Insulation vapor barriers?

Question

A friend is insulating the attic for heat loss in her cabin in northern Michigan. Above the living space ceiling is some blown insulation covered by wood on the top of the truss base (to allow for attic storage). A contractor is rolling R25 plain insulation over the top of this wood planking. Is this wise and/or okay?

Answer 1

Seems a bit daft to me because doing it in this manner will prevent you from now storing things in the attic. Technically as long as the vapor barrier is facing the side that is heated it should be ok, but unless the old insulation also has a vapor barrier you could see condensation trapped and sogging down the old insulation. Sounds like this is about the laziest way to do it. Of course, to replace the old insulation, which may be packed down and full of mouse droppings, you'd have to take up the planking and maybe even elevate to accomodate extra thickness of the new insulation, and that's a lot more time and effort and money.

Answer 2

It depends on where you live. The paper on insulation is a vapor barrier and in MOST parts of the country it should face the living area. Most insulation with a paper vapor barrier will actually be labelled "THIS SIDE FACING LIVING AREA" or someting like that. If you are using bare insulation without a vapor barrier and then add the vapor barrier on your own (like stapling vis-queen over the insulation) the same rule applies- the vapor barrier usually faces the living space. In certain parts of the country with a lot of humidity (like Florida) the vapor barrier will be installed facing away from the living area. Contact your local building department or a local insulation company to find out what is recommended in your climate. clpmjb20

Answer 3

I don't think it's a good idea to roll out on top of the plywood. The part of it that concerns me is the air space between the top of the blown insulation and the bottom of the plywood sheets... if the plywood is tight against the blown insulation below it, no problem rolling on the plywood... if there is a space between or cracks between sheets laterally, you might find some moisture issues (then mould...) down the road...
The only solution would be to put a vapour barrier on top of the plywood... the unfortunate part of this scenario though is that you would then have to open vents to the blown insulation area between each rafter and below the plywood. The vents would need to vent into the space below (finished area) and NOT to the outside. This would effectively raise the attic to above the plywood and introduce the lower part of the attic into the airflow on the inside of the house... not an easy chore... not a good plan.
I would remove the plywood sheets, install the new insulation over the blown and then replace the plywood for storage.
Good luck!

Answer 4

If there is a vapor barrier at the "bottom" this is ok. The vapor barrier should always be as close to the heated space as possible. One way to do this is to use a vapor barrier paint on the ceiling. If there is no vapor barrier under the blown in insulation, the paint would be the thing to use. It's basically a good alkyd enamel, with a matte finish so it works well as a primer for the finish coat.

Answer 5

Look up " Radiant Heat Barrier" on the web. I highlt recommend this product. You will too.