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Insulating a Heated Workshop

xtal_01 | Posted in General Questions on

I am confused  and need advice.  I am building a large workshop … just over 3000 sq ft on the bottom floor and just over 1000 sq ft of attic storage.  It has 16 ft walls.  I have divided it into three bays.  There are two 14 x 14 doors in two of the bays.

I live in Vermont .. we can hit -30 … only average 5 days above 90

Standard stick construction … 2 x 6’s with lots and lots of blocking … covered with 1/2″ OSB.

We used a thicken edge pad (hmmmm … long story  how I was talked into this, but it is what it is) … 2″ of foam under it … 2″ around the outside edge  …. plus 2″ starting at the bottom of the edge and going out 2′

So, I am planning to heat it.  I have natural gas already to the building.  I will use three zones (or three furnaces or  three ???).  Since I do not use it every day, I want to keep it as say 45 or 50 deg  … just so the coolant in my machines, my  paint …. anything in the shop will not freeze.

On the days I go out to the shop, I will heat the bay I am working it.

So, I have been careful to try stopping air leaks … used a gasket and caulk under the sill.

I had planned on just doing a standard insulation job … batts between the studs … blown in insulation in the ceiling (I have a 24″ energy heel so with 18″ of insulation I will have a 6″ air gap … great since I am planning on a metal roof over purlins.

As I was looking at the building I realized just how much bridging I will have because of the blocking, lvl’s and such.

I have a tight budget … building it all myself … so I can’t get extravagant and use something like spray foam or build a double wall.

I thought maybe I could buck out the windows 1 1/2″ (2 x 4 on flat) … put 1 1/2″ of iso or xps reclaimed foam on it … plus the batts … strap the foam, cover with vinyl siding … all set.

Then a friend who is very  eco building smart said NO! First he said I should not heat the building … ok, not an option.

Then he said either use 6″ of foam (out of my budget) or use none.  He said I would rot my walls away … the water would condense on my wall sheathing.

I even saw one person put the insulation on the inside …. but might have the same problem?

I tried reading up on this … it looks like it might be true but then there is always talk about water load (should not be much in the shop)  and use of a vapor barrier inside.

Just not sure where to go from here.

Here is a link to some videos my wife put up showing the project.

Thanks ….  Mike

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  1. brian_wiley | | #1

    Hi Mike,

    I'm not entirely sure why your friend said to leave the shop entirely unheated, but he is right about needing more than 1 1/2" of exterior insulation on your proposed assembly. I believe that you're climate zone 6. If that's the case you need around r-11.25 with your 2x6 walls. There's more information here: Depending on which insulation you pick—eps vs polyiso, for instance—that may be closer to 2 inches, but more is always safer to a point.

    While it can be difficult to track down sometimes, used polyiso may be an option. Checking craigslist might turn up some options that'd keep the budget in check.

    I wonder if you'd be better to put an inch of foam on the inside of the building before you hang sheetrock to address the thermal bridging? Definitely check in with a more advanced member, but it is something I've seen brought up here before.

  2. xtal_01 | | #2

    You read my mind!

    I was wondering why no one ever puts foam on the inside.

    My fiend keeps trying to convince me that a shop that is only used a few days a week does not need to be heated ... he has never had expensive machines that have coolant in them ... or cans and cans of paint that he does not want to freeze.

    Like I said ... I think keeping it at say 45 deg should be reasonable ... then just turn up the thermostat in the bay I will be in that day (all my thermostats are on wifi ... easy to do as soon as I get up).

    I don't expect a super efficient shop ... but if for a few thousand more I can cut the heating cost ... over say 25 years (I am almost 60 now ... hoping to use it till them) ... I would put in the investment now.


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