Miscellaneous

Costs

As of writing, &shed has cost £17,000 and some change. Does that represent good value? For a shed, that’s a lot of money. For a garden room, it’s around £10,000 less than you’d pay for a cedar-clad, EPDM-roofed bifold-doored cube. What value do you place on your time? I’d estimate that, over the last year, I’ve spent at least one day a weekend working on &shed (so North of 50 days) and lots more time thinking about it. Since I’m happy to work for free and have enjoyed the vast majority of the work, I’m happy enough that &shed will probably pay for its raw materials if we ever move.

Let’s compare to the cost of &shed to that of other domestic construction, but be aware that a small building is necessarily less efficient as the ratio of surface (which determines the cost of the structure) to volume (if that represents the value you get from the structure) is worse for smaller buildings. Conversely, buildings a shed might in some ways be more cost-effective than, say, a house as there no need for building control and there is no expensive kitchen or bathroom.

&shed is approx 10 m² and cost around £17,000, which equates to a cost per unit area of £1,700 per m². Various website, which I suspect are mostly repeating each other without original research, place the cost of a single storey extension with standard finish anywhere between £1,250 per m² and £2,500 per m². &shed comes in close to the mid-point of those two figures (£1,875). I could argue that &shed is particularly good value, being in the South East of England, where costs are typically at the higher end.

Breakdown

I had some funds set aside for the project and, because it took a year, saved up more as time passed. Let’s see how much I spent over time from day one, the planning submission:

The big jumps around days 150 and 250 show me ordering the SIPs and the glazing, respectively. Later in the build, costs rise more gradually.

A breakdown by category shows us that the SIP shell and the person power needed to assemble it account for almost half of the total cost. This tells me that building with a more conventional timber frame would be the first step in bringing down the costs, at the expense of a longer build time. I went with the cheapest SIP quote and don’t think that the labour was overpriced so there isn’t much room for manoeuvre there. Glazing was an unavoidable cost unless I wanted to buy a used door speculatively and then design the opening to fit. Flooring and foundations were both particularly good bargains, I reckon, since I could have spent double on each.

Thermal Efficiency and Comfort

Passivhaus

I’ve long been intrigued by the Passivhaus standard. Passivhaus is a standard for energy efficiency and comfort, which requires to you careful model and measure your building to make sure that it requires minimal energy to heat and cool, providing a comfortable and low-cost living environment. Passivhaus has been shown for years to work very well in residential buildings, where thick layers of insulation and mechanical ventilation with heat recovery result in extremely low energy bills and year-round 21° C comfort. In order to achieve the standard, you need to model the building in a tool like PHPP (Passivhaus Planning Package) and then measure the building’s performance to confirm that it meets the standard. This assurance step gives the homeowner confidence that the building was built correctly, and it also gives valuable feedback to the designers, builders and the Passivhaus standards body. In my opinion, it’s a shame that we don’t have a reliable testing and certification step applied to all new domestic builds in the UK. Instead, we have building control sign-off, which does not verify the performance of the building at all.

Passivshed

Unfortunately, as you make a building smaller the ratio of surface area to volume increases, so that you lose more heat through the walls and roof per unit of volume than you would in a larger building. For this reason, I don’t think a full Passivhaus build is practical for most sheds. Having said that, there are always exceptions and the smallest Passivhaus that I know of is in France and measures just 12 m². See the great write-up in this PDF.

Not only does it become harder to achieve the standard for smaller buildings but many of the costs of Passivhaus do not scale down with the size of the building. For example, the cost of the PHPP software and the cost of an airtightness test are all fixed costs that would be much the same for a shed as for a house.

Instead, I’ve stuck to the spirit of Passivhaus, if not the letter. I’ve aimed for a well-insulated, airtight building with mechanical ventilation and heat recovery. A high-performance shed, if you like. It has been designed with enough insulation, and built carefully enough, that the heat given off by an occupant and a valve amp should be enough on all but the coldest days.

Modelling

There don’t seem to be many software options for shed builders to calculate the performance of their shed in the design phase. The most obvious option is to use PHPP, version 10 of which can be bought for £216. If you are self-building a house, this seems like a very reasonable investment. For sheds, I wish there were something a bit more rough-and-ready. The main FOSS (free and open-source) contender seems to be OpenStudioApp, which appears to use EnergyPlus to do the back-end calculations.

If you have experience using building energy modelling tools and would like to help me model &shed, please get in touch via email.

Less accurate figures can be had, though. For example, h2x Engineering have a Heat Loss Calculator, which estimates a heating demand of 545 W for &shed, with an internal temperature of 21° C and an outside temperature of 5° C. That is certainly in the realm of the power consumption of some valve amplifiers,

Real-World Performance

It’s summertime as I write this and I haven’t finished the valve amp, so I can’t say much about the heating demands yet. Check back here in the winter for some real-world data on the heating and general comfort of the shed. Until then, I can comment on the thermal and acoustic performance:

• On the hottest days, &shed as reached as high as 32° C. This is at least as warm as our brick-and-block built house. There isn’t much cross ventilation and the MVHR, even on the higher setting, doesn’t make a noticeable difference to the temperature. A small heat pump or portable air con unit would make the world of difference, I expect.
• CO₂ climbs above 1000 PPM if the window and door are shut for extended periods, even with the fan on the higher setting. I have yet to find out exactly where it levels off but there’s a chance I’m going to have to either fit another Europlast MVHR, find some way to hack the one I have to add a third speed or invest in a lot of houseplants. With the window tilted, CO₂ stays below 1000 ppm with the door shut and the fan on the lower setting.
• Some noise from neighbours playing, parking and gardening can be heard through the walls. Although I haven’t measured the sound transmissivity yet, I plan to when time allows.
• Outside the hottest days, general comfort is excellent with pleasant humidity levels, a good view of the garden, a faint smell of timber and reasonably solid floors with a little bit of spring between joists.
• So far, everything external has stood the test. I expect the OSB mounting panels for the gutters and floodlight will be the first to need attention. I’m planning to reconfigure the gutters to collect rainwater so that is not a problem.

Tool Reviews

Tripod and Laser Level

These two get their own dedicated mention because they have proven themselves invaluable throughout the build.

Manfrotto 190XPROB with 486RC2 Ball Head

The Manfrotto tripod is an example of excellent design and build quality. Though I’m sure there are fancier tripods, this one is a joy to use and there is almost nothing I would change. All height and angle adjustments are intuitive and pretty quick, and they generally exude a feel of quality. In addition to holding the laser level for getting the foundation heights right, the battens horizontal, the cladding screws aligned, the doors square, the conduit level and the flooring straight, it also works OK to hold a camera. Although mine are getting on for 15 years old, it looks like Manfrotto either still make them or make very similar models, which tells you all you need to know.

Huepar 703CG

This laser level is the opposite of the Manfrotto. The Manfrotto is a luxury, where everything is just-so. With the Huepar, everything from the case to the fine adjustment knob feels cheap. However, as much as it’s a plastic-y eyesore, the 703CG gets the job done and is packed with features for the price:

• It self-levels in all three axes, allowing you to get things plumb, level and square with minimal effort. The gimbal can be locked if you like by turning the power slider to OFF and then holding one of the buttons on top. It’s probably mentioned in the manual, but I only discovered it by accident.
• It has several brightness settings, so you can (just about) see it in broad daylight, and you can also use it inside with less of a chance to blind yourself.
• It comes with a little reflector that makes it easier to find the line in bright light and at longer range.
• The battery life has been very good thus far, requiring only a handful of charges.
• I almost always use the tripod mount underneath but there are also magnets if you have a steel post nearby.
• It has a fine adjustment knob which might be less useful if you have a tilt-and-pan head but is great when you want to make small horizontal changes with a ball head.

You can greatly reduce the number and severity of eye-zaps by turning the laser power to the lowest setting and blocking off unused segments of the beam with masking tape.