtightness.