[Strawbale] Greenhouse retro fit

[Andrew Webb]

Hi Kate,

The priorities for your retrofit depend where you are exactly; how cold 
is too cold?  What latitude, how much sun, extent of cloud cover, etc.?  
Anyway, I'll assume too cold means below freezing.  I'll also assume the 
GH is oriented to the south (obvious, maybe, but I've seen plenty of 
them that aren't). 

I'd venture to say that thermal mass is more important for the 
greenhouse than insulation.  Insulation keeps heat in but is no good if 
there is no heat to keep in.  If you just line the wall with insulation 
you would be relying on only air to hold the heat, assuming your gravel 
floor is covered with plants and therefore not exposed to direct sun.  
Line the inside of the north wall (in the northern hemisphere) with 
something like, in approximate order of effectiveness - water 
containers, poured concrete, rammed earth, fired brick, mud brick, 
concrete block -  to absorb heat during the day and radiate it out at 
night (equally true if you have a heater inside).  In a mild climate the 
render on straw bales may be enough but not if you have very low 
night-time temperatures.   The thermal mass will even out the internal 
temperature throughout the day & night, which is what plants need.  If 
the mass wall is thick enough and the sun exposure long enough, the 
thermal lag will compensate for a lack of insulation.  If you can 
insulate the exterior of the mass wall, all the better.

Another simple and effective method is to place large water containers 
around the greenhouse, not only on the north wall.  This distributes the 
night-time heat more evenly.

The next step is to insulate the roof, because that will be your biggest 
heat loss.  You can insulate the northern half of the roof with whatever 
insulation material/system you want, perhaps cellulose, batts, 
straw-clay or bales (suitably detailed and protected, of course).  
Assuming it's not a mild climate, you can then insulate the southern 
half of the roof with movable blankets or panels which you put in place 
before sunset and remove after sunrise.  If you just want to lengthen 
the growing season and not necessarily use the GH all winter, you could 
just add an extra layer of glazing in the form of UV resistant poly, 
inside the glass.

Something else to consider is that the thermal mass will be more 
efficient if it is dark, preferably black - a mixture of charcoal / soot 
and linseed oil can be used on earth walls/render.  Black paint or black 
plastic water containers are good.  However, so that your plants grow 
relatively straight you need to distribute the light around the 
greenhouse also.  This can be done by having white side walls and black 
north wall or by painting the north wall with black and white stripes.  
If you had to choose one, the effect of the white is more important than 
the black.  A white ceiling is best.

Humidity is a concern with whatever insulation you use.  It sounds like 
you have good ventilation; which is important in winter and summer.  
Putting bales in a GH would probably mean they have a limited life-span 
and need replacing at some point.  That may not be a major problem 
though and you can probably use the mulch.  Others may have more advice 
on the detailing to mitigate that; I haven't used bales in this 
application but I have insulated GH roofs with straw and wood shavings 
with a vapour barrier underneath.  Personally I would look at putting a 
mass wall inside (which would also be narrower than bales and give you 
more internal space), leaving the glass outside of that as a vapour 
barrier and wrapping the exterior with bales, keeping about a 1 3/4" - 
2" gap between the bales and glass.  You could dip the bales before 
placing if you can't easily remove the glass temporarily to render 
inside.  If you don't have the space or budget to add the bale wrap, 
then as I said, the thermal mass is the priority.  A half-way point 
would be to coat the exterior of the mass wall with a thick straw-clay 
mix; heavy on the straw.

The wall to ceiling joint would need careful consideration and that 
depends on the angle of the roof and the method of insulating / covering 
the roof which depends on the structure you have to work with.  Also, 
why are you going to put wood on the outside?  You don't need it from a 
thermal point of view.

There are a lot of assumptions here, but I hope it helps.

All the best,
Andrew


Kate Bastedo wrote:
>    Hi all,
> ..   I have a greenhouse.