Uvalue Steel frame cavity wall.

marco_polo

Just reviewing a BER report and this was one of the points regarding wall insulation, I am trying to establish if this relates to some or all the wall construction of the house. 

“Part of the wall area in this dwelling has a U-Value of less than 0.6 and greater than 0.27.”

The house itself is steel framed. From exterior to interior the total of the walls is approximately 33-34 cm.

As best I ascertain the construction wall construction is exterior plaster, 4” outer leaf block (100mm) a 50mm cavity -> 50/60mm Kingspan -> 90/100 mm mineral wool and 12.5mm plasterboard. It seems to a “warm steel” frame, as in the Kingspan is attached to the outer face of the steel frame.

The mineral wool is embedded in metal studs on the internal frame to which plasterboard has been mechanically fixed. The spacing between the studs appears to be 40cm. Not sure if relevant but the first floor is concrete, but I do know know if this is supported from the inner frame or extends to the outer block layer as well.

I know there are indicative uvalue calculators out there but I have no idea how to account for the thermal bridging aspect of the steel stud. Just wondering if anyone would be able to put an approximate uvalue on this wall buildup.

The house is also 1/3 dormer, 2/3 two story (Airtightness implication of this is a whole other conversation in itself ) so I am not sure the “part of the wall area greater than 0.27” applies to some of the dormer wall or the whole thing.

   

 

Mellor

There are calculators that account for the stud. It’s literally just the stud and insulation resistance proportionally to their widths.

But I feel steel studs are handled poorly. The width is usually calculated on stud flange width, but surely the heat pass through the web, and the insulation goes right up to the web. Using the stud width does not seem accurately account insulation area

sydthebeat

https://www.brebookshop.com/details.jsp?id=140115

https://www.steelconstruction.info/images/0/09/SCI_P312.pdf

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marco_polo

That great information thanks. As luck would have it a near identical house in the estate has just gone up for sale, and looking up their BER report listed we are talking about 0.36 it would seems, with overall HCI 1.8 W/(K-m2). Windows are just under 2.2 as well. Overall these house are C1 territory so not great by current standards but not terrible either.

Mainly why I am here is that we are looking at doing some renovations this year, an attic conversion, new kitchen and insulating the dormer room (budget permitting). Without O$$ levels of grant access the retrofitting budget would be fairly limited going with independent local contractors. I am hoping to ensure anything done now will not need to be partially redone if/when we are in a position to undertake additional retrofitting improvements down the line.

We did recently have a thermal imaging survey done, and the main issues this identified were poor insulation and airtightness in the dormer room, and air infiltration around external windows, doors, sockets, skirting boards, pipes right throughout the rest of the house. Apart from the dormer section, the inspection didn't really pick any issues with the external walls or existing attic insulation for example.

The survey showed that by and large the windows themselves are still in reasonably good condition with the exception of a few seals could that be repaired,  but in general the windows would be modest enough in size so probably not the highest priority if other airtightness issues could be addressed first. It would be a similar story with the external doors. I have internally sealed a few of the worst offending windows and it seems to have had a reasonably significant impact already, and I see no sign of any external sealant between the external plasterwork and windows so there is probably more low hanging fruit to be had there as well.

I just had a couple of questions really, firstly while doing the dormer rooms + attic there would be good access to install ducting to all wet rooms and commission a demand controls ventilation system situated in the small loft above the dormer. Would this be a crazy idea without fully addressing airtightness upfront?

Granted this would not normally be a good bit lower down the list, but would become a much bigger job after the attic has been converted. We would be sealing up the chimney in this first phase as well as we never use the fire.

The other main doubts I have are concerning airtightness, as it is not that common a construction type there is not much information out there for the layman on what a good strategy would look like. Could a VCL be installed inside the current mineral wool / steel studs. Would this be a bad idea as inner PIR leaf has a foil covering to the inside? Or would it be a matter of taking down the mineral wool and tapping joints / gaps on the existing PIR layer as best can be done?

That in itself of course begs the final question, since EWI or a cavity pump are off limits for a steel framed house, if one was to start taking down 12.5 plasterboard to do some airtightness work, there is not much putting 12.5mm back up when the uvalue is 0.36?