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Posi-Joists will cost more initially (a 4.2m posi-joist at 225mm depth will be around £50 compared to £30-£35ish for a solid joist), but the cost savings come once you factor in labour for installation and routing services. Typically you'll find posi-joists work out cheaper in the end unless doing the work yourself and not paying yourself a wage.

Ours are manufactured in the UK, but MiTek do seem to have contacts in the US that would suggest they have manufacturers over there too: www.mitek-us.com/contact/