Insulation Required to Limit Thermal/Condensation Issues with Steel Beams
Hello
I am hoping for some advice that I can’t seem to find searching the articles I’ve read.
We are planning a small 1 bed 400sqft cabin that for various reasons is likely to require a steel ‘portal’ frame super structure. I won’t get too into the details of why we need steel, but essentially due to the design and a very high wind location timber frame won’t seem to cut it structurally according to the structural engineer. Location is south coast of the United Kingdom, call it marine 4c I’d guess.
It is looking like we will end up with about 6 vertical steel posts and a steel box frame in the roof. The steel frame will be infilled with standard 2×6 walls on 24″ centres to be filled with Rockwool.
My question relates to how best deal with the thermal issues of the steel posts and beams.
We can wrap the entire structure in 1-2″ of high spec R8/inch Kingspan Kooltherm. Or, as is commonly done here, we can also insulate the interior with 1″ of Kooltherm prior to drywall. I understand this latter approach doesn’t warm the sheathing but it is a common method here.
Is there a minimum external insulation value to limit/prevent thermal bridging at the steel sections? If we have a continuous air barrier (Intello for example is widely available) on the interior side of the steel would that also alleviate risk of condensation forming on the steel?
Could we also insulate to the interior and keep all the steel on the outside of the thermal envelope and outside our air barrier? If warm moist air isn’t reaching the steel shouldn’t that alleviate condensation risk?
Please let me know if clarification is needed, any advice appreciated as always.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I would try for as much exterior insulation as you can. This is basically what is done on commerical steel-framed structures too, they try to get all the insulation on the exterior as much as possible so that the steel is entirely within the building envelope.
You can try to mitigate moisture risk with vapor retarders, but remember that steel is MUCH more thermally conductive than wood, so the issues are MUCH more severe. Wood has insulating value itself, about R1-1.5 per inch or so. Steel has essentially no insulating value at all by comparison. If you were going to try for moisture control instead of exterior insulation, I'd apply an adhered insulation (that means spray foam) directly to the steel. Even a thin layer with a few points worth of R value would make a HUGE improvement in terms of condensation risk here.
Bill
Without seeing the structure it's hard to know what to recommend specifically, but keeping the steel 100% outside the envelope or 100% inside the envelope are the two best strategies. If you can't do that, the insulation thickness to make the problem go away would be enough to get the same R-value you have in the walls and roof. So you likely aren't going to make it go away, but will only reduce it. Which is why it's better to go to 100% outside or 100% inside.
Thank you Bill and Charlie.
I will try to get a drawing put together to clarify things. The roof is low pitch, so I am likely to externally insulate in keeping with the ratios discussed here at GBA, then the walls I might be inclined to insulated internally if that is appropriate. However with these points in mind let me work out a few details and revert. Thanks again