Hardie Board over foam board

You should call Hardie tech support and ask why they have the restriction. 1.5" foam isn't much more likely to crush than 1" foam, so I'd think that it has to do with the weight of the fiber cement siding. Maybe the siding sags over time as the actual point of attachment is further away - the foam wouldn't support the nail by itself and you can't nail it tighter to hold it more securely.

tombsy had written this in response to

I think they probably said furring strip. Google it. That would address the sagging situation.

What country are you in?

In the UK we would use battens to stop the foam being compressed.

Furing strips in the UK are normally tapered (eg as used to form a 1:40 slope on a flat roof) but I see in the USA Furing strips = battens.

I'm in NY. That Wiki article had the UK usage of firring strips for the tapered roof battens. I think there is a lot of phonetic spelling in construction. Many times I have to pause and translate to determine what a person meant from something they heard from something that person heard from...

You should call Hardie tech support and ask why they have the restriction. R

Thank you req

You need to know how to respond to posts in a Usenet group. You need to know how to use quoted text properly. You need to learn how to only add useful information.

Don't blame me if the commas didn't go in like they normally do. req

You need to know how to begin a sentence without "You need to know how to".

(__ Nehmo __)

Oh, we're all so needy...

I put 2" of R7.2" foamboard on an exterior then Osb, tyvek and Cedar shingles. I would not from what little I know put any siding directly on foam. It will compress unevenly as your fasteners cant all be secured equaly, then your finish siding will be wavy viewed from a bistance as it bows in and out. Cover it with osb, tyvek, then siding. To do furring strips how can you get them perfectly even, you cant go over the foam because you cant fasten each one without bowing it in at areas, you would have to cut out the foam and insert the furring to the solid structure, then its not cheap furring, its an expensive 2 x since dimension is less. It was alot bigger job then I expected and payoff was less than expected due to 100 yr old design. Hardi board, isnt that heavy concrete board? You should reconsider a lighter material as I dont know how Osb will react to all that weight, I used the thinnest Osb, but Hardiboard and its weight could over many years ruin the thin stuff, and you will need alot more fasteners on the Osb to hold hardiboard. Overall the job is a headache and you are experimenting with something that wont last if your choises are incorrect. Have you also considered insulation on the exterior in extremely cold zones like Zone 5 allows winters condensation to possibly form on the foam touching the exterior, and your house non breathable. With new windows and foamed walls and a condensing heat system you will need a heat recovery fresh air system, or leave a window a bit open all winter. The head aches of converting old houses to be efficient can be agravating.