The easy questions first. The Stormdry products do not block the pores, unlike sealants. This brings me to my first post above where I distinguished between sealants and waterproofers. Tests show that about 2% of “breathability” is lost. Sealants loose about 98% to 100% breathability if I remember correctly.
Any building has to be watertight, as in liquid water, but see below about vapour.
Now the trickier bit!
A structure that was designed to “breathe”, although unintentionally, needs to breathe unless a very skilfully designed and executed conversion plan is carried out by absolute experts. Breathing buildings can be airtight ones at the same time. This is because what these breathing buildings “breathe” is water vapour, not air.
It is a common belief that buildings need air but that isn’t so.
So, how is airtight and breathable possible at the same time? Easy peesy. The gaseous molecules of air are mostly larger than water molecules, so all you need to do is make your home airtight with layers that water can get through (so will hydrogen but we can ignore that) but atmospheric gasses can’t.
Although modern products are available that do this there are traditional ones that coincidentally do too. Or maybe it isn’t coincidence. Our ancestors would have discovered what products lead to a lasting comfy home by trial and error without understanding the science.
As luck would have it, 2cm or more of lime plaster or lime render meet the requirements. Assuming no gaps or cracks. In practice you need gaps to get in and out of and to let the light in, so special attention needs to be given to joins. Also, you can’t quite touch floorboards with the plaster as the boards expand and contract and damage the plaster, but get as near as possible and seal the gap with a suitable modern product. I suggest laying some cardboard on the floorboards and plastering to them and then after removing the cardboard add flexible sealant.