Retrofitting an apartment in an existing metal-sided building
Greetings and thank you in advance! I’ve been an avid reader of many articles on this site, and now it’s finally time to put them to practical use…
I will be constructing a 700 sq. ft. apartment with a loft area in an existing wooden pole barn structure with metal siding. I have some questions on the best way to insulate the retrofitted space. Here are as many pertinent details as I can think of without burdening you with unnecessary details:
Climate Zone 3
The building is framed with 6×6″ wooden posts spaced 10′ apart.
The rafters are 2×6″ spaced 5′ apart, so I will be using 2×4″ strapping spaced 24″ O.C. for ceiling finishes.
There is no insulation currently.
Once the space is enclosed, it will be possible to provide roof ventilation entry at the gable eave. The ceiling in the apartment will slope with the rafters (I’m not putting in a drop ceiling in order to have space for a loft).
Since the metal siding and roofing is already there, I will essentially be constructing from the outside – in, rather than vice-versa. I’m looking for the best way to ensure a tight building envelope and prevent moisture damage. My biggest concern has to do with the ceiling.
I’m currently planning on buying used Poly-iso sheets in 2.5″ and 3″ thicknesses for the ceiling insulation. This will fit perfectly inside the thickness of the 2×6 rafters. It will be cut-and-cobbled basically, but shouldn’t be too difficult since the rafters are 5′ apart. The roof purlins running the length of the building above the rafters will create a 1.5″ air gap between the insulation and the metal roofing, which, once the eaves are vented, should create a sound design against moisture trapping, correct?
But if I understand correctly, If my roof is designed to dry to the outside (it would be), then I would want to take steps to avoid moisture traveling through the walls from the inside, correct?
I can air seal all of the joints of the Poly-iso with tape, but that would be fairly expensive. Using the air-tight sheetrock method would be another option. The thing is, I was hoping to use sheet metal on the ceiling underside rather than sheetrock, which of course is NOT airtight.
What about using an air and moisture barrier like Grace Ice and Water shield on the underside of this assembly? Thus making the design go from top to bottom:
Rafters with 5.5″ Poly-iso rigid insulation
Grace Ice & water shield air & water barrier
2×4″ ceiling strapping/ air space
Sheet metal ceiling finish
Would that not ensure that no air or moisture could ingress/egress through the sheetmetal ceiling, thus allowing the the whole assembly to dry to the outside?
Grace is expensive, but it seems like this would be a more cost effective way of air sealing than buying several hundred feet of expensive tape and cans of spray foam, based on my calculations. Is there some reason this would NOT work or be a bad idea? Do you have any suggestions that would be better suited to my application?
Many many thanks!