It wasn’t supposed to be a living wall

Whilst at a straw bale building event this summer, I heard someone claiming that straw bale walls were of no interest to rodents because there was nothing for them to eat in the straw. Based on the evidence I and others have seen at a few sites, this doesn’t seem like a particularly sensible claim to make. On the last project I visited, a left over barley bale was abandoned outside in the weather, and had sprouted.

sprouting barley
The top of the photo shows left over barley straw from a building project that had sprouted when left outside. The bale in the foreground was a different type of straw (Miscanthus) which does not set seed in UK conditions.

I’ve also seen photos of sprouts emerging from straw bale walls whilst the clay plaster is still wet; the moisture in the plaster allows grain to germinate, and if there’s one thing that sprouts are good at, it’s working their way through to the light.

This clearly demonstrates that there is grain to be had in the bales, although whether or not it’s actually particularly tempting to a hungry rat I don’t know. It’s possibly also a reason to consider building out of non-cereal straws (i.e. ones with no grain feed value). Good straw bale builders will always take care to incorporate mesh at vulnerable points (e.g. at cladding junctions, or other junctions between posts and straw) to prevent rodents from nibbling away at the walls. I recently spoke to someone who deliberately left their building bales in an open barn for a year before building with them, in order that rodents would have attacked any bales with feed value in before the bales got as far as being in a wall.

Combine harvesters are designed to separate out as much of the grain as possible in order that the tonnage of grain per hectare is maximised, and that virtually none ends up in the straw. There is really very little way of knowing how much grain does end up in straw, but incidents like this do suggest that it’s an issue. Sprouting bales is certainly a phenomenon that farmers are familiar with, as are nests of rats in straw barns. Even if there is no grain, a straw bale wall would be a nice cosy place for a rodent to nest, so it’s really important to prevent them from getting into the insulation in the first place.

It’s worth emphasising that this is not a problem unique to straw bale building; I’m sure we’ve all heard stories of rats in lofts and underfloor insulation systems in buildings with conventional insulation, so it’s not a reason to avoid building with straw. This is simply a plea for straw bale builders to stop claiming that rodents wouldn’t find straw bale walls attractive; there are more than enough well substantiated reasons to build with straw, without the need to resort to dubious claims.

This post is based on work enabled by the BEACON project, which is funded by the European Regional Development Fund via the Welsh Government. If you’re interested in building with straw, the SBUK website is a useful starting point.

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