We have several problems that we need help on.

1.
Sub floor.
The house is raised (above a double brick garage and concrete stilts). The sub floor is floating pine, no underfloor insulation. The pine is not completely sealed around the edges.

How do we approach this situation, just lay underfloor insulation, or air tight then insulate? If we air tight what do we use, acrylic caulk?

2.
Thermal mass.
The sub floor is double brick, half of it is acting as a retaining wall for the soil as its built on a slope. There is a stairwell in the middle of the house that goes downstairs into the garage, half of it is exposed double brick. It's literally freezing, often feels colder than outside. There is no space in the stairwell to insulate it.

There is no cavity between the double brick.
Could we use a reverse brick technique where we wrap insulation around the outside of the brickwork and use it as a thermal storage?

Is there any point as it always feels cold because it's below soil level, so not sure if insulating will do anything.

Also, in order to promote thermal storage, wouldn't it need to gain the heat form somewhere for a period of time.

Hatice, you should post a dilemma, saying which country you are in, so people can get the details right. :)

@Hatice Chooky is right. That's quite the dilemma. Put it out there. Far fewer people will see it here to offer advice than if you post your own dilemma.

Chooky?...I was wondering if double hung means the same as sash windows?

In double hung you can open at top and bottom. Good for letting the heat out top and cold air in bottom.

Here is the answer. We have double hung here.

https://en.m.wikipedia.org/wiki/Sash_window