BISF house insulation

Postby martynfogg » Wed Nov 14, 2007 4:54 pm


I own a BISF house, essentially a steel frame prefab. The steel frame is covered with sheet metal cladding and/or render on the outside and stud wall on the inside.

What would be the best way to add or increase wall insulation?


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Simply Build It

Postby Simonhayes » Tue Jan 01, 2008 7:31 pm

I own one of these houses too and I am wondering about increasing the insulation in the walls. If you take the plasterboard off on the inside of the wall you will see that there is rock wool insulation hanging inbetween the framework. It is tempting to just get some thicker rock wool insulation and just pack the void out. However I am concerned that this could cause the timber and metal frame to 'sweat' if water vapour gets in and this could lead to rot/rust. At the moment the thin insulation allows air to circulate.

I am not an expert, but if you ever get any guidance from an expert, perhaps you could let me know. At this point I would suggest that the best way to insulate the walls would be to replace the pasterboard with some new plasterboard that has insulation fixed to it, I think its called Jablite. Infact rather than removing your existing wall covering, you can probably fix Jablite to the existing wall.
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Postby martynfogg » Fri Jan 04, 2008 3:50 pm


Thanks for the reply. I had the same concern as you about adding to the 'cavity' insulation, I think it may form a bridge for moisture which would be lethal for the metal frame.

Other options I have considered are an external cladding with integral insulation, which is expensive, or dry lining all the walls. Insulated plasterboard would be good, although I think Jablite is just insulation, you have to plasterboard on top of that.

If I get any good advice or other info, I'll post here. It's good to get in touch with other BISF owners. I'm in Bath, where there are around 20 BISF houses (to my knowledge), although mostly Housing Authority owned. Whereabouts are you?

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Postby noseyparker » Mon Aug 17, 2009 3:04 pm

guys, you are both right to be concerned about condensation forming in the cavity if you change the insulation. dont have any experience of these houses in particular, but if you were to increase the insulation you may also need to get the work checked and signed off by the Council's building control dept. as the work is covered by building regulations.

you would certainly need a vapour barrier if you were taking the plasterboard off and increasing the insulation behind it; this could be as simple as using foil backed plasterboard, or fixng polythene sheeting on the room side of the insulation and beneath the plasterboard. you could consider using an insulating plasterboard (polyurethane foam on the back of it), or lining on the room side of the existing plasterboard with "Sempertap" from Mould Growth Consultants Ltd. which is itself a vapour barrier. You may need to provide some ventilation into the cavity too.

strongly recommended that you get some expert advice from specialists on what to do, before you do it. should not be insurmountable. but "interstitial" condensation - when it forms in the cavity - can be very damaging, causing wood rot (rust in a BISF house?) etc.
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Postby BrianM » Thu Mar 04, 2010 7:53 pm

The problem with attempting to insulate inside is that any moisture within the building from cooking,showers etc will still condense on the inside of the steel sheets and render. (similar to what used to happen on single glazed windows.This then ponds and can cause corrosion at the bottom of the steel columns. The only way to overcome this is to use a Structural Cladding System - see Structherm's website - these are the only company that has a system approved for these house types. There are various other insulation systems that may be cheaper to install - but these aren't designed for this house type and will probably fail due to the expansion/contraction. The BISF houses were only meant to last 20-30 years - they're 50 years old now and really need a bit of money spent on them to extend their life and bring up to modern day insulation standards.

Hope this helps
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Postby booshank » Thu Nov 03, 2011 8:56 pm

Actually the idea that BISF houses were only intended to be temporary is a complete myth and probably arose because there were other types of prefabricated house that were only temporary. BISF houses were designed for, and have been found to last as well as traditionally built houses of a similar age, provided that they have suitable maintenance. The reason for their construction was not because they were cheaper (they were in fact slightly more expensive than traditional houses) but because they could be built quickly, as they required less skilled craftwork.

I do not understand why increasing the internal insulation should lead to more condensation. I would have thought it should reduce condensation problems. I am no expert on insulation, but I'd like to see an explanation.
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Postby booshank » Thu Nov 03, 2011 9:04 pm

I'm buying a BISF house in Bath, and as they are all in the same area I guess it's pretty close to yours Martyn. Did you get anywhere with the internal insulation, as it is something I'm interested in.

I'd rather try and insulate from the inside, because it will probably be cheaper and something I can do more of myself rather than having to get a specialist in. Also, although many people seem to think tacking some sort of cladding on the outside of the house improves its appearance (perhaps because they want to hide the construction method as shameful) I think it usually makes the house look worse.
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Postby welsh brickie » Fri Nov 04, 2011 7:27 am

you could remove the plasterboard,the fibreglass and staple iso super10 to the studding then plasterboard.
Removing the fiberglass will allow adequate airflow for any moisture to evaporate.
welsh brickie
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Postby booshank » Fri Nov 04, 2011 9:23 pm

Thanks for your suggestion Welsh Brickie. This is pretty much what I was thinking of doing, but I was a bit concerned about reports of condensation problems. I take it that it's necessary to leave an air gap between the super10 and the external leaf of the wall to allow airflow?

Sounds as though it should be a reasonably straightforward and cheap job for me and my dad, who is a builder but hasn't come across a BISF house before.
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Postby Perry525 » Sun Nov 06, 2011 6:43 pm

There is every possibility of condensation forming inside a wall.
The way this works is, water vapour is created inside the home by the people who live there.
Water vapour is programmed by nature to move from warm to cold or a low pressure area or both.
Homes are in the main usually warmer and wetter inside and the world outside is usually colder and drier, add to this, the passing wind creates an area of low pressure above and to the lee of the home.
Moisture moves outwards.
If you add a closed cell insulation, polystyrene/Styrofoam/jablite etc: more than three inches thick to the outside of a wall/roof/floor the fabric of the home stays warm and the dew point moves outside.
Condensation does not form on any surface where the temperature is above the dew point.
In most homes condensation forms on windows as these are usually the coldest surface in a room.
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Postby booshank » Sun Nov 06, 2011 8:38 pm

My thinking was that the exterior steel cladding will be such a good conductor of heat that the surface that faces the wall cavity is going to be cold no matter how poorly insulated the interior leaf of the wall is. Therefore, improving the internal insulation should not cool that steel surface significantly and therefore there should be no significant increase in condensation problems... or, if condensation is going to be a problem it should have shown itself by now.

I'm just thinking of the physics of it, but I don't have any experience of this, so I'm wondering if anyone else has experience of this type of insulation problem.
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Postby booshank » Sun Feb 12, 2012 9:13 pm

When topping up my loft insulation I found that the cavity in the walls of a BISF house extends right up to the loft, which if the roof has its original asbestos sheeting is extremely well ventilated. Therefore I would say that wall cavity is very well ventilated (if you make a hole in the internal plasterboard lining you get a strong cold draught coming in). Thermally, you could almost treat that cavity as external in my opinion - the outer steel cladding just provides waterproofing. Makes me feel much easier about the risk of condensation.
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