Expert: Stephen Major (Owner--Major Design Group) - 6/8/2007

Question
Dear Mr Major

I am currently taking over the building work to a half completed barn conversion in the UK (ie, turning an old agricultural barn in to a residential dwelling) and I am concerned about the quality of workmanship in the construction to date.

One of my main worries is insulation-related and I am concerned that the current insulation work will ensure that this conversion is far from energy efficient and this is obviously a very serious matter in this day and age.

The barn is of masonry construction and an inner leaf has been built out of 100mm thermalite (lightweight concrete)blocks on a concrete slab. The plans stipulate that the cavity between the blockwork and the brickwork should be 90mm. 40mm Celotex (a foam board) should be tied to the blockwork to leave a 50mm air gap in the cavity. The plan was then to plasterboard the inside with 12.5mm board and skim on to that to achieve a u-value of 0.30.

Please find some links below to photos of the Celotex work to date.

Could you please let me know if you think this insulation will work properly?

Could you please let me know if you think this is acceptable workmanship done with reasonable care and skill?

Could you let me know if you think the wall design is sensible?

How difficult is this construction in reality?

If you have any questions, please feel free to email me or you can call me on 011-44-1948-890192.

I look forward to hearing from you.

Yours sincerely

Ed Corbett


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Answer
Ed,

This system is fair as long as two requirements are met:
(1) the inner air space is sufficiently sealed against air infiltration, and
(2) (related to (1)) there is no air-air connection between the inner and outer spaces.

Unfortunately such air sealing is probably impossible in the real world.

Looking at your photos I would have approached this a little differently.  I would have installed the foam insulation  tightly against the outer masonry, as is (sort of) done in photo 54.  This becomes the building envelope.  Anything installed inside of the envelope is in "conditioned" space.  This will work best if the insulation is continuous, with few or no gaps.  And air is free, in fact encouraged, to move along the inner layer of insulation.  

If moisture in a problem in any of the exterior walls I would use extruded polystyrene, ("Dowboard" or similar -- it's more waterproof).  Preferably, remove any moisture problems in the exterior walls.

As far as rating the work, the work as shown in your photos is very sloppy.  I would consider it to be below accepted standards of workmanship.  

Let me know if you need additional information or want to discuss this further.

--Steve Major