Strangely, by trying to keep the roots cool, this can actually slow growth.
In the wild, cannabis prefers open areas and does not grow in rocky, cold crevices.
Concrete is also highly alkaline and this can lead to major pH problems, again slowing growth.
They LOVE sun and can cope with amazingly high temps too, so no need to "baby" them or wrap them in cotton wool.
Open, friable soil structure is best for rapid growth.