Our plan is to remove the old asphalt shingles, then installing 1.5" styrofoam panels between 2x4's on top of the excisting plywood to add more insulation to the roof since the roof has no attic space on the lower 2/3 part of our roof and only has attic space above the bedrooms upstairs.
We also plan to ventilate the small attic space in the top by installing 2x vents on either side of the house.
This house was built in the early 40's and have shavings insulation.
My only concern is if this could create a condensation/moisture problem underneath the Styrofoam?
The winters out here in Alberta are cold.
First I would have an energy audit conducted to see where you are actually losing the most heat.
The cost to upgrade may be more than the energy savings gained and possibly comfort levels.
You may be further ahead by weather stripping, foam gaskets behind outside wall outlets and switches, and upgrading windows if not already done and adding additional insulation if possible in the attic area.
Also insulate the basement rim joist areas since this is were you will have cold air entry, and insulate basement walls if not already done.
You can not just place the foam on top of the roof, you will have condensation problems as you suspect. it will also do nothing for the top 1/3 of the roof.
For the ventilation, yes add the vents on top but you need a continuous air flow from the sofits all the way to the top vents in order to properly vent the roof.
If you want to upgade the insulation, your limited on this style of house. Start with you attic and add as much blown in insulation as you can.
The other sections of roof will require that you take off the drywall or the roof plywood to gain access.
Spray foam insulation would be best for the sloped sections in the bedrooms and blown in or regular wall insulation for the short walls in the bedrooms.
Just remember you need continuous air flow to the peak of the roof. If you want the absolute most insulation, you can take off all the roof plywood and nail 2x4 or 2x6's on top off your rafters to give you more space for insulation and air flow.
You would gain nothing from placing a Styrofoam insulation over a vented roof system. I am a past representative of Dow Chemical who are the manufacturers of the product. The use of Styrofoam does work in 'compact' or cathedral roofs which is not your case.
We have recently build a metal house building and after facing two major problems of condensation and loss of heat, we have decided to put insulation and among many options we ended up with spray foam insulation. Insulation is done for the purpose of reducing problem of condensation so there is no problem of moisture under Styrofoam.
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