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Old 08-19-2010, 10:32 PM   #1
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Sloped ceiling insulation retrofit

I have a 1940's 1 1/2 storey house - ie sloped ceilings in the bedrooms. I'm pulling out some shabby old built-in closets to give us more floor space, and had to remove some of the sloped ceiling too. I want to do some upgrade of the insulation while I'm there. I now have removed the drywall and layers of original ceiling tile and fibreboard. I'm down to the foil, which has quite a few holes, and behind it I can see the paper of the original batts. Roof structure is 2x4 rafters with a knee wall half way along the span from eave to cross-member.
I guess I am wondering what people would suggest - I'm in Ottawa, Canada so R values are important but I can't see how to get much with 2x4 rafters.

Thanks for ideas


Last edited by Favouritesnail; 08-20-2010 at 05:41 AM.
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Old 08-20-2010, 08:57 AM   #2
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As you can see the original insulation was fairly useless.

If you are going to insulate, then do the best job you can.

The recommended insulation is 14 inches of polystyrene = this will mean that for most of the winter you will use, just the heat from your bodies and any other electrical equipment you may have switched on to be comfortably warm.

While this may seem a lot of insulation, it will be a once off job, and will quickly pay for itself, then you will have years of miniscule power bills.

If you do not wish to insulate to the best standard, then use at least six or seven inches of polystyrene.

Remove the existing insulation, then carefully leaving a one inch gap between the existing roof and the top of the new insulation (to enable any water that may leak through the roof to run down and escape)
fill the spaces between the rafters with sheets of polystyrene, cut to a push tight fit with a sharp knife.
Then fix a sheet of plastic below the polystyrene and the rafters followed by another layer of three inch thick polystyrene, this is to stop the heat from your home being conducted through to the roof via the rafters to the outside air, then finish with drywall.

Buy an infrared temperature gun, this will indicate the surface temperature of anything you point it at.
Use it to scan the inside walls, ceilings, floors and outside walls and roof to be able to measure the before and after effect of what you are doing.

It is quite likely that the walls of your home are poorly insulated as well. When you have experienced the benefits of the improvement to your roof, then move on to the walls and downstairs floors.

Most of the heat is lost, due to the effect of the wind, pulling the warm air from your home through various gaps and cracks, try to fill as many of these as you can before winter.

Using a closed cell insulation like polystyrene, will ensure that the wind cannot pull the warm air from your home and the extra internal layer of polystyrene placed below the rafters and on the room side of your walls, will stop the heat that is lost by conduction from escaping.


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Old 08-20-2010, 09:24 AM   #3
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Thanks Perry525

Doing a bit more research I think I have an unvented roof design (a 'hot roof'). There is no airflow between soffit and ridge - the batts basically fill the space but are old and baggy in places (there'd be even less room for insulation with an air channel / baffle in there). Would the gap you are suggesting change that basic design? Other than that it sounds like your suggesting upgrading the existing fibreglass to rigid foam, then vapour barrier, then more rigid foam (3 inches might mean too much loss of headroom... but the old layers of ceiling tile etc were about 2" so it might not be too bad). Is a foil layer in there a good idea too? If so, as the vapour barrier or where?

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Old 08-20-2010, 10:10 AM   #4
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That design is typical of the day, it is only in recent times that the idea/theory, that the passing wind will pull the water vapor (and your warm air) from the home during winter to avoid condensation and ice in the roof came about. The idea does not work, as there are many days and nights when it is freezing and there is no wind.

However, while you can fit the insulation up tight against the inside of the roof, if there should be a leak, then it will (probably) only be when a lot of damage has been done that you will know.

leaving a space for the water to run down and some air to move through, will result in a minor leak not doing any damage for a long time and maybe you finding you have a leak sooner rather than later.

From a insulation point of view, then filling the space with a closed cell insulation, will give you the best result.

The problem with fiberglass is that both air and water vapour can and do move though it and more important round it, it is impossible to make fiberglass and almost air tight fit, and it is the air being pulled through and round it, that makes your home cold, you also have the problem that in freezing weather, water vapor will freeze inside the fiberglass turning to ice, that then thaws making the fiberglass wet.

Water is a better conductor of heat than the air that normally fills the holes in fiberglass, it is the air that is trapped in the fiberglass that provides the insulation, when the fiberglass is wet it is useless as an insulation

Water is an express escape route for your expensive heat, it is 4,000 times better at conducting heat than dry air.

Polystyrene is 2% plastic and 98% air, that is why it is so good.
Polystyrene can easily be fitted to a tight almost air tight fit, by cutting it slightly oversize at a slight angle then placing a piece of wood over it, and hitting the wood with a hammer and forcing it into place.

As polystyrene is comprised of thousands of tiny bubbles of air each surrounded by plastic the air is trapped and kept dry by the thin plastic skin.

Never the less polystyrene is strong and you can stand on it.

The plastic membrane is a fall back, to help prevent the water vapor in your home from getting into the wood frame making it damp and possibly leading to mold and wood rot.

I understand that the only way to have the perfect home, is to design and build it yourself, working with something that's already built, does mean making a compromise, you trade off saving money on heating for a bit more space.

With the walls, some of the insulation can go on the outside.

With the roof, there is a system called SIPS, you can use this at some point in the future when you are repairing/replacing your roof.

Foil is used to reflect the sun or other source of infrared heat, it needs to have a clear space on the side you are reflecting, either upwards for the sun or down to keep the heat in. From what you write, you do not have enough space.
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insulation upgrade , sloped ceiling

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