To poly or not to poly ....

[tundradude]

I have a newly constructed garage with a loft. We are about to lay the floor for the loft. I'm not sure if I should put poly under the floor or not. (or insulation for that matter)

Here are my factors:

The garage is 38x28. The loft is 14x38. The garage has a heated floor. The loft will have it's own heat source. (probably baseboard) The garage (incl loft) will be fully insulated.

I live near Winnipeg, so we see some very extreme temperatures. +35 Celsius in the summer, -35 Celsius in the winter. I plan to keep the garage at around 5 degrees in the winter (and heat it up when required) The loft will be about 2/3 rec room and I plan to keep that about 10 degrees (and heat up when in use)

So should I poly & insulate the space under the loft floor or neither?

 

[kbs2244]

Water vapor go to cold.
If it hits something it cannot pass through, but is cooler than where it came from, it condenses into moisture. (Water)
But if the humidity is low enough in the cold area, it can pass through and stay vapor and blow away.
That is why they put vapor barriers on the warm side of home walls but not in the attic.
The vapor goes through the insulation to the dry, cold, winter air and is vented.
If you are going to keep both spaces heated all the time, and are not going to be washing cars, then you do not need the vapor barrier between them.

If you are going to be washing, put in the poly and also a powerd vent fan to exhaust the garage. You will "waste" some heat. But you will not rust all your tools.

You will have to have some way for the vapor to get out of the top floor.
Depending on what you are useing it for, and how much vapor you are creating, will decide that.
But, from the sound of it, I would treat it like a house.
Vapor barrier on the walls, but not in the attic.

 

[Steve in Mi]

I would recommend the insulation between floors for two reasons. 1.) in those cases when you are heating the upstairs only, w/o insulation in the second floor you will have a cold floor in the recreation room - not comfortable.
2.) in those periods that you only want to heat downstairs, w/o insulation between floors the heat will migrate upward (heat rises) to the rec. room - so you in effect will be heating both areas.

I put poly on the warm side of walls and ceilings. The idea of moisture rising thru the insulation and being swept away is bogus - the moisture vapor will travel into the insulation until it reaches the dew point and then water will drop out leaving you with wet and/or ice laden insulation - which is poor insulation.

 

[kbs2244]

From the text from the first site:
(Bolding by me.)

Condensation of moisture can be a concern when a radiant barrier is installed on the attic floor directly on top of conventional insulation. During cold weather, water vapor from the interior of a house may move into the attic. In most cases, this water vapor will not cause any problem because attic ventilation will carry excess vapor away. During cold weather, a radiant barrier on top of the insulation could cause water vapor to condense on the barrier's underside. Condensation of large amounts of water could lead to the following problems: 1) the existing insulation could become wet and lose some of its insulating value, 2) water spots could appear on the ceiling, and 3) under severe conditions, the ceiling framing could rot.


http://www.insulation4less.com/install_attic.asp

http://www.ankn.uaf.edu/publications/VS/insulation.html

http://www.owenscorning.com/around/insulation/faq_attic.asp

http://www.owenscorning.com/around/insulation/fallpromo/InstallAttic.asp

http://www.ornl.gov/sci/roofs+walls/insulation/ins_05.html#moisture

http://www1.eere.energy.gov/consumer/tips/insulation.html

http://www.weatherization.org/atticinsulation.htm

http://www.habitat.org/env/pdf/ceiling_and_attic.pdf