Ventilation fan

Lime Sublime - This article is part of a series.

Part 11: This Article

Oops, forgot to add fresh air.

The hut has had a couple of weeks of use now (as a podcast studio). It works, but needs ventilation. Leaving the door wide open is not a great option in summer (heatwaves, mosquitos) or in winter. And anyway, passively relying on the wind doesn’t always work.

Of course the CO₂ levels rise quickly when such a small space is occupied with door and window closed. What is more interesting is that the CO₂ levels drop when it is unoccupied. I have seen it as low as 185 ppm (the outdoor CO₂ level is around 430 ppm). This is due to carbonation of the lime in my walls, ceiling, and floor. From a health point of view, while that might reduce direct effects of high CO₂, it is wise in any case to avoid re-breathing air too much.

The ideal solution is a Heat Recovery Ventilation (HRV) system. But for this tiny occasional-use hut a simple intake fan will be fine.

To minimise noise, I went for an inline fan: then fan itself can sit outside through the wall, and connect via a duct to an inlet. Fantech’s VKO 100 is small and quiet and cost just $69.

There’s no way around the fact that I need to make a hole in the wall. Not so easy when your wall is 280mm thick. It definitely would have been easier if I had planned this from the beginning. Still, I managed it. I got through the plaster with a hole-saw bit, and then traced through to the other side with a 300mm extension bit.

Then I could cut out the plaster on the other side; the relatively weak hempcrete in between came out with some easy chiseling.

I mounted the fan onto the underside of a rafter, blowing into the room through a length of PVC pipe. This is known as positive pressure ventilation. The incoming air will be balanced by outflow through gaps around the door.