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Discussion Starter · #1 ·
Hi, I'm building a small workshop accessory structure at the back of my property. I do plan on having a heat pump for it when I am using it. I'm in southern Ontario. First step is the concrete pour, and I'm wondering if it's worth it to put 2" of rigid insulation underneath first? My research thus far has differing opinions on whether it is worth it or not. Thanks for your time.
 

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What size of building?
What type of foundation are you planning?
What is the ground like underneath and do you have an idea of the water saturation (are you high on a gravel hill or low in a boggy area)?
Are you in a location where you'll be taking out a building permit (so some of your choices will be made for you)?
 

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Discussion Starter · #3 ·
16 x 20 foot building. 6 inch concrete slab with 4 inches of gravel underneath. 12 inches of concrete at the perimeter. I'm in a high area. I've taken out the permit already. It doesn't require rigid but it's up to me whether I want to add it or not.
 

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Discussion Starter · #5 ·
What size of building?
What type of foundation are you planning?
What is the ground like underneath and do you have an idea of the water saturation (are you high on a gravel hill or low in a boggy area)?
Are you in a location where you'll be taking out a building permit (so some of your choices will be made for you)?
16 x 20 foot building. 6 inch concrete slab with 4 inches of gravel underneath. 12 inches of concrete at the perimeter. I'm in a high area. I've taken out the permit already. It doesn't require rigid but it's up to me whether I want to add it or not.
 

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Thanks. What if I just did the sides (not the bottom)?
The floor mostly is the same temp year round . We do some around the perimeter in a basement when it is at ground level. I would challenge any one to be able to tell which house has it and which doesn't.
 

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I'm in Maine somewhat similar climate and have see condensation at times when the ground is cold and humid air gets in. Rather slippery. When heating, it will take longer to warm up a slab on top of that gravel. The insulation would help if you are turning the heat off and on, maybe.

Bud
 
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The floor mostly is the same temp year round .
Six or eight feet below ground will be closer to constant temperature, but the top foot of ground, in Southern Ontario, in January will be frozen solid.

2" of rigid XPS is not cheap. I can't answer if its "worth it". If you heat the building all winter to 72F, then definitely I would put the insulation. I don't see how that is any different than should the walls be insulated. If you are more concerned of the comfort of standing on the cold slab, well I don't know how sensitive you are, but likely you will be wearing heavy shoes or boots, if its a workshop.
 

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Six or eight feet below ground will be closer to constant temperature, but the top foot of ground, in Southern Ontario, in January will be frozen solid.

2" of rigid XPS is not cheap. I can't answer if its "worth it". If you heat the building all winter to 72F, then definitely I would put the insulation. I don't see how that is any different than should the walls be insulated. If you are more concerned of the comfort of standing on the cold slab, well I don't know how sensitive you are, but likely you will be wearing heavy shoes or boots, if its a workshop.
Inside a shed with no heat?
 

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Whether you insulate or not you will need a vapor barrier. If you don 't keep it conditioned the slab will take on the temperature of the ground below it.
That will happen slower with the insulation. I have worked in areas that are conditioned year 'round and the slab never reaches the temperature of the air.
A thermal break will make it possible for the concrete temperature to be closer to room temperature rather than ground temperature.
 

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Per the OP, there WILL be heat in the building when in use. I am in SE New England and we put 4” of foam covered by 6 mil poly with taped seams and then wire mesh in basement to all new builds.
 

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I have 2 outbuildings that are heated and neither has insulation under the floor. I have seen rare cases where frost goes under an insulated floor and heaves it (unusually cold weather for a long time without snow cover). If I were to provide insulation it would be vertically at the wall line to the maximum frost depth to keep frost out from under the building. For occasional heat it won’t matter.
I heat my buildings to 45F minimum all the time, warmer when working in them. I run ceiling fans all the time to push heat down and dry the floor quickly if a snow covered car is parked inside.
 
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