Exterior GPS Insulation and Empty Stud Cavities
Empty stud bays with drywall + continuous exterior insulation
Is it possible to do a wall enclosure with 3″ exterior GPS insulation, no stud bay insulation, and drywall?
I can’t pour concrete until spring, so I’m playing with different enclosures to find the best price/performance ratio. I like the idea of R14.1 GPS exterior insulation. As an added bonus it comes treated with insecticide for termite resistance.
I’ve seen enclosures with similar lack of stud bay insulation, but they don’t have drywall. I’d like to use drywall for fire resistance. Would I run into any moisture issues with the lack of stud bay insulation? What about critters? Would I want to put blocking in there for extra fire safety? Or can I skip the drywall if I use cement board siding?
This is for a 9’x13.5′ conditioned woodworking shop in climate zone 4a.
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Replies
shedworkshop,
There are two aspects of fire safety.
One is to protect from fires originating outside the building, and for that you use largely non-combustable materils for cladding and trim, and protect openings against embers.
The other is to limit the damage from fires starting inside. That's what drywall helps with. So it really depends on what you are trying to achieve whether you need drywall or not.
Frame your walls, add fire blocking, and protect against pests the same way whether you are adding drywall or not.
Malcolm, that makes sense. Since it's a wood workshop, I'll error on the side of caution and install some lightweight drywall on the interior. The exterior will have fiber cement siding so should be good on fire resistance there as well. Thanks!
There are no likely moisture or critter issues with leaving the stud bays open. In fact, putting all of the insulation on the outside means that the studs will be completely inside the thermal envelope, nice and warm and dry.
How are you conditioning it? This is a very small space, so most systems will be overkill. Winter only? With such a small space, electric resistance is probably the best bet. Not the most efficient, but payback on anything more sophisticated is probably forever. Even if you will have no A/C, you will probably need dehumidification to keep the rust off the tools.
Good to hear. My insulation values will be lower than you'd want for a house (R17 wall + R17 roof + R6 slab edge), but since it's just a small workshop the building code won't apply.
The plan is to use a 1,000-watt in-wall heater in winter and a 5000 BTU window AC in summer. Not ideal for summer, but wall ACs are hundreds of dollars more. Open to advice if anyone's got it. An 80 CFM bathroom exhaust fan will be used for air supply. I'll add in a dehumidifier as well.
Depending where you are located you could look at the quik-therm material and their 936 connect wall systen. It should be very cost effective.
https://quiktherm.com/product/936-connect-composite-wall-weatherization-system/
For the cost of fiberglass or hemp batt insulation I would insulate the stud cavity.