Insulation questions

[JBOD]

I'm building a small 10 x 15 shed to be used as a motorcycle garage. I want to heat it to keep from freezing in winter and insulated from heat in the summer as well as controlling humidity to prevent rust mildew etc. I'm in Montreal, cold winters and humid summers. So far I have 2x4 walls 16''oc framed on top of a 2x6 16''oc base on skids. Walls will be sheathed with ½'' osb and wrapped with tyvek. I've built trusses for a gable roof with vaulted ceiling (raised the bottom chord about 12 inches just for some extra storage) and I'll have a ridge vent with truss vents running the lower section of the roof.
The base I insulated with 3½' roxul Batts beneath 2'' eps between the joists, and on top some bubble foil I found for cheap, then ¾'' plywood. Also put wire mesh on the bottom face to stop any animals getting into the roxul. I realized after that a better design would have been to put the eps on the bottom of the joists with roxul batts above and left a space in the top of each cavity for an air space for the radiant barrier, oh well what's done is done. The bubble foil should at least act as a vapor barrier beneath the ply.
So I'm wondering about vapor barrier for the walls and ceiling. With the cold winters I understand that vapor barrier is necessary to prevent condensation moving outward through the walls but what about the summer when it's hot and it's the reverse? Won't condensation just for on the backside of the vb? Assuming vb is installed on the inside of studs and ceiling beneath drywall.


Also I'm kind of wishing I'd framed the wall 2x6 to allow for more insulation. I'm considering adding another layer of 1'' foamboard on the inside studs, walls and ceiling, and then ¾ strapping over that and then gyp. If I used foil faced foamboard or put a layer of bubble foil (I got alot for cheap) beneath the strapping, that should give another layer of insulation plus provide an air gap to actually use the radiant properties of the foil which will also act as a vapor barrier.

 

[PoorUB]

With such a small shed, sealing it up is probably more important that the R value of the insulation, at least within reason. I would just use a higher R value fiberglass insulation in the walls and ceiling, 4 mil plastic, and sheet the inside with your preferred materials. If you really want to, you can put foam sheets on the inside walls and ceiling, but i wonder if the cost of the foam will ever offset the heating cost.

What ever you use for a door will be the larger problem with air leakage.

Have you decided on equipment for heating and or AC?

 

[billconner]

I don't know what your siding plan is but continuous foam on exterior will do a lot more than on inside or between studs, and if 2" or so solve moisture problems but keeping sheathing above dew point.

 

[pcmeiners]

With such a small shed, sealing it up is probably more important that the R value of the insulation, at least within reason

Fully agree with PorUb, sealing is critical. You can have the perfect insulation job, but with air changes due to poor sealing you will get rust.

 

[JBOD]

Ya ok, I agree sealing it up as airtight as possible is most important. For heater I'm looking at pouring a thin set concrete with heating wire set in or possibly infrared, also have baseboard heaters, might just go with those to start since winter is coming soon. As far as as foamboard, i would rather put it on the exterior but I think that might increase the sq.footage just enough that I would need a permit

 

[billconner]

As far as as foamboard, i would rather put it on the exterior

I get the sq ft limits. Even an inch would be beneficial and with taped seams it's easily seals the building.

 

[JBOD]

Would tyvek be necessary over exterior foam board? I'm thinking not

 

[dcg9381]

With the cold winters I understand that vapor barrier is necessary to prevent condensation moving outward through the walls but what about the summer when it's hot and it's the reverse? Won't condensation just for on the backside of the vb? Assuming vb is installed on the inside of studs and ceiling beneath drywall.

If you're cooling it, HVAC is a de-humidifier.

Also I'm kind of wishing I'd framed the wall 2x6 to allow for more insulation. I'm considering adding another layer of 1'' foamboard on the inside studs, walls and ceiling, and then ¾ strapping over that and then gyp. If I used foil faced foamboard or put a layer of bubble foil (I got alot for cheap) beneath the strapping, that should give another layer of insulation plus provide an air gap to actually use the radiant properties of the foil which will also act as a vapor barrier.

Most of your heat gain/loss is in the roof/ceiling. So concentrate on that. There are products like Zip-R that are foam board + OSB and you can get them in various sizes (if you're ready to spend more money).

But at the end of the day you've got 160 sqft and It's already probably R-15 or so, that's better than most RVs. It's going to heat and cool just fine if you throw capacity at it... And you don't need much capacity, so the ROI for adding more insulation is relatively low here.

 

[JBOD]

Thanks for all the feedback, got a few things to consider. I'm going to double check the max area allowed and see if I can't squeeze in a layer of exterior foam. I just got the roof trusses up today, here's a few pics