Creating access hatch in existing beam and block floor

[embra]

Hi everyone,

 

Looking for some advice on creating an access hatch in an existing beam and block sub floor. There is currently a ventilated void of 300 to 380 mm (depending on interface with beams) beneath the subfloor, with no access to the void space. Our property was originally built ~1990 and is located in Edinburgh, Scotland. Ideally I would like to create an opening at least 400 x 400 mm.

 

Is this as simple as cutting a couple of blocks out with a disc cutter? Anything else to be aware of? Beams would be left alone, so the opening would be limited by the beam centres in one dimension.

 

Thanks in advance!

[ETC]

Why do you want access?
 

I presume you’d be cutting through the screed, insulation and DPM?

[Dave Jones]

depends which way they laid the blocks,  if its longways then you need to be skinny <200mm

 

You will also be comprimising the DPM. What reason do you have for accessing it ?

[embra]

29 minutes ago, ETC said:

Why do you want access?
 

I presume you’d be cutting through the screed, insulation and DPM?

 

6 minutes ago, Dave Jones said:

depends which way they laid the blocks,  if its longways then you need to be skinny <200mm

 

You will also be comprimising the DPM. What reason do you have for accessing it ?

 

Reason for access: I'm investigating adding insulation below the beam and block floor, since there is none at present.

 

Current floor construction from top to bottom: carpet, 18 mm chipboard, ~50 mm wood battens @ 400 mm centres, concrete beam (180 mm) and block (100 mm), 300 mm void, DPM, ground. Therefore I would not be breaking any existing DPM. 

 

There is more than 200 mm between beam centres, however I haven't managed to measure centres exactly. 

 

[Dave Jones]

your going to have the same issue as the other guy here trying to do the same thing. you somehow have to get a continuous dpm below the insulation lapped into the inside skin.

 

somehow need to keep those blocks dry after covering them with insulation.

 

 

[ETC]

Look into this very carefully and read the fine print from the installer closely.

 

From the previous poster the system seems to need to have the DPM overlap the DPC and totally encapsulate the proposed insulation in a DPM/DPC barrier - not easy in a refurbishment project.

Building Control will want a BBA Certificate for the system and will want to ensure that the system has been tested and will not contribute to any future deterioration of the existing building fabric.

 

Why on earth is there no insulation in your existing floor?

 

You might want to look into:

 

Laminated or solid timber floor planks on

a VCL on

50mm floor insulation (Kingspan OPTIM-R or other high performance insulation).

[Redbeard]

I have no experience of Scotland but in England it was still OK in 1990 (+) to use 'teeming and ladeling' with Bldg Regs, so you were allowed to get away with not insulating the floor if you (allegedly?) bumped up the wall insulation. I worked for a hsg ass'n at the time and although we used to build our owmn we were outbid on one site and ended up buying the pair of spec-built semis which were built on it. I soon got in the habit of taking off my jacket and one pullover before going in because the house was always at 25+ degrees. Why? Because they were always trying to get warm feet. Never did, really; just sweated from the knees up.

 

Re the suggestion of floating floor the problem is that it puts your stair out of compliance with Bldg Regs unless you have the space to make the bottom stair a 'landing' (can't remember the dimensions)

[ETC]

1 hour ago, Redbeard said:

Re the suggestion of floating floor the problem is that it puts your stair out of compliance with Bldg Regs unless you have the space to make the bottom stair a 'landing' (can't remember the dimensions)

Not necessarily - work from the existing floor level down and increase the floor insulation accordingly.

[Redbeard]

@ETC, I take it you mean 'work from the existing *first* floor level down'. Of course we don't know the ceiling heights, but losing 200 might be significant.

[ETC]

12 minutes ago, Redbeard said:

@ETC, I take it you mean 'work from the existing *first* floor level down'. Of course we don't know the ceiling heights, but losing 200 might be significant.

No - incorrect. This post is about insulating the ground floor and the poster has posted:

 

“Current floor construction from top to bottom: carpet, 18 mm chipboard, ~50 mm wood battens @ 400 mm centres, concrete beam (180 mm) and block (100 mm), 300 mm void, DPM, ground. Therefore I would not be breaking any existing DPM.”

 

What I am suggesting is removing everything from the top of the block and beam flooring and installing a floating floor above this - OP has approximately 68mm-78mm to play with (carpet (8-10mm) + 18mm chipboard + 50mm battens). Replace all this with timber flooring (approximately 18mm - 22mm) and insulation (whatever space is left). 
 

If I had time I’d draw a sketch - but I’m a bit busy at the moment.

 

Staircase and doors not affected.

[Redbeard]

@ETC, sorry, got that now. I had missed the ref to the lay-up on top of the B & B. That would work quite successfully. Without knowing the house we can't know the perimeter/area ratio, so cannot calc the U value with, say, 50 - 60mm insulation, but it will be a hell of a lot better than it is now.

[embra]

18 hours ago, ETC said:

No - incorrect. This post is about insulating the ground floor and the poster has posted:

 

“Current floor construction from top to bottom: carpet, 18 mm chipboard, ~50 mm wood battens @ 400 mm centres, concrete beam (180 mm) and block (100 mm), 300 mm void, DPM, ground. Therefore I would not be breaking any existing DPM.”

 

What I am suggesting is removing everything from the top of the block and beam flooring and installing a floating floor above this - OP has approximately 68mm-78mm to play with (carpet (8-10mm) + 18mm chipboard + 50mm battens). Replace all this with timber flooring (approximately 18mm - 22mm) and insulation (whatever space is left). 
 

If I had time I’d draw a sketch - but I’m a bit busy at the moment.

 

Staircase and doors not affected.

 

50 minutes ago, Redbeard said:

@ETC, sorry, got that now. I had missed the ref to the lay-up on top of the B & B. That would work quite successfully. Without knowing the house we can't know the perimeter/area ratio, so cannot calc the U value with, say, 50 - 60mm insulation, but it will be a hell of a lot better than it is now.

 

It's an end of terrace, ~9m front to back, ~8m side to side. Party wall is ~9m in length.

[Redbeard]

So area is 72m2, exposed perimeter 25m. P/A ratio = 0.35. Therefore likely base-case U value 0.56W/m2K (R = 1.78m2K/W). On the face of it adding 50mm PIR (R=2.27m2K/W) seems to give a U value of around 0.25 (a pass!), but how it actually performs will depend on a number of factors including whether the B & B floor is ventilated. 

[ETC]

7 hours ago, Redbeard said:

@ETC, sorry, got that now. I had missed the ref to the lay-up on top of the B & B. That would work quite successfully. Without knowing the house we can't know the perimeter/area ratio, so cannot calc the U value with, say, 50 - 60mm insulation, but it will be a hell of a lot better than it is now.

Irrespective of the p/a this project will be limited to the physical space available - that’s why I am suggesting something like the Kingspan Kingspan OPTIM-R or something similar. The poster will only get a U-Value that can be obtained by the floor build up with an insulation with the greatest thermal resistance.

 

A bit of research is required.

[embra]

10 hours ago, Redbeard said:

So area is 72m2, exposed perimeter 25m. P/A ratio = 0.35. Therefore likely base-case U value 0.56W/m2K (R = 1.78m2K/W). On the face of it adding 50mm PIR (R=2.27m2K/W) seems to give a U value of around 0.25 (a pass!), but how it actually performs will depend on a number of factors including whether the B & B floor is ventilated. 

 

8 hours ago, ETC said:

Irrespective of the p/a this project will be limited to the physical space available - that’s why I am suggesting something like the Kingspan Kingspan OPTIM-R or something similar. The poster will only get a U-Value that can be obtained by the floor build up with an insulation with the greatest thermal resistance.

 

A bit of research is required.

 

I spent longer than I care to admit running through the MC heat pump / heat loss calculator yesterday. This allowed me to carry out a sensitivity analysis of insulation thickness vs whole house heating cost, and therefore determine payback period. I used Kingspan K103 (thermal conductivity 0.019 W/mK) as a basis, with 25 mm insulation as a starting point. Helpful perspective to avoid chasing diminishing returns.

 

@Redbeard the beam and block is ventilated - is there a recommended approach for factoring this into u-value calculation?

 

I did investigate Optim-R in the past (thermal conductivity 0.007 W/mK), initial discussion with Kingspan revealed the cost would be ~5x that of K103, meaning the payback just isn't there in our case.

 

 

 

Edited April 17, 2023 by embra