Steel Barn conversion

[jen and mark]

Good afternoon!

 

We are in the process of converting a double steel portal barn in to our new dream home. I have a quick question that I hope you guys can shine some light on. Looking at Part B, fire regulations, do we need to paint our frame in Intumescent Steel Paint? It is a single storey, single dwelling.

 

Many thanks

 

Jenny

[IanR]

I don't believe you need to fire protect the steel structure, in order to protect the property itself, but you do if there are other buildings close by that yours could collapse on.

 

That was the rationale that the Building Regs chap took with ours, so I didn't ask too many more questions.

[saveasteading]

I know about new steel buildings but not if these rules apply to conversions.

If it was new, then it has to be built such that it does not allow the spread of fire across the boundary. For that, it is necessary to either:

 

1. protect the whole steel frame with either intumescent paint (very thickly and probably expensively, or plasterboard/masonry/whatever.

OR

2. Use an extra strong column and very big foundation so that it stands in place* whatever the fire condition. 

AND fire-proof the wall internally on that face.

ie your building is allowed to collapse internally but one wall stays and the neighbour is protected.

A few openings are allowed, depending on distance to boundary.

 

Option 2 is pretty well impossible as a retrofit, as the steel is lighter weight than required, and there will be normal or light footings.

Assuming it is an agricultural building, it will be much lighter  steel than would be required if it was new for domestic.

So anything partly towards this is a compromise.

 

If your construction is a fireproof box within the structure, and you can't see any steel, then that may suffice.

 

Jargon for the discussion that BI may throw in.   Boundary condition, Constrado.

 

If the BI lets you have any relaxation on the above then that will be good news.

However, if I was your Engineer or BI I wouldn't be terribly worried as it is single storey. Depends a lot on the construction.

 

*This is not to stop your building falling into the neighbours. That does not happen as steel buildings soften and collapse vertically or crumple inwards. It is to keep the wall standing as a fire barrier.

 

 

 

[jen and mark]

Thanks very much for the prompt replies! Luckily, our barn was built on sufficient pads and the SE was happy for no more additional work. The double barn was erected in 1990.

 

We are having a steel composite roof so we can expose all the purlins and some sides of the main frame/columns. We have no other properties around us apart from a neighbours shed - our barn is 4.5m from the boundary fence so the BI is not too worried about  the spread so because of this, I didn't say anything more but did wonder of we needed to treat the exposed steel with the very expensive intumescent paint!

 

The construction is.....

We have raised the  original floor by approx. 420mm - usual hardcore (MOT type 1), blinding and DPM - Gen 3 with fibres being poured on 23rd August. This meant we did not have to dig out the already solid base. We have a single skin block wall ( about 4ft high) between the steels which we will add a timber stud on top and then add a second timber stud wall, internally, to create a cavity, we will insulate the 2nd stud wall. The outside will then be clad with a combination of steel and wood cladding.

 

Another question - would you cover the existing block wall (no existing damp proofing) with the same visqueen that is going under the slab and then add a DPC on top before we build the external stud wall?

 

Many thanks in advance

[saveasteading]

It doesn't matter what buildings the neighbour has, as they could build nearer to the boundary.

From inside, will we see plasterboard on your walls? Will the columns be visible?

 

btw it is very unlikely that the foundations are big enough for the new-build rules...they are huge. But the SE can probably see that there is no real risk, so has sensibly let it go.

 

I was told, by national experts on fire, that the rules for protecting steel are way over the top. The concrete lobby was trying, with some success, to get the steel industry to box every steel member with concrete, to make them less competitive. So the steel industry came up with these relaxations....which are mostly unnecessary.

Basically if you have a solid wall that is non-combustible from the inside, then the risk of spread to the neighbours is gone. Also your 4.5m allows any fire through windows to vent to the sky.

 

If the neighbours ever build close to the boundary then they have to protect you.

 

Visqueen: would have to see more details. Do you mean the bitumen sheet that Visqueen and Bituthene make, or polythene?

On my project there will be polythene under the slab and bituthene for a metre up the wall, to compensate for there being no DPC in the existing wall.

[jen and mark]

Thanks @saveasteading I think you are right regarding pads and SE

 

The DPM below slab is 1200 gauge Visqueen Ecomembrane Damp Proof Membrane ( polyethylene). I will look in to  the bituthene - is it like tanking for a cellar? Would you install before the concrete is poured? 

 

Cheers

[IanR]

1 hour ago, jen and mark said:

We are having a steel composite roof so we can expose all the purlins and some sides of the main frame/columns.

With the columns likely bolted straight down on to their original pads and no insulation between the pads and ground, the steel frame is going to act as quite an efficient heat sink. Are the purlins also steel, or timber?

 

Are the columns visible on the outside at all, or will they be insulated from outside air temp so that it's just ground temp they won't be insulated from?

I had to have portions of my columns visible from outside, so are effectively at air temp, therefore in winter they can be below freezing. To avoid heat losses through the steel frame, I've put all the original frame outside the thermal envelope, which was tricky for four of the columns that are internal, within the building.

If yours are insulated from outside air temp, and only un-insulated from the ground, your issue is not quite as bad, but the columns will still be at around 6°C - 8°C at the point they touch the ground. Any of the steel frame that is below 14°C and exposed to inside air is at risk of condensation.

I also wrapped the bottoms of the columns in Celotex, where they came through the floor, to thermally break them from the insulated raft, that includes the UFH.

[jen and mark]

Thanks for the info @IanR Thermal bridging is a nightmare for steel framed building! On one side of the barn, the purlins are wood and on the other side (double barn) they are steel z purlins.

 

The columns will not be visible from the outside and will have some insulation outside and more in the internal stud wall. The 'saleperson' regarding the composite roof informed us that the roof would be more thermal efficient as there would be no thermal break. When I asked about condensation he said the MHRV would sort this out. You have me worried now!

 

Great tip about wrapping the columns in celotex - we will definitely do this!

 

[IanR]

When I first started thinking about my own conversion I did think I'd have the primary frame structure visible internally, in some areas and have an industrial look to the interior, but I couldn't mitigate the thermal bridging, so ended up covering it all up internally and fully insulating it from the inside air, including vapour barrier.

 

The sales man is not wrong saying the roof will be thermally efficient, without cold bridges. Assuming you are looking at Kingspan insulated, profile sheeting (or similar), if you go thick enough it can give decent U values and the marketing suggests decent air tightness is possible. It fully wraps the steel structure so the roof beams and purlins are fully within the thermal envelope.

 

You may have an issue though in the detailing of the roof structure to to the columns. The roof has the steel frame within the thermal envelope, but your description of block walls between the columns and a timber stud structure internally to create a cavity suggests the columns will be outside the thermal envelope in places (although not visible from outside). You have a difficult transition at the point the columns move from being outside the thermal envelope to inside in order to meet the roof portals.

Since the columns are not visible from outside, if there's enough room to the cladding (or however you are finishing the outside) perhaps you can wrap the columns to well insulate them from outside air temp.

 

But that doesn't resolve the thermal bridge to the ground.

Unfortunately the salesman is incorrect regarding the MVHR. In winter it will help reduce the relative humidity of the internal air, but should help to keep it in the 50%-60% range for comfort. But 21°C air with a 60% relative humidity has a dew point of 12.9°C, so any surface it comes into contact with, below this temperature, risks condensation.

To add another worry bead, I also have a concern with the insulated profile sheeting. They are very lightweight, and while they have reasonable U Values (if you go thick enough) the insulation type has a very short decrement delay. I'm not sure how they will perform in a domestic setting. I have no experience of them, so for me it is just a concern, but if you haven't done so already I would try and get the views of someone that has used them in a similar way.

Edited August 14, 2021 by IanR

[jen and mark]

The roof panels are Kingspan with a u value of 0.15 - well below the regs of 0.18 which is why we thought it would be a good idea plus the underside of the panels are the finish so no need to plasterboard and can keep the vaulted ceilings to the hall, kitchen, lounge and bedrooms (although we will have standard ceilings in the bathrooms, ensuites, utility etc to accommodate the MHRV ducting)

35 minutes ago, IanR said:

You have a difficult transition at the point the columns move from being outside the thermal envelope to inside in order to meet the roof portals.

We will also have insulation, vapour, air barrier and cladding externally on top of the steel and block walls. This will be minimal though as we cannot go (much) beyond the original footprint so we are insulating the inner skin to building regs. The external insulation is a bonus and to help with thermal bridging.

40 minutes ago, IanR said:

 

But that doesn't resolve the thermal bridge to the ground.

I know - what a nightmare and I cant think of a way around this - we are deffo NOT going to dig out the pads to put in thermal breaks! I do like your idea of wrapping the steels to UFH level. We are currently living on site in a mobile so hopefully the barn will be warmer that this is in winter!

 

43 minutes ago, IanR said:

Unfortunately the salesman is incorrect regarding the MVHR

Hopefully the insulation wrapped outer steels will mitigate this

 

44 minutes ago, IanR said:

try and get the views of someone that has used them in a similar way.

Great idea-will do

 

Many thanks for your time and knowledge

 

[IanR]

2 hours ago, jen and mark said:

We will also have insulation, vapour, air barrier and cladding externally on top of the steel and block walls. This will be minimal though as we cannot go (much) beyond the original footprint so we are insulating the inner skin to building regs. The external insulation is a bonus and to help with thermal bridging.

 

I had the same restriction as mine was a Class Q PD Conversion, so no opportunity to insulate around the outside of the columns.

 

If you are only getting a small amount of insulation around yours you need to give this some serious thought. Steel conducts heats 22 times better than concrete, so the equivalent cold bridge, if it were in concrete, would be 22 times the size. With your current plan I don't see how you are going to avoid condensation on the steel frame, and possibly forming within your walls.

 

2 hours ago, jen and mark said:

Hopefully the insulation wrapped outer steels will mitigate this

 

You can see the base of the columns wrapped on mine, but this just thermally breaks the UFH from the columns so it doesn't pull the heat directly out of the floor to the ground, but it hasn't broken the cold bridge from the column to the ground. As you say this is almost impossible.

 

 

My internal columns ended up full encased in a sprayed on, closed cell foam insulation to exclude them from the thermal envelope and ensure no moisture could get to them. Here's a before and after.

 

 

[jen and mark]

Thanks for this - I think we need to seriously consider our options - luckily we are still in the early stages!

[saveasteading]

18 hours ago, jen and mark said:

The 'saleperson' regarding the composite roof informed us that the roof would be more thermal efficient as there would be no thermal break. When I asked about condensation he said the MHRV would sort this out.

 

This is nonsense.

As stated above there are difficult details but the roof is not one of them.

I have experience of exposed double skin, and composite roofs, in buildings for people use. Never had any condensation,

Theoretically there is a cold bridge through the screws, but this is a tiny area and contact.

Even used in a swimming pool, and had no issues...as long as the environment is in control it will be ok.

 

Steel columns go into concrete and cannot have a thermal break. All you can do is insulate the steel itself, and the losses through steel then concrete, then earth are small, to negligible.

You could calculate it and see how little it mattered. If concerned you can expose the outsides of the pads and insulate with polystyrene, but I wouldn't bother, and the money is better spent elsewhere, especially on airtightness, and especially at joints and junctions.

 

[saveasteading]

The bituthene liner seems to be an accepted detail for a solid wall. I only found it in a blog by a self-builder in Aberdeenshire, and it seemed 1. to be  a standard procedure and a great idea. 2. to be accepted by Building Control. 

We have not submitted the design yet, and welcome any comments from anyone who has done it or knows more.

There seem to be several manufacturers and specification levels. As it is not a dam or water tank I think the thinnest will do fine.

Adhesive is applied to the wall first, then it is stuck on to the bottom metre. it should overlap with the dpm at some stage, and underside of slab seems appropriate.

This photo seems to be inside a water tank or retaining wall , and there is still a concrete slab and dpm to go in.

The cheapest I have seen so far is  "Hyload"  £186 inc VAT for 15m roll, and including primer, so at about £10/m is a considerable cost.

Probably tricky to fir to a masonry (not very flat) wall.

 

 

[jen and mark]

1 hour ago, saveasteading said:

This is nonsense.

Could you clarify which bit is nonsense - is it the condensation issue or the MHVR. Would you clad all the columns or would it be ok to expose any? My thoughts are the steel would be inside the envelope even though the outside steels would have limited insulation.

 

Have you found the composite roofing system ok?

 

Love the bituthene liner - bit expensive though! Will look forward to anyone else's view on this

 

Many thanks 

 

 

[saveasteading]

I think both. There should not be condensation under the composite cladding, so MVHR should be irrelevant.

I should add that I have found reps, even at exhibitions, to be rather ignorant of their own products, as well as their aspect of building. I expect this is because there are very different skills in selling and technical understanding, and they may not have been briefed on what doesn't suit in selling.

Easy though if your product is wonderful.

 

How so? The cladding sits on a steel purlin but then there is the layer of foam. The only link to the outside is the screw that comes from outside and into the purlin. On some cladding systems even that is partly concealed on the outside.  Some very approx sums:    About 3 screws per m2, and they are say 8mm  diameter on the outside  50mm2 per 1000x 1000mm: ).   0.005%

But heat on the inside is contacting the screw not the head, so even less.

 

Composite is good. You can also use built up systems, where any thickness of fibre insulation can be added. This may be the only option if the roof is thick (and heavy) and there is no crane access.  PIR is more idiot proof but more difficult at details. PIR is almost impossible to change if damaged by , say, a falling branch.

 

On cladding: I recommend only buying top quality products. This may be an unfamiliar name but big on the continent, or a big name.

The cladding you see in agricultural adverts is usually much lower quality, in thickness, galvanising and colour coating (product and thickness).

Only PIR or rockwool type filling is suitable, and cheaper products may have a more flammable material.

Also there is 'non prime' cladding':   'seconds'..don't go there for a house.

For plain metal sheeting, some suppliers offer the best down to the worst.....depends what the client wants.

 

More than you wanted to know at this stage perhaps,  but important if looking at prices.

 

[jen and mark]

Great advice @saveasteading. We are thinking of using http://sasprojects.com/roofing-cladding/ They are a commercial company rather than domestic - we met them at our storage facility as they were adding another self storage unit, but do do domestic as a bit of a side line. They design and fabricate all of the steel work in their workshop.

 

We have had a quote of £45,000 for the roof (354m2), insulated valley, hopper etc and includes telehandler and installation  etc. This works out at approx £127m2 but the roof would be complete - no need to plasterboard or plaster inside so seemed a good option. We are still debating the wall cladding!

 

Many thanks

[IanR]

5 hours ago, saveasteading said:

The only link to the outside is the screw that comes from outside and into the purlin.

 

You may have missed that while the roof portals are under the insulated profile sheets, the columns are outside the thermal envelope, which of course join to the roof portals at the haunches.

 

I didn't take a risk with mine, so had some condensation risk analysis done, which showed that condensation was a certainty. Maybe you can get enough insulation around the outside of your columns to reduce the risk.

[saveasteading]

IanR what did you do to overcome the 'certainty' of condensation?

 

I was only addressing the comment about condensation under roof insulation. It may be a 'certainty' in theory but seems very unlikely to me and never happened in reality on my projects. Lucky 400 times perhaps?

It happens, of course on non, or poorly, insulated roofs with humid interiors.

 

How many metal roofs on retail parks and supermarkets have condensation? It would run down to the next purlin, then to mid span and drip noticeably I think.

 

As you say, the junction of freezing cold columns to warm rafters would be  big heat loss and a problem. Encase and insulate the columns?

[jen and mark]

We hope to mitigate this by having a large overhang on the roof so the outside walls meet the roof on the outside of the frame.

 

[IanR]

29 minutes ago, saveasteading said:

IanR what did you do to overcome the 'certainty' of condensation?

 

I was only addressing the comment about condensation under roof insulation. It may be a 'certainty' in theory but seems very unlikely to me and never happened in reality on my projects. Lucky 400 times perhaps?

 

All my steel frame is outside the thermal envelope, with vapour barrier in place to stop warm moist air getting to a cold (sub 14°C Surface). 

 

29 minutes ago, saveasteading said:

Lucky 400 times perhaps?

 

Not at all, I'm sure they are purpose designed and built. If you have the same or similar insulated panels on the sides of a retail park/supermarket building then you have a continuous thermal envelope outside the steel frame and no cold bridging to outside air temp (just the cold bridge to ground).

With conversions, planning are not always open to an increase in footprint to allow insulation around the outside of what was previously an agricultural building and had no such insulated panel outside of the steel frame.

 

This was my issue: no opportunity to insulate the steel frame (on the sides) externally, so the condensation risk is not just a theory, it was very much real.

The OP is only able to get minimal insulation around the outside of the columns, and the columns are outside of the main wall cavity that will carry the wall insulation.

Edited August 16, 2021 by IanR

[saveasteading]

3 hours ago, IanR said:

same or similar insulated panels on the sides of a retail park/supermarket building

You are right of course. The high quality buildings are sealed at all joints and have a solution at the eaves etc to ensure reasonable airtightness.

Lesser buildings used to leak at all joints, then sometimes added a dpm in the roof.

Columns outside buildings, fancy steel entrances linked to the structure, parapets etc are challenges. In a huge shed, any such problems are less important overall.

 

I don't know the full circumstances for the OP here. It appears that there will be heat loss, and it is a matter of reducing it.

[jen and mark]

Ok - lots to think about here. Steel is not my area of expertise as you may have guessed!

Would this work:

 

- Vapour/air barrier

 

- walls having similar composite cladding to roof all round  -  40mm PIR core with a u Value of 0.46 - (minimal wooden cladding with similar insulation behind to break up the steel box effect)

 

Not sure how to make where the walls and roof meet airtight - tape?

 

Second internal stud wall insulated to 0 . 21 w/ m² k - airtight with vapour/air barrier - or am I fighting a losing battle here? 

 

Thanks for any advice!

 

 

[IanR]

Are you getting any help with the detailing of areas like this? Have you or they prepared any sections that help visualise the region.

A Section at the eaves, through one of the steel columns and showing the proposed wall and roof build-up would help to ensure the issue is understood and perhaps how a fix can be developed.

 

It may be that 40mm PIR will be sufficient to stop the steels, where they are "internal", dropping below the dew point, but a condensation risk analysis would be required to know for sure. There would still be a sizeable cold bridge though, even if there was little risk of condensation.

 

There may be an issue though with the walls having composite panels outside of the wall build-up you propose. Quite rightly you are positioning a vapour barrier on the inside surface, I believe the effect of placing a further layer on the outside that is also vapour closed (composite insulated panel) will stop any moisture that does get into the wall from being able to get out, and therefore pose a mould risk.

 

These are aspects of my build that I was not confident in making the correct decisions, so I got some help in.

[saveasteading]

20 hours ago, jen and mark said:

Not sure how to make where the walls and roof meet airtight - tape?

Best draw a cross-section and the gaps become more visible, and also more readily resolved.

Whether you close off with tape or something bigger, it is often worth stuffing the junction area with compressed fibreglass. This insulates and reduces draughts.

 

Could you draw up what you have described above? i a too buy doing my own details to do yours, but checking is easy.

Basically protect the steels (primary and secondary) from the outside weather.