I am in the lucky position of building my own house in Southern Ireland and have have a 'winter garden' room which will have three tall walls of glazing with one side facing south. The roof is very high so a lot of light will be let in. I plan on growing various shrubs, climbers and plants that grow in temperate regions such as Kenya (where I am originally from hence the choice of planting) + some tropical plants.

I have two main questions:

1 - The glazing in the house will be triple glazed with a UV % rating of 20 Tuv - I know that the temperature, humidity and light the room will receive will dictate what will grow well or at all in this region - but what impact will this type of glazing have? I may be able to opt for different glazing (e.g. double glazed instead). The glass has a U-value/Light/Energy rating of 0.5 / 62 / 42

2 - I will have limited access to fancy soil - can anyone recommend a sensible soil mix that can be sourced here in Ireland for this type of planting and anything I should be aware off (generally, I am asking what soil composition I should be choosing).


3 Answers 3


more light = more growth, although I'd imagine if you are growing this for personal edification rather then produce yield, this will be much less of a factor. If I understand the Light value of 62 to mean 62%, this figure does seem very to me to be very much on the low side - I'd have expected figures > 80%.

I suspect you are aware that light has multiple wavelengths, but I'm not sure if you are aware that plants respond mainly in certain wavelengths (blues and reds) - so it is the transmission of light through these wavelengths that is important.

Another consideration might be artificial lighting - Googling leeds me to believe you only get about 1600 sunshine hours per year, and, of-course, very short days in Winter. Plants like a LOT of light (a typical sunny day would be 50000-100000 lux, a well lit indoor area typically 1000. If money is not an issue you may want to consider getting some growing lights [ height adjustable ones, or woven among the plants ] so you can get adequate lighting in the middle of winter. (I doubt it makes sense to try provide that level of lighting to the entire place - and the further the light from the plant, the more wasteful it is).

Also consider ventilation - I understand that in the height of summer it can get well over 25c outside - unless you have good ventilation the area could get to hot for the plants to handle ! (I don't know how expensive triple glazing is to double glazing - but it might be worth considering getting a heat pump instead - which will give you better control of temperatures throughout the year - including cooling in the heat of summer - which (at least for Eggplants, which are a tropical crop I'm familiar with) would be advantageous.

  • Thanks for the comments/feedback. I was looking at indoor growing lights and there are plenty of options/types to consider but my main concern is the amount of lighting needed to be of any impact without costing a small fortune in electricity bills (even using LEDs). Regarding ventilation, I have 4 large windows that will have motors on too open and close them depending on the temperature
    – Damien
    Apr 10, 2016 at 15:28
  • The house is a passivehaus with a heatpump heating / cooling system built in with airvents and extraction points in various rooms. However, the winter garden is not considered part of the airtight envelope of the house so currently has no vent/extractor in it. The reason is that it has the potential to overload the system and we wanted to be able to seperatly open windows just in this area....but thinking about it, a I might look at a standalone heatpump in just this space - good thinking! However, it is veeery rare for 25deg days here - perhaps a few per year :)
    – Damien
    Apr 10, 2016 at 15:35

Each pane of glass will reduce the number of lumens coming in which means less light for your plants, which as close to the arctic circle as you will be will starve your plants for light during the winter months.

I would opt for double glazing, take the savings from that and invest in a high quality wood stove that will allow you to keep the glass house warm and also allow you to keep large pot of water on top to keep the humidity up and give you warm water for your hands.


First of all, I want to caution you regarding the amount of humidity a grow room is going to produce, especially if it's heated. Depending on what your walls are made out of, the humidity may eventually damage them or encourage mold on them. So, make sure that's accounted for. I've had a problem similar to that before, but you probably have more light in the room to help keep mold away. If you can get heat mats, or heated pots, that may be a lot safer than heating the room, since it shouldn't make the room as humid. Edema should be less of a problem if the air is cooler, and less humid, too. Insulated pots can also be helpful. The coldest air goes to the ground (that's how frost happens outside when it's not freezing where the thermometer is). So, raising plants higher may help them stay a few degrees warmer, too.

Secondly, I'm guessing the UV index is pretty low where you are in Ireland. You know where you are specifically; so you can find out. Although UV rays do damage plants, they can also make them more nutritious. They can also kill microbes that could be bad for your plants and house. I imagine they can also damage (crack/fade) things like wood and plastic over time, unless it's designed to reflect the rays, maybe. They can also help humans produce vitamin D. So, if you have stone walls/floors that wouldn't be damaged by UV rays, I recommend not worrying about blocking them much. If you have lots of stuff that would get damaged, blocking them would be pretty smart. The glass may or may not already block UV rays without the glaze, too. I'm not sure how you'd test it.

Stores sometimes have large bags of compost you can buy. I'm not sure about in your area. That might help out with the soil fertility considerably. You might visit a garden center, see what they have, and ask them what they think.

  • All four walls in the room at glazing. There is some low level block work that will be rendered. Mould and humidity has been considered in the finishes of the room. The beams supporting the glass are timber but treated as well as the soul plates. The glass as spec'd at the moment has a high level of UV blocking (I read it as 20% will make it through). I have chatted to a few garden centres locally but they're pretty useless on advice so might chat to a national botanical garden staff and see what they suggest.
    – Damien
    Apr 10, 2016 at 15:39

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