If a roofing contractor offers an insulation material with an R value of 2.5 instead of 3?

Question

Would it be worth forwarding the offer to the client? Is the difference that substantial?

Answer 1

You don't want that.  If he offers 3.5 instead of the required 3, that is a different story.  Otherwise he should give you more material thickness to provide an equivalent R of 3.

Yes, the difference can be substantial and I wouldn't forward it to the client.  The smaller the R value of the entire roof sandwich the larger the difference it makes. Like the change from 5 mpg to 4 mpg is a 25% worsening in fuel economy, but a change from 100 mpg to 99 mpg is 1%.

It will mess up the heating/cooling of the building and the mechanical systems might be inadequate.  They could save money on the roof and complain how the building is too hot/too cold or the mechanical systems never shut off and their energy bills are too high.