Canvas is a wonderful product that is very healthy for our horses because IT BREATHES. The drawback to this breathability is that in certain conditions canvas can also let water through.
When we line a canvas with wool or felt, the wool/felt acts as a buffer or air lock, which is most circumstances keeps the water on the outside of the rug.
With a UNLINED CANVAS rug we have two situations that can cause the canvas to let the water in.
First, if the canvas is touching the horse, the warm body heat of the horse actually draws the water inwards (the same as a tent does if you touch the inside of the canvas wall or roof with your hand).
Second, our horses produce a lot of body vapour that can be trapped on the inside. If you have a cotton sheet under the unlined canvas this may help to stop the water drawing through to the inside because the canvas is not in direct contact with the skin, but may absorb any vapour or condensation from the horses body before it has a chance to escape. This happens when the rug is very cold on the outside and warm on the inside (from the body heat of the horse). Have you ever experienced condensation on the inside of an unlined stable or shed. You can get rained on inside because of the inside warmth and the outside cold coming together.
I use my unlined canvas' extensively. I always have a cotton sheet against the horse, no matter what is on the top. They are used most nights through the summer months and days when there is a possibility of a shower. Sometimes the weather can be very warm and showery, I find even if some water has managed to get through the horse is still dry. Spring and autumn often finds my unlined canvas over a woolen under rug, firstly for the extra warmth and also because there is more possibility of rain or showers at this time of year (bearing in mind I live in South Gippsland).
Naturally, if the weather is cold or wet enough then the horses go into a light lined canvas or their Millenniums, whichever is appropriate for the time of year.