Insulating a Finished Attic


Old 03-10-10, 05:32 PM
Thread Starter
Join Date: Mar 2010
Posts: 3
Insulating a Finished Attic

i just bought a house with a finished attic. It looks like the livable area has been insulated with R-19 at the walls. There are two crawl spaces that run the length of the house with no insulation at all. The attic already has Durovent installed the entire length.

The two crawl spaces have no insulation above the floor boards. Like an idiot, I began laying R-30 above the floorboards without knowing a lot about roper insulation. I do know that there is some old insulation (its yellow with a vapor barrier) beneath the boards. I have done a lot of reading but can't get a good answer. Am I wasting my time rolling the R-30 on top of the boards? I live in NY, it gets cold.
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Old 03-14-10, 12:39 PM
Join Date: Feb 2008
Location: Lake Wales, FL
Posts: 463

The modern way of dealing with this, is to think of the living space as a comfort zone.
The comfort zone needs to be isolated from the outside cold/hot world.
As most of the heat escapes from the home through holes and cracks, they need your first attention.
Once all the holes are blocked, then it doesn't matter how hard the wind blows, your warm air will stay in the comfort zone.

The next consideration is the heat loss/gain by conduction.

The Durovent is too far away from the comfort zone to have any impact on heat loss or gain.

It is easy to understand how the sun shining on the roof, heats the timbers underneath and these in turn heat the rooms below.
The winter uses the reverse system where the room heats the ceiling that heats the timbers that heat the roof that looses heat to the sky.
Insulation needs to be as close to the room/comfort zone as possible, so that you do not loose heat by conduction.

When you place insulation between the joists, that slows the heat loss, over that space, but there is still the large amount of heat being lost via the joists themselves and the parts of the roof and attic floor, connected to the joists.

As you guess, putting insulation on top of the attic floor, is a waste of time and money. The heat just slips out underneath.

The most effective way to insulate is to use closed cell polystyrene and to place it across the ceiling in the room under the joists. This stops the heat loss through the ceiling and through the joists.

Then if you want to improve on this, then fit more tightly fitted polystyrene between the joists (as much as you can pack in)

You need to treat the attic walls the same way, with polystyrene between the uprights and rafters with another layer of polystyrene below the rafters and on the front of the uprights.

This will keep your expensive heat inside the comfort zone and will save a lot of money.
Old 03-14-10, 01:32 PM
Join Date: Oct 2008
Location: New England
Posts: 9,995
Hi Yaemish and welcome to the forum. Sounds like you have a cape with kneewalls and side attics, very common for us up north. Here is a link specifically on capes to get you started.
Welcome To Home Energy Magazine Online

Air sealing, as mentioned, is very important and a lot easier to take care of before all of the insulation goes in. One of the unseen characteristics of a home is how air easily moves through the structure of the house once it enters any wall or ceiling cavity. It may follow an adventurous path, but air entering around any electrical box or other penetrations can easily find its way into ceiling cavities or the attic. Here is another link on air sealing. Slow to open, but good information.

Like building anything, it is how you get started that sets the stage for what you do. Asking now will really help you choose what is best.


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