BISF (steel-framed house) insulation

[bath_ed]

Hi all,

I have recently bought a British Iron and Steel Federation (BISF) house, a type of steel-framed house built in the late 1940s. The construction is unusual in that the house has a steel frame in the external walls. Downstairs they have an outer layer of concrete render on a steel latticework, while upstairs they have a profiled steel cladding. Inside, the walls are fibreboard on a timber studwork and between the inner and outer layers there is some 1940s rockwool insulation.

Now, the house is not nearly as cold as I expected - certainly better than the Victorian house with solid, non-cavity walls I have moved from. Nevertheless, I would like to improve it. The fibreboard walls are also inconvenient as they cannot be skimmed, wallpaper has to be applied directly.

My idea was to strip off the fibreboard from the walls, insert a better type of insulation and then put up plasterboard in place of the fibreboard. I have found the site www.bisfhouse.com very useful, and the administrator has done something very similar but was wondering if anyone here has done anything similar.

The usual advice is to reclad the outside of the house, but I'd rather not do that as it's expensive, non-DIY and I think looks ugly (basically it removes or covers up the only features of the house that give it any interest).

Here's a diagram of the construction:

[wookey]

The usual advice is correct.

External insulation is better than internal in all ways except cheapness. It is DIYable if you are reasonably competent and in many ways it's easer than inside because you don;t have to take you house to bits in the same way. The final result is better because you don't have remaining cold bridges at all internal wal junctions. Normally you can put either better or cheaper insulation on due to having more space.

You avoid the potentially serious complications of interstititial condensation, which may be particularly bad in a steel house, because anything vulnerable is on the warm side.

It usually isn't difficult to put back 'features of interest' or a reasonable simulacrum on the outside so no-one will be any the wiser.

I know EWI is a big jump becuase you get attached to the 'look' of a building, but it really is a lot_better so do think quite hard about it, the risks of IWI, and how you might feel about this in say 15-20 years time.

I started off thinking IWI was clearly the way to go, EWI was 'too hard', changed the look of the house, might annoy planners, not DIYable, etc. But I've changed my mind over last 5 years, and despite having IWIed part of the house, I'm planning to EWI the rest (the front in fact, peversely).

[A.L.]

hello,

I would not have a problem with internal insulation in this case as long as you did nothing to reduce external air getting behind the metal skin.

Assuming the diagram is reasonably to scale I would leave the existing mineral wool and fill between it and the plasterboard (100mm?) with more of the same or similar. "Better" insulation is only better in a ratio of 50mm being equal to 75mm of mineral wool and the cost/performance ratio unfavourable.

Interstitial condensation will only occur  where it occurs already (if it does), either on the external surface of the mineral wool or more likely on the internal surface of the metall skin where air movement removes it. You could use foil backed plasterboard or a polythene vapour check behind the plasterboard for increased confidence.

If the existing mineral wool is 25mm thick you could see a 2/3rds reduction in heatloss with an additional 100mm of insulation.

[bath_ed]

Thanks Wookey and A.L.

I realise external insulation is more efficient, but I don't think the savings would ever justify the cost. I use little gas to start with for heating. I have only been here since 16 December but so far we've only used about 40kWh per day. For a quoted price of £8000 for a 3 bedroom semi-detached house, the external insulation would have a repayment time of decades! Covering three external walls of a double-storey house I think would be beyond my DIY skills.

External has the advantage of not disturbing the house interior, but as the house needs refurbishing, that's not really an issue. Where it says 'plasterboard' in the diagram is actually fibreboard, pretty much just hardboard, which makes redecorating difficult. My thinking was if it's coming off to be replaced by plasterboard, I might as well improve the insulation at the same time. We would do this one room at a time, to make it more manageable.

I do not yet know the thickness of the existing insulation. I can see it where I have removed an electricity socket. On the BISF House site they report that it has often collapsed inside the wall. I was thinking of using the foil backed foam board type insulation, but the BISF House admin reported good results using loft type insulation, chosen as it's much cheaper. I'm still waiting for more details at the moment, as I'm not sure how one prevents that from collapsing. Also he reports adding external air vents to increase ventilation to the internal cavity and prevent condensation. At the moment each room has a ventilator just below the ceiling, but these do not make sense to me (though original) as I would have thought you would want ventilation with colder, drier air from the outside to that cavity, not warmer, damper air from inside the house that would tend to condense on the steel.

As for the aesthetics, I realise a lot of people don't like the look of BISF houses, probably because they look different from a 'normal' house and that makes them think they must be bad, and therefore the external insulation is often sold on the basis it being a facelift (fake brick, fake render etc). Also a lot of the insulation companies mislead people by saying the house was only meant to be temporary or intended to last 10 years, making the owner think renovation is urgently needed to prolong the life, when in reality they were always intended to be permanent and studies done in the 80s gave an expected life similar to a traditionally built house.

This is a BISF house soon after construction:-

[wookey]

OK, so IWI it is.

You could use foil backed plasterboard or a polythene vapour check behind the plasterboard for increased confidence.

Sorry, it's absolutely vital. The existing insulation is probably thin. If you are doing all this work you should insulate to a decent standard. (I'd say  U <=0.2 is acceptable). That will make the structure much colder than it has been to date and thus will be below the dewpoint more of the time. If there is air getting into the wall from the house it will be full of water and will condense. You can get an impressive amount of water into the wall this way, which is why airtightness and vapourtightness are very important.

The steel frame will be a monstrous thermal bridge, but if you can go all round the inside and put in a full insulation layer (even past internal walls) it could be made into quite an efficient house. What happens round doors/windows and at eaves/footings?

Don't forget to the bit between ceiling floor level. A lot of the worst drafts go in here (at least in brick buildings)

You shouldn't really be thinking in repayment times. Nothing else you do to your house repays itself (new kitchen, decorating, bathroom, patio, decking, heating). And plenty of people are happy to pay 8 grand for those. Why dislike insulation so much? You should be able to get EWI for £60/sqm.

On the other hand IWI will probably cost about 2 grand (and an awful lot of weekends).

If you want to use fluff (and it is cheap, but obviously doesn't help with airtightness, nor hold itself up) then I guess you'd start by putting up OSB sheets where the fibreboard is, then using something like knauf's ecostud (XPS+OSB) with fluff in-betweem and panels on the front. plasterboard, multipro (rather stronger) etc.

[bath_ed]

I don't dislike insulation, but I don't consider it the same as redecorating, a new patio etc. One doesn't think of a repayment time for things like that because you have them done for the enjoyment they give you, the enjoyment of having a redecorated room is the 'repayment' and you have to decide how much that is worth to you (I certainly wouldn't spend £8000 on a new kitchen for example). Insulation is not typically done for its enjoyment value, one doesn't put up say loft insulation so you can have a look at it every so often and think 'aah that's some good insulation'.

I would certainly put up a vapour barrier to control condensation and that is why I was wondering about removing the internal ventilators as they would defeat it. I'm not sure what happens around the doors, windows and eaves until I remove some board. The internal walls are all board on timber studwork (the house is unusual in that none of the internal walls are structural) so it should be possible to put some insulation in them too where they join exterior walls.

My thinking is that steel is such a good conductor of heat, especially upstairs where the outer skin of the walls is steel cladding, that the outer layer is going to be cold despite any heat coming through from the house due to poor insulation and therefore if there are condensation problems they should have developed already... ie if the insulation is improved, it should not get much colder.

[clockmanFR]

For me, the main concern is the vertical steel I beam, these are problematic (actual experience) regards condensation and you don't want pools of water at the base mount joints.

Therefore for me, i would insulate on the inside.

[bath_ed]

A quick update with my latest findings. While topping up the loft insulation I had a closer inspection of the structure up there. I found that the wall cavity extends right up to the loft ie you can actually look down into the void from up there and see the original fibreglass insulation in there. Therefore there must be excellent ventilation to combat the risk of condensation. In the meter cupboard under the stairs I found a very strong, cold draft coming through the hole in the plasterboard where the electricity comes in, which I plugged up.

[bath_ed]

Just been up in the loft again on a cold day and found there is condensation on the inner surface of the steel cladding in the gable. Inside the loft the gable wall consists just of the steel cladding on the steel frame - the inner layer stops at ceiling height. I also put my arm down into the wall cavity and there is condensation there too. This is despite there being good ventilation in the loft - the roof has all sorts of gaps in it at the eaves and around the gable, some big enough for birds to have got in and nested.

Not sure how concerned I should be - there is little evidence of corrosion on the exposed steel after 62 years, but it does make me want to install a vapour barrier.

[Sean]

just out of curiosity, did you get a mortgage for this purchase - often up here folks cant, due to the lenders surveyor not liking the non standard construction methods

[bath_ed]

just out of curiosity, did you get a mortgage for this purchase - often up here folks cant, due to the lenders surveyor not liking the non standard construction methods

This is an interesting one. I approached all the main lenders to ask if they were happy with a BISF house and they all said yes. Attitudes to BISF houses appear to have changed quite a bit as they have been found to have similar life-expectancies to traditionally-built houses, structural engineers report few to no serious problems even after more than 60 years.  I chose Nationwide because they offered the lowest rate for the loan-to-value ratio but then their survey declined it at the survey because of the asbestos roof (which they had not mentioned they would not accept). They said they would have been OK with it apart from the asbestos roof as they will not lend for any house with an asbestos roof.

In the end I could only find Halifax would accept an asbestos roof.

You do have to be a little careful of surveyors not knowing about the different types of non-traditional construction. I've heard of surveyors rejecting BISF houses as they've wrongly identified them as a type of house made of precast concrete panels that I forget the name of, which had serious structural problems by design, even though it looks completely different from a BISF house even to the untrained eye! (You'd think a surveyor could tell the difference between corrugated steel cladding and concrete panels).

The trouble is that there are a lot of non-traditional types of construction and they don't really have much in common apart from being non-traditional. Some were found to be seriously defective and were listed as such in the Housing Defects Act 1984, but a lot of people and even some surveyors etc don't distinguish the good from the bad and mix them all up.

The suspicion of non-traditional construction was actually a great help to me as of the six(!) houses I could afford in Bath it was the only one I actually liked - 3 bedroom semi of a reasonable size, useful outhouse, nice size garden backing onto fields, nice neighbours with gardens not filled with junk 

There's a very informative site about the BISF house at http://bisfhouse.com/

[bath_ed]

Just out of interest, where about are you, as I'd be interested to know where else there are BISF houses.

[Sean]

Just out of interest, where about are you, as I'd be interested to know where else there are BISF houses.

Orkney, but my question wasn't related specifically to BISF here

[bath_ed]

Just out of interest, where about are you, as I'd be interested to know where else there are BISF houses.

Orkney, but my question wasn't related specifically to BISF here

I know there are some in Port Glasgow, but that's the only place in Scotland I know that has BISF houses.

[A.L.]

as I'd be interested to know where else there are BISF houses.

- east end of glasgow and small numbers elsewhere in central belt of Scotland