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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: 472

Here's the wall with the glassfibre insulation removed. You can see the trellis-like original timberwork better now and the red-painted back of the steel cladding. The splashes on it are mortar from when the party wall was built.

I cut out all of the original timber except for the frame of each panel (ie the bit against the ceiling, floor and steel uprights), as it's very thin. The wood is held to the steel uprights with special clips so it's best to leave those pieces in and screw your new timbers into them, as attaching to the steel would be difficult.

A little tip I got from an energy efficiency and insulation forum was to insulate the back of the steel uprights with 25mm expanded polystyrene (jablite). I cut 60mm wide strips and applied them with instant-grab adhesive and wedged them in place where necessary. I'm not sure how much difference this will make, but the jablite is very cheap though it is a fiddly job!

Next I put in 70mm Celotex PIR insulation board. It comes in a number of other brands such as Kingspan and different thicknesses, but 70mm seems the ideal width to fit into the cavity and still leave a 55mm cavity in front of the steel cladding for ventilation. 70mm also fits into the steel C-section uprights if you notch the edge slightly.

I forced the Celotex down below floor level until it sits on the steelwork just above the ceiling in the room below as I thought that would mean it's accessible when I do the downstairs. Ideally I think you'd do downstairs first. I also put it so it was sticking up into the loft a little so it forms a little barrier to stop the loft insulation blocking the ventilation under the eaves (as the pitch is shallow, it's hard to get under there to see if you've left enough of a gap when putting the loft insulation down).

In front of the Celotex I've started building the new studwork to put the plasterboard on using 63x38mm CLS timber. I have formed a frame screwed to the original timber at the floor, ceiling and steel uprights and then put in uprights at 600mm centres with noggins roughly 600mm too (as I found, it seems better to make the noggins very slightly closer together to fit the height of the inner insulation rather than to have to use two pieces for each opening).

As a slight change to my original plan the inner insulation is 50mm Kingspan PIR board which came cut to 1200x450mm pieces.

This post was modified 4 months ago by Admin
Posted : 13 March 2012 9:18 AM
Posts: 68

Hi Ed, is it OK to post here?

Once again a very nice set of pictures, there has been much talk about insulating the internal walls of BISF houses but this may well be the only step by step style project to date.

I imagine this will be an extremely useful page for those wishing to carry out this work on their own BISF houses and I can't wait to see the finished article :).

Do you have anything special planned for the party wall such as sound proofing etc?

Posted : 14 March 2012 3:21 AM
Posts: 472

Hi Denton, of course it is!

I was a bit surprised that i couldn't find anything about this on the internet and Marc was the only person I could find who had any experience of it. There must be over 30 000 BISF houses still in existence so I expected to find more information.

I hope it will be useful to other BISF residents, as it should be within the means of many DIYers (bearing in mind I was shown how to build the studwork by my dad, but after that have been doing it myself).

Also, replacing the hardboard with plasterboard is a good thing to do in itself for redecorating and adding the extra insulation doesn't make it that much more difficult or expensive.

I haven't decided how to soundproof the party wall. Any suggestions? It's not in much need of extra soundproofing as there is a double layer of concrete blocks plus an air cavity and then board on each side, so I think it's better than an average party wall, but I thought while I'm reboarding I might as well improve it. I was thinking of maybe polystyrene-backed plasterboard.a

Posted : 14 March 2012 10:08 PM
Posts: 68

Hi Ed,

Indeed, all I could find during the (lengthy) purchase of my house seemed to be unanswered questions and opposing views on insulation/condensation problems.

I would have been thrilled to have found this website when researching BISF houses for the first time & project pages like this would have been marvellous.

Replacing and adding insulation to the internal walls is definitely a good idea, I'll get a few pictures when trying this myself as our houses seem to be different in many ways.

I haven't yet had chance to look at sound insulation in depth but disconnection of an attached solid surface (such as plasterboard) using some sort of sandwiched dampening membrane seemed to be a route. It's something I need to look into for the neighbours' sake rather than mine as they seem pretty quiet.

Posted : 15 March 2012 1:42 AM
Posts: 472

Just a quick update on how I'm building the new studwork. Here it is almost finished. I have put in a second layer of 50mm Kingspan board between the timbers instead of the expanded polystyrene I originally planned. I found it easier to put the Kingspan in and then the noggin on top and so on rather than build the studwork first and then put the Kingspan in afterwards.

You can see the Celotex behind where I haven't quite reached the top yet. This bit was a bit trickier as the original woodwork at ceiling level was not attached to the steel joist above.

This post was modified 4 months ago by Admin
Posted : 18 March 2012 9:14 PM
Posts: 1223
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Considering this is your first project I must admit that I am very impressed by your high standard of workmanship and attention to detail.

This is probably the very first BISF house, internal insulation guide that has ever been published and your project certainly sets the standard. I sincerely hope that other BISF owners find your project as useful and as interesting as I have to date.

I know how much research you have undertaken to get to this point and this is clearly evident in many ways. I think the Jablite on the back of the steel uprights is an excellent idea and this should eliminate all risk of thermal bridging. Your stud-work looks clean and precise and I have no doubt that despite your outlay on materials so far, you will easily recoup this cost in a relatively short time.

Your steel cladding panels and support rails also appear to be in excellent order. It can often be beneficial to give the visible steels a quick coating with a rust prevemtion paint at this stage but to be honest your steels don't look like they need this. Whilst you have the structure exposed it's a good idea to pay particular attention to any nuts and bolts that may be visible as these were often not coated or galvanised but judging by your attention to detail so far you have probably already done this. Smile

Keep up the good work my friend and I look forward to seeing more of your work!


This post was modified 4 months ago 3 times by Admin
Posted : 20 March 2012 12:07 AM
Posts: 68

I agree with Marc completely - the quality of work and attention to detail is astounding. I bet the room feels warmer already! :D

Finnegans (now Hammerite) No.1 could perhaps be a good paint to treat any visible rust spots, I've used it on cars in the past and it seems to perform well.

Looking forward to your next update.


Posted : 20 March 2012 10:47 PM
Posts: 1223
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Hi Ed,
How is the project coming along?

Did you overboard the ceiling or did you opt for a clean install? I ask because sadly I have the exact same job coming up very soon and I haven't yet decided which way to go.
I know over boarding will be less messy but those noggings can be pesky blighters to accurately screw into. Smile

Posted : 12 April 2012 11:27 PM
Posts: 0

Hi Marc, sorry for the slow reply! Things are coming on a bit slowly as I'm having to fit it in between work and don't want to start too early or carry on too late to avoid upsetting the neighbours, who have young children.

In the end I removed the fibreboard from the ceiling completely and replaced it with plasterboard. It's very soft and spongy and can be broken up by hand.

I found a few things that complicates it a bit (well, I knew they were there, but hadn't really thought of them before).

Firstly, the noggins are just notched and fitted into the steel beams so they can be moved around. They are not attached at all. The old boards span the whole room, so there are no cross timbers to butt two plasterboards up against (they are bigger than modern plasterboard sheets). Also, as the noggins are loose, I thought the ceiling would probably crack if you ever stepped on the noggins in the loft.

Secondly, the old ceiling boards were obviously put up before the walls as they are continuous from one room to another. It's the same with the hardboard on the party wall, which continued behind the built-in cupboard.

What I have done is put new timbers in at right angles to the old noggins to stop them from moving about and to provide something for the boards to meet on. It's also quite fiddly to break off the old ceiling boards above the internal walls without leaving any sticking out.

It might have been easier to put up the plasterboard on top of the fibreboard, but it doesn't seem like such a good way of doing it.

This post was modified 4 months ago by Admin
Posted : 13 April 2012 5:11 PM
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