A Hot Composting Saga (Part 6) - Draughtproofing

 I was doing a bit of admin and realised I hadn't completed this series of posts. See here for Part 5 and links to the earlier articles. This is the final one on this particular topic though there will be plenty of future posts on hot composting.

To bring you quickly up-to-date, I have three hot composting bins: one is an early version of the HotBin (possibly Mark 1 or maybe earlier) along with two prototype hot composters (Super Compost Bins) supplied by Tony Callaghan, inventor of the HotBin. In spite of its age, my HotBin is still working well, enabling me to successfully hot compost through the winter months due to the excellent insulation provided by the one-piece expanded polypropylene body (no leaky joints!).

By way of contrast, the prototype Super Compost bins require self-assembly meaning there are plenty of potential cold spots at the joints.

Photo 1: Unpacking the Prototype Super Compost Bin

After several years of untroubled hot composting with the Super Compost bins, this Spring I was having all sorts of problems getting the bins up to working temperatures (50 ℃ - 65 ℃). Figure 1 plots the daily compost temperatures (measured at depths of 10 cm and 30 cm) of Super Compost Bin #2 along with the cumulative volume of added garden waste. This was a successful hot composting process with temperatures in the active (top 10 cm) section of the heap remaining above 60 ℃ for 40 days and 40 nights; there were a couple of periods, around day 22 and day 35, when the temperature dropped below 60 ℃ because the bins were left unattended due to holidays.

Figure 1: Daily Compost Temperatures (℃) and Cumulative Garden Waste Added for Super Compost Bin #2 (10th October 2022 to 20th November 2022)

Table 1 presents the equivalent data for the subsequent hot composting run in the same Super Compost bin. Despite daily attempts to boost the temperatures using hot water bottles, the active zone only achieved temperatures in the mid-forties (℃).

Table 1: First Hot Composting Attempt in 2023 with Super Compost Bin #2

DAY	Added Garden Waste (Litres)	T oC @ 30 cm	T oC @ 10 cm	Minimum Daily Temperature (oC)
1	94*	not measured	not measured	6
2	18*	19	35	7
3	20*	23	37	3
4	10*	26	34	4
5	8*	27	44	1
6	8*	32	40	6
7	0	36	43	4

* Two hot water bottles added daily to kickstart the hot composting process

Having fixed the first Super Compost Bin with some self-adhesive draught excluder, it was time to turn my attention to Super Compost Bin #2. As there was insufficient draught excluder for the second bin, I decided to utilise some thick capillary matting previously used as a tray liner for controlled watering of potted plants.

The Super compost Bin was dismantled and cleaned before reassembling using strips of capillary matting to fill the joints in the four side pieces (Photo 2) ...

Photo 2: Capillary Matting used as Draught Excluder in Super Compost Bin Joints

The lid of the Super Compost Bin rests on rubber tubing seals that were not sealing properly. Some old packaging material (recycled jeans!) was used to fill any gaps and reduce the escape of hot air (Photo 3) ...

Photo 3: Operating Super Compost Bin with Extra Lid Seals

A similar system was used on the first Super Compost Bin. Photo 3 was taken when the Super Compost Bin was in operation (details below); note that the hot compost is covered with a sheet of capillary matting to help retain heat and stop the compost drying out too quickly.

After the refit, the Super Compost Bin #2 once again worked to specification (Figure 2) with temperatures in the active region (upper 10 cm) ranging from 57 ℃ to 70 ℃ for the first 28 days.

Figure 2: Daily Compost Temperatures (℃) and Cumulative Garden Waste Added for Super Compost Bin #2 (8th July 2023 to 8th August 2023)

A better test of the 'leak-proofing' would be a hot composting run when the weather is colder. Figure 3 summarises such a run that started on the 7th November 2023. The green line represents the daily minimum outdoor temperatures that dropped below 0 ℃ towards the end of November. The yellow line, representing the upper (top 10 cm) active layer of the compost heap, maintains a mid-50 ℃ to mid-60 ℃ temperature range for the first 25 days until the bin is full.

Figure 3: Daily Compost Temperatures (℃), Outside Minimum Temperature and Cumulative Garden Waste Added for Super Compost Bin #2 (7th November 2023 to 3rd December 2023)

The three hot composting runs shown in Figures 1, 2, & 3, consumed around 400 litres of shredded garden and kitchen waste - equivalent to between 600 and 800 litres of pre-shredded waste. I will leave the full compost bin until spring to continue maturing before transferring the contents to my maturation bins for finishing off.