We respond to a reader's question about this.
In our recent post Prefab Passivhaus CLT Tiny Homes being built in Britain, I described how the modules were completed in the factory right down to the floor finishes, but wrote, "Note how at the end the exterior finishes are done on site, which avoids a lot of problems.” A reader wondered in comments:
What is the benefit of brick cladding on site? This would seem to be totally against the ethos of off-site construction. Why not ship factory finished wall units with insulation and timber cladding? or factory fitted ceramic tiles or similar?
It’s actually a very good question and deserves a longer answer. There are a couple of reasons that I can think of:
Prefabrication can be done in the factory to very high degrees of accuracy, but it’s not perfect. Even steel structures can compress a bit under load, so when wooden modules are piled on top of each other the bottom one might end up being a tiny bit shorter than the top one. Not much, but enough that the cladding might not fit.
When they built 461 Dean, the big modular prefab in Brooklyn, they had endless trouble getting the facade panels to fit together properly; it demanded incredible precision in placement of the units.
When they built the other important New York City prefab, Carmel Place, they did brick cladding on site, which covers a multitude of sins. The brick is a small enough module that you can adjust it to cover up defects and inaccuracies.
If you are going to get anywhere near the air tightness standard demanded of the Passivhaus certification, which is only 0.6 air changes per hour, you need to seal the gap between the units perfectly and have special tapes covering the joint. It is good construction practice to have a rainscreen design with an air gap behind, so you cannot seal it at the face of the cladding.
There are ways around this. In a modular Passivhaus project in British Columbia, Mobius Architecture designed a panel that was installed on site between sections of siding that was done in the factory. Then, as shown on the detail above, they could seal and insulate the joint. Note that the covering panel was designed to adjust for a tolerance of 3/4 of an inch, which is a pretty big number when you are talking about accuracy. But it is totally realistic in the field.
Most of the cladding systems that can be done in factories are lightweight and are often paired with rigid insulation. Even if the cladding is non-combustible it might still not have any kind of fire rating.
I suspect that, in light of the Grenfell disaster, people in Britain will distrust anything other than good old fireproof brick, and that architects will be loath to specify even supposedly inflammable foam insulations like polyiso, which proved to be deadly in its own way. It will be back to rock wool and fibreglass, which are squishier than foams and more suited to a site installation.
Philosophically, I like the idea of doing the cladding in the factory for the same reasons that the modules are done there -- better accuracy and quality control. Look closely at Carmel Place in New York, and you can see what can happen with site built masonry. It can be a mess.
But practically -- and I have not built a modular building in a decade, so I might be out of touch a bit -- I suspect that you get a better building with better performance if you do the cladding on site.