Insulation, Radiant and Vapor Barriers - Gambral Roof Insulation

We own a 2 story Gambral roofed cottage on an island in Georgian Bay. It was built in 1975. We have renovated the main floor installing new insulation and vapor barrier on all exterior walls. Next spring we will start the upstairs reno work. My question is what is the proper way to insulate a Gambral attic. The current insulation is the old paper backed glass bats type and is stapled to the undersides of the roof rafters and continue down the second slope of the gambral to the soffits which are not vented to the outside. I have always thought that the attic side of the ceilings should be insulated as well as the attic side of the interior knee walls as this will form an envelope. What does anyone else think??

Hi w1 and welcome to the forum. I'm glad you are here to ask as most show up after the work is done, at least the upstairs is yet to go.
The knee wall on the sides can be done either way, but probably not both. I will add some links to explain. Depending upon your rafter sizes, 2x6, 2x10, and such it is usually difficult to get enough insulation inbetween the rafters. Thus a layer or two of rigid before the sheetrock can be a great addition. Check out the reading and post more questions if needed.

Also, if the upstairs is just craft and fiberglass right now, as cold weather sets in, there will be ice forming on the inside of the sheathing. Warm moist air leaks up there and finds it's way behind the kraft and freezes on the cold surface. Seen it all too many times. Since it may not be well heated up there, even adding a plastic vapor barrier may not help as the moisture will just condense out on the surface. Anyway, check the links:

Welcome To Home Energy Magazine Online (http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/95/950309.html)

Energy Savers: Vapor Barriers or Vapor Diffusion Retarders (http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810)

Bud

We own a 2 story Gambrel roofed cottage on an island in Georgian Bay. It was built in 1975. We have renovated the main floor installing new insulation and vapor barrier on all exterior walls. Next spring we will start the upstairs reno work. My question is what is the proper way to insulate a Gambrel attic. The current insulation is the old paper backed glass bats type and is stapled to the undersides of the roof rafters and continue down the second slope of the gambrel to the soffit which are not vented to the outside. I have always thought that the attic side of the ceilings should be insulated as well as the attic side of the interior knee walls as this will form an envelope. What does anyone else think??

My opinion has always been that a attic should be a cold space and that the insulation should only be to the room side and not on the roof side.

You need air flow inside of the attic to take away the heat in the summer and the winter to protect the shingles and the roof.

So basically anyone who builds a house like this and is determined to turn it into a living space is totally stupid in my opinion.

Gambrel Roofs are fine for a shed, but were never intended as a main roof for a house. It seems that some people are bound and determined to do everything wrong and then they hope for the best and when it doesn't work out they sell it to someone else and get out from under it and make it someone Else's problem.

Holmes on Homes did a remodel job for a homeowner who wanted a second story addition and a contractor came in and insulated the attic and Mike had to take it all down, restructure everything, put up a pitched roof. It was a good idea gone bad. Even his own workers - who left unattended put up a new roof and did not know how to figure pitch and built a roof with the wrong pitch and he made his own guys tear it all down and do it again.

Basically you want a cold zone in the attic and you want a warm zone where you are going to live and you want a certain amount of head room and a certain amount of space when it comes to building a house and a Gambrel Roof is not the way to do it - unless you plan to use the upstairs as a attic and not a heated living space.

I agree with Mr Fix It - except for the part about gambrel roofs. I have a two story gambrel cape in New England where that roof style is very common. The insulation is installed between the ceiling joists and not between the rafters. Unless you plan to heat the attic, putting insulation against the rafters is a bad idea.

You might want to evaluate the ventilation also. You can google attic ventilation and find all sorts of sites that will help you calculate how much ventilation you need based on the size of your attic. Mine has gable, ridge and soffitt vents.


If you insulate and ventilate this way the attic will remain cold in the winter and you won't get any condensation.

I agree with Wayne, the insulation should be on the ceiling of the first floor and not on the second story ceiling / against the roof and the second floor should be used for storage and not a living space.

When you insulate - you need a cold zone to protect the roof and you need adequate ventilation.