floor void sleeper wall access / lintels

I am looking at insulating my ground floor in a 1930’s house, which has a suspended timber floor. I plan to install 75 or 100mm rigid insulation between the 4 inch joists resting on battens (I believe there is no need for a vapour barrier).

Last night I opened up the floor void in one of the rooms to find out how big the floor void is and what condition the void area was in.

Thankfully it’s a pretty deep void (36cm clear under the joists), dry and joists are in good condition, there are clear air bricks In the external walls and ventilation is good.

What surprised me was the amount of sleeper walls in the one room I looked at, in a 3.5 x 4m room there are 3 sleeper walls (across the 3.5m).

The size of the void means that I can get under the floor and insulate from below, which is a relief as I don’t want to be pulling up all the boards.

However what I do with all the sleeper walls I have to go through is a concern and I need advice on.

I plan to put an access / hatch in the cupboard under the stairs which is in the centre of the house, and access each room from this location. Getting access to each room by going through sleeper walls under the doorways to each room (not under load bearing internal walls!), and then through the sleeper walls in each room.

I want to make the access permanent so that I can get access easily in the future, so I am thinking that I put a concrete lintel above the new accesses (40cm wide).

The sleeper walls are 4 bricks high so where I put the access I would remove the bricks and put a lintel over the top (DPM between lintel and joist).

http://www.diy.com/departments/expa...d-concrete-lintel-l600mm-w100mm/150789_BQ.prd

Would I need to do this in all access holes, or could I just limit it to the doorways (assuming floor access holes under different joists)?

Thoughts and suggestions?
WN