cost efficient wall insulation for add on?

[ShellbackBill]

Hi,

Long time lurker. I think my first question.

Getting ready to do an addon to a pier and beam house. I want it to be energy efficient, but don't want to break the bank.

South East Texas, swampland in hurricane country. Hot in summer, some freezing temps in winter usually short periods, as in only a few hours at a time below freezing. Add on is north side of house.

I'm thinking either 2x6 with fiberglass or 2x4 with spray foam.

Either would be covered with 2 inch Polyiso.



I'm strongly leaning toward 2x6 walls with fiberglass, and two inch Polyiso.

One worry is fastening the HardiBoard siding well enough that the first hurricane won't blow it away.

I guess, I'm asking what's my best bang for the buck?
Any thoughts or suggestions?

 

[Sparky617]

You're going much heavier on the wall insulation than your region would indicate.

https://www.greatdayimprovements.com/insulation-r-value-chart.aspx

A 2x4 wall with fiberglass meets your requirements. As to spray foam it would depend on the type you use. Polyiso is about r-5.6 to r-8 per inch. Open cell foam would only get you r-12 in 3 1/2". Adding 2" of polyiso board would get you an added r-13-16. Spray foam will make a tighter house.

Hardi is very hurricane resistant, I'd think you'd want to sheathe the polyiso with OSB screwed into the studs to give you a nailing surface or strap it with furring strips vertically to give you a nailing surface. Trying to drive nails through the Hardi and the insulation would be a challenge.