# Insulating a 900 sq ft workshop



## James414 (May 27, 2020)

Greetings! I'm new here so first let me say hello to everyone! 

I'm planning to finish the inside of a 900 sq ft workshop (30 ft x 30 ft) that comes with a house I'm purchasing and I'm trying to decide the best way to insulate it; I wonder if anyone here can offer some advice. This space will need to have walls, floor and ceiling that are smooth and easily cleanable, so my material decisions revolve around that requirement.

Here is a short video of the workshop to give an idea of its current state: 




I plan to finish the interior walls with this plywood: https://www.homedepot.com/p/15-32-in-x-4-ft-x-8-ft-ACX-Radiata-Pine-Sanded-Plywood-Panel-80496/206093575

I will screw ceiling panels into the underside of the rafters, these are the panels I will use: https://www.homedepot.com/p/Suntuf-26-in-x-8-ft-Polycarbonate-Corrugated-Roof-Panel-in-White-101890/100087758

My plan is to:
1. Move all electrical outlets outwards (so they are in line with where the plywood paneling will be)
2. Add insulation (foam or roll - if it's foam I'll hire a pro for that part)
3. Panel the walls with the above-mentioned plywood
4. Seal the concrete floor
5. Install a split a/c unit (will likely do this after step 1 but still undecided on exactly what step this will be)

As you can see from the video, the roof area is ventilated (you can see the outdoor light coming in). I was originally leaning towards using open cell foam to insulate the walls and roof, but I'm not sure if this will work with the vents. Do they need to be prepped somehow if we use foam? I assume they are ultimately plugged with the foam, but is there some modification needed first? And if we do use open cell foam, will the air get stale? Do we need some kind of vent system for this little building?

I could use roll insulation everywhere, I'm just weary that it won't be very effective. However it would actually make use of the vents that are present since it would be rolled up between the rafters. We're in southern Alabama and want to try to get our utility bill down (the building will be occupied by one person frequently, two people occasionally).

Any advice is appreciated, as I'm not the handiest of handymen.


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## James414 (May 27, 2020)

Also I should mention that I'm open to advice on other parts of the project as well (wall finishing, ceiling etc.) if anyone has a suggestion. I'm trying to keep costs down while still ending with a nice looking space.


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## Nealtw (Jun 22, 2017)

How is the out side finished, type of siding and roofing?


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## James414 (May 27, 2020)

Thanks for your reply!

It's vinyl siding with a shingle roof. Here's an image (sorry you can't see the roof):


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## James414 (May 27, 2020)

Well the image didn't work... here's the URL: https://ibb.co/Bz5CDFV


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## Nealtw (Jun 22, 2017)




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## Nealtw (Jun 22, 2017)

You want the roof vented to add life to the roofing and allow radiant heat to escape.
When you cool a space, you are making the air smaller and the room will suck air in. If you allow air to get passed insulation it will bring with it humidity and dust which are both food for mold. 2 layers of thin plywood with offset joins would do a better job on the walls and I would reconsider the ceiling it needs to be sealed.


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## James414 (May 27, 2020)

Thanks for posting the image. Not sure what I did wrong. Anyhow that's the only image I have or I would post one showing the rooftop, but it's a standard asphalt shingle. I haven't moved into the property yet so I can't get more pictures, I'm currently trying to plan the project before we move in next month.


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## Nealtw (Jun 22, 2017)

James414 said:


> Thanks for posting the image. Not sure what I did wrong. Anyhow that's the only image I have or I would post one showing the rooftop, but it's a standard asphalt shingle. I haven't moved into the property yet so I can't get more pictures, I'm currently trying to plan the project before we move in next month.


Why not just do it with drywall?


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## James414 (May 27, 2020)

The plywood aesthetic is more what I'm going for, things can be hung from anywhere without regard to studs, not as susceptible to humidity/moisture. These are the main reasons. Also the price isn't much different, especially when you consider the finishing work drywall requires, and I'm willing to spend a few extra bucks per panel for a space this size (we're not talking very many panels either way).

But the insulation is the thing I'm still trying to figure out.


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## Nealtw (Jun 22, 2017)

James414 said:


> The plywood aesthetic is more what I'm going for, things can be hung from anywhere without regard to studs, not as susceptible to humidity/moisture. These are the main reasons. Also the price isn't much different, especially when you consider the finishing work drywall requires, and I'm willing to spend a few extra bucks per panel for a space this size (we're not talking very many panels either way).
> 
> But the insulation is the thing I'm still trying to figure out.



I would look at insulation as R value per dollar or dollar per R value.
They all fail with air leaks.


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## Bud9051 (Nov 11, 2015)

2x4 or 2x6 walls?
Will you be adding heat and what type?
Will you be heating and cooling this 365 or intermittently as needed for use?
Will any of the activity in there be moisture related, wet cars or other?
Do you know what was used for rafters, trusses, 2x4's, 2x6's or other?
Are you planning on any use for the attic space?

I'm guessing that summer heat is your main concern and if so as Neal mentioned you will want ventilation from those soffits up to a ridge or high vent. The answers to above questions will help with the recommendations.

Bud


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## Nealtw (Jun 22, 2017)

Bud9051 said:


> 2x4 or 2x6 walls?
> Will you be adding heat and what type?
> Will you be heating and cooling this 365 or intermittently as needed for use?
> Will any of the activity in there be moisture related, wet cars or other?
> ...


Vaulted trusses so insulation and venting is not a problem.


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## Bud9051 (Nov 11, 2015)

You watched the video, that's cheating .
Also saw the skylights in the roof that will need a solution.

Thanks,
Bud


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## carpdad (Oct 11, 2010)

Foam has more r value per inch but not greatly more. I think fiberglass gives you more value. I think even more value in your area and without any shade cover would be powered attic exhaust fan or even big cfm bath fan high in the gable and adding some intake mushroom vents on the other side. 



I would:
1. gable vent fan (both heat and moisture) rated close to attic space you'll have after closing off with insulation. Equal or greater intake vents. All measured in CFM. You'll want to reframe for access later for replacement/repair. About 30x30 inches and put a ladder there to see if more is more comfortable/access.



2. Spacer baffles along the eaves. Minimum required is 1.5" but 2 or more is better.


3. fiberglass insulation. Along eaves, as much space as it will fit. Stuffing more does not give you more r value.


4. As spaces get bigger, switch to at least r-40. I read that after r60 there is no more return on cost.


5. Cover the whole thing again with xps foam board. Min 1.5" or more if budget allows. XPS will give you greater air seal too. If xps, use faceless fiberglass insulation to avoid double vapor barrier.


Don't use inside vapor barrier on walls. South is a region where "vapor drive" is out to in, so you want inside sheathing to allow vapor to pass.


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## James414 (May 27, 2020)

Bud9051 said:


> 2x4 or 2x6 walls?
> Will you be adding heat and what type?
> Will you be heating and cooling this 365 or intermittently as needed for use?
> Will any of the activity in there be moisture related, wet cars or other?
> ...


I'll run through them one by one:
2x4 or 2x6 walls? 2x4
Will you be adding heat and what type? Will use a split a/c unit with heat.
Will you be heating and cooling this 365 or intermittently as needed for use? Daily during business hours
Will any of the activity in there be moisture related, wet cars or other? There will be a utility sink, drying rack for dishes, and dishwasher. Not sure how much moisture these give off but they will be installed.
Do you know what was used for rafters, trusses, 2x4's, 2x6's or other? I'm not sure, I would have to check.
Are you planning on any use for the attic space? Probably not. It appears to be too small to be useful in our case.


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## James414 (May 27, 2020)

carpdad said:


> Foam has more r value per inch but not greatly more. I think fiberglass gives you more value. I think even more value in your area and without any shade cover would be powered attic exhaust fan or even big cfm bath fan high in the gable and adding some intake mushroom vents on the other side.
> 
> I would:
> 1. gable vent fan (both heat and moisture) rated close to attic space you'll have after closing off with insulation. Equal or greater intake vents. All measured in CFM. You'll want to reframe for access later for replacement/repair. About 30x30 inches and put a ladder there to see if more is more comfortable/access.
> ...


I'm new to this stuff so I don't follow along very well.
1. By 'rated close to attic space' you mean the CFM rating is similar to the cubic feet volume of the attic space? For 30x30 inches are you talking about the access port dimensions?
2. Ok
3. Ok
4. Here I assume you're talking about between rafters in the open ceiling area. Correct me if I'm wrong.
5. Are you referring to the ceiling or the walls, or both? If ceiling, I'll have to drop the light fixtures somehow (maybe hang them so they dangle) if the walls I'll have to extend the electrical outlets significantly since this will add a lot more gap.

It seems like all these thick batts with the xps foam board will cost more than an open cell foam job. Also, we have some oddly sized/shaped spaces between studs (near corners or doors especially) and they will be hard to insulate properly, among other places (electrical, wiring, etc.) so I'm questioning how well it will work. 

Is it your opinion that open cell foam will have moisture problems? Would open-cell foam need some kind of a vent system in a building this size?


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## Bud9051 (Nov 11, 2015)

Thanks,
As conditioned living space you will want to address attic ventilation.
2x4 walls will only allow space for 3.5" batt insulation but you can use something like Roxul that has a bit higher r-value, r-15.
Baffles from the soffits up to above whatever you install for insulation.
Not sure what you will want to do with the skylights, possibly box them down to below the ceiling and insulate them very well.

Bud


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## carpdad (Oct 11, 2010)

1. The air space (or attic) that's left after closing off the roof with insulation.
That roof, you can think of it as triangle minus the insulation sizes. Triangle area x length = volume (cubic feet). I say a fan because I like fans for quick exchange of air that can cut down a lot on radiant heat you can feel through the insulation. If lots of insulation, you won't feel (human feeling) the heat. Exhaust fan must have a intake air which can be done if enough ridge venting or mushroom vents. Soffit vents can be intakes too, which yours seem to be. Intake cfm should be close to outtake cfm. So if your attic is about 1000 cubic feet, you want close to 1000 cfm (per minute) fan and intake air vents that lets 1000cfm air through.


2. I'm not familiar with open cell spray foam. Only things such as lower r value and it lets water leak through. A quick search about it got me opposing answers. That it is fine and sheathing will dry out, that it will rot the sheathing. Sorry, nothing definitive from me.


Only definite thing I know is you want wood material (and other things that can rot) to dry if they get wet. As such, separating them is about the only answer, giving space for drying out.


Leaking rain is one problem. Another is vapor condensing into water. Vapor is everywhere, esp outside. This vapor is driven into the indoors when hot/humid outside. My "suspicion" is that open cell in contact with osb/ply sheets will not allow the wet sheets to dry quickly. I admit that this is in contrast with some of the pros. But remember that many pro "opinions" come from incentive to sell their product. If open cell is as breathing as the claims, it also brings in lots of moist air, my logic tells me. So, walls, I'd use fiberglass for vapor permeability because you can't add more drying space. But roof you can. One thing you can do is sacrifice r value along the wall and add air baffles and fiberglass or just couple feet of closed cell spray for highest r value/air seal. Then go to cheaper fiberglass where you can have the air gap as well as r40 or higher. And attic fan to remove moisture/heat. Not a pro but that would be my setup. No plastic vapor barriers anywhere. 



Your walls are sheets and vinyl. Air seal the stud bays by spray foam/caulking every joint. Kraft paper insulation which absorbs some of the vapor and air seal from inside. Spray foam every electric box openings and gaps against insulation. With plywood inside sheathing, not sure how you'd air seal. Do the best you can.



So lower insulation value along the walls and sacrifice some of the energy for cool/heating, but give it some air gap for drying. If in snow area, handle the ice dam with ice barrier on top.


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## carpdad (Oct 11, 2010)

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/14/aft/85623/afv/topic/Default.aspx


3rd reply post by dana1. Esp interesting is, "zone 5, r20 closed cell for dew point control", paraphrased, and spraying foam in lifts (or layers). Layer spray is esp a good reminder. If spray foam is applied too thick and in one shot, center may not cure properly. R20 dew point is specific number.



Above site doesn't look that active. Hope that gives you more ammunition for research. I don't really understand what dana1 is talking about. Sorry but try to keep my life simple. About the only thing I try to remember, initially, is drying air gap.


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## James414 (May 27, 2020)

Bud9051 said:


> Thanks,
> As conditioned living space you will want to address attic ventilation.
> 2x4 walls will only allow space for 3.5" batt insulation but you can use something like Roxul that has a bit higher r-value, r-15.
> Baffles from the soffits up to above whatever you install for insulation.
> ...


There are no skylights. They may look like that in the video but those are just regular light fixtures fixed to the rafters. They're bright though.


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## James414 (May 27, 2020)

carpdad said:


> ...


Ok, I'll have to do more research on the options. I may call an insulation contractor and have a chat. My understanding of open cell is that is that it's far superior for above-grade because it allows water leaks to pass through. Without that feature you can never find your leaks and your exterior wood ends up rotting away (also there can be a delay in when the leak begins and when you even notice it, since you effectively have a foam "shell" with closed-cell insulation).

In terms of air permeability, I'm not sure just how permeable open-cell is. I'm thinking not very. Certainly not enough to keep the air from going stale. However imperfections in the door area, roll-up door, window etc can fix this issue easily. Also we can always crack the window if this becomes a big issue. As I said, I'll have to call a contractor and get an opinion (keeping in mind they are always selling something...).

I'll report back with any insights.


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## Bud9051 (Nov 11, 2015)

Thanks, sure looked like daylight. That will make the process a lot easier.

Support from a thousand miles away with limited text and a few pictures is really tough so thanks for being patient.

Bud


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## Porsche986S (Dec 10, 2017)

James414: I live in north Georgia and here is what I did on my 24x26 man cave garage . Mine started as a pole barn with a concrete floor . And I have a metal roof/siding vs your shingles and vinyl siding but the build process is similar .

1.I have 2x6 walls with 1/2 " plywood on the interior and exterior . Exterior was covered with Tyvex then metal siding installed vertically . I have 4 windows and a door for natural ventilation and light .

2.The walls have standard R19 kraft faced insulation . When I installed the interior plywood I caulked every joint/seam . Then painted white for a bright finish .

3.For the ceiling I used white metal roof panels , similar to what you linked but metal vs plastic . The metal is much stronger and is an excellent reflective finish for lighting . Plus after install you are done there is no painting .

4.My rafters are wooden 2x4 construction . I have full length soffit vents on each side and a full length ridge vent . On the underside of the rafters I stapled 4 ' wide reflective radiant barrier running from front to back . When you are done you have built a cocoon of silver foil . It makes a 3 1/2 " air gap between itself and the bottom of the roof . It allows air to flow up from the soffit vents to the ridge vent . It also keeps the attic space cooler . I finished off the space with R19 fiberglass insulation laying on top of the metal roof panels . I will add more insulation in the future .

5.I have a concrete floor that is sealed with a standard clear sealer .

6.I added a 24K Mitsubishi mini split to heat and cool the space . It does a phenominal job at both . I did a 6 month test running the mini split 24/7 and compared electric bills prior to install . On average it cost $20.00 - $30.00 more per month to heat/cool . And the inside is very dry , you know how hot/humid it gets in the south .

7.I have no idea if " experts " would say what I did was right or wrong , all I know is I built this 2 years ago and have not had any issues show up like mold or leaks or anything like that . It can be 95 degrees out and stinkin humid and I can turn the AC on , set it to 70 degrees and 15 minutes later it is VERY comfortable inside . I then adjust upward to stabilize . Same thing for heat , I can set for 70 and it's 28 degrees outside and in 10-15 minutes I can work inside in a tee shirt . 

Hopefully some of what I have said makes sense to you and can help you make some decisions . Good luck with your build .


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