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DIY Bedroom Internal BISF Insulation Project by Member ED

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Internal Insulation Project of BISF Bedroom by Ed
BISF House has a number of valuable members who take the time to share project ideas, thoughts and designs for the benefit of other members and readers of the site.

In this latest update we follow one such member as he undertakes a project to insulate the rear bedroom walls of his newly acquired BISF House .

Ed knew that his new home required a lot of internal renovation and from the very start he has been determined to keep the buildings original features. He knew that an external insulation system would strip the property of its unique character so internal insulation would be his only option.

Ed obtained as much advice as possible and drew up detailed plans outlining his chosen construction method. He also opened an inspection hole into the existing hardboard wall to expose the timber stud-work and calculate the thickness of insulation required The wall cavity was already well ventilated so solid insulation panels were chosen as they provided excellent insulation properties and continued air circulation.

Ed has recorded his ongoing project in the Your Home section of the website but we feel that his work and dedication requires its own dedicated page so that other members can follow the project. The original posts made by Ed will stay on the Your Home post.

We Shall pick up at the point where Ed starts to check the structure of the bedroom walls.

All of the text below has been written by Ed himself and re-posted here with only minor edits to aid continuity.

I've cut out and removed a piece of hardboard from the back bedroom exterior wall in the corner next to the party-wall to investigate the construction and shed more light on how I'm going to insulate it. I took it out from just above skirting board height and about 50cm high by 100cm long.

It's not quite what I was expecting but it gives me a much better idea of what I'm going to do. I took some photos but they don't really show it very well so I've drawn a cross-section sketch. The timbers are smaller than I imagined, but closer together. Each one is roughly 45x25mm and they are about 50cm apart horizontally and 30cm apart vertically arranged to form a grid. At the steel uprights there are two against the steel vertically. At the base there is a floor plate. The cavity between the hardboard and the inner surface of the steel cladding is 175mm (ie where it is nearest in the corrugation).

My plan is to place Celotex/Kingspan PIR board in the cavity, up to 75mm of PIR would still leave a 55mm cavity for ventilation but the exact thickness will depend on availability/cost. Then 50mm of expanded polystyrene board in the gaps between the studs, then 12mm plasterboard.

I used this u-value calculator:  ..and with the materials described I should get a value of 0.18 W/m2K.

In reality it will not be quite as good as there will be a little cold bridging, but for comparison a new build house has to be no worse than 0.27. I think (the lower the value the better) and inputting the values for the original upper storey BISF construction gives 0.55 assuming the glass fibre insulation is 50mm thick (it's hard to tell how thick is is, as it seems to have collapsed a bit).

Here's my plan (steel and timber original“ rest to be added):-

I may not bother with the polystyrene, as it was a bit of an afterthought., just to fill the gaps between the studwork/noggins.

I'm thinking if the plasterboard is laid horizontally as was specified in the plans, the timber will probably be in the right place for fixing.

I've also made a diagram of the original construction, but I haven't included the glass fibre insulation.

Ed March 12, 2012 at 11:41 am

Yesterday we started rebuilding the back bedroom, and as promised I took plenty of photos. My aim is to refurbish completely to modern standards with extra electrical sockets, ethernet and wall insulation to make the walls at least as good as a newly built house without destroying the BISF character that attracted me to the house to start with.

Here's the room before, decorated in badly hung floral wallpaper that has been stuck over the picture rails etc

On the plus side there is a nice original "shaker" style door and another on the built-in cupboard and a number of other original details

The first job was to roll up the carpet and carefully prize off the skirting boards, picture rails and other woodwork for re-use later. They came off very easily.

The picture rails removed showing edges of hardboard wall covering.

Then the hardboard could be stripped off the walls. It could mostly be removed with bare hands (or gloved to protect from the glass fibre behind).

The original insulation is glass fibre in a brown paper casing. It was actually in much better condition than I expected. It is nailed to the back of the timber studwork with a washer so the nail head doesn't go straight through. The damage in the far corner was where I first opened the wall up to investigate the structure


As you can see the timber studwork/noggins are pretty small and flimsy. They do not match what is in the architects plans, so I guess this part was modified when hardboard was used instead of plasterboard. Interestingly, from the way they are nailed together from the outside, you can tell that each section was prefabricated off-site a bit like a garden trellis, and the whole panel installed in one piece.

The original insulation is glass fibre in a brown paper casing. It was actually in much better condition than I expected. It is nailed to the back of the timber studwork with a washer so the nail head doesn't go straight through. The damage in the far corner was where I first opened the wall up to investigate the structure.

The party-wall is a bit different as the board was nailed to battens applied directly to the concrete blocks.

Next came the messy and itchy job of pulling out all the glass fibre insulation. Underneath the steel is in very good condition it seems.

More to come later as I'm still rebuilding the studwork and putting in insulation, so you can see that too.


Posted : 12 March 2012 11:06 PM
Posts: 1

good job ED , just inherited mother in laws bisf in inverness and looking for all / any info.
do you intend to replace heating system ,ours is mains gas with original ruby cylinder totally goosed .

Posted : 20 January 2023 8:55 PM
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