Insulating your Attic – Part 2 by Richard Komorowski
Cornwall ON – At the end of the last article, I promised to deal with basement insulation. However, because of space and time constraints, I left a lot unsaid about insulating the attic.
Last time, we dealt mostly with fibreglass insulation, which is relatively cost effective, and easy for the home owner/handyman to install, thus saving labour costs if the budget is tight.
However, there is another form of insulation which deserves more attention – blown cellulose. This insulation needs little energy to produce (as opposed to fibreglass, which is made from molten glass), and the major material is almost 100% recycled. The Ad Bags, Seaway News, Standard Freeholders and Guy Lauzon broadsheets can achieve a long and useful second life, keeping the heat in.
This insulation is made basically by grinding up newspaper into little pieces, and treating it with chemicals to retard mould growth and fire. Like fibreglass, it is compressed and packed into plastic bags.
Fibreglass is normally laid down in batts, by hand, although it can be chopped and blown into an attic. Cellulose is normally blown by a machine which works like a high power vacuum cleaner in reverse. One can rent these machines, although it is probably better to leave this kind of insulation to an experienced contractor. For the handyman on a really tight budget, it is possible to open the bags in the attic itself, and spread the contents around evenly, with a garden rake.
There are advantages and disadvantages to both fibreglass and cellulose insulation in the attic. Fibreglass is (relatively) easy to install, (but very itchy). Cellulose is more work, but has a higher R value per inch – around R3.7 per inch of cellulose vs R3.15 per inch of fibreglass. In addition, cellulose, being denser, is somewhat more airtight than fibreglass.
When it comes to upgrading my own attic sometime this summer, I shall probably use cellulose. There are 4 reasons.
- Budgets are limited, but I can install fibreglass or cellulose myself. With fibreglass however, once the level goes over the joist height, it will be hard to find them later on when I want to put in more insulation. With cellulose, however, I can simply blow in more, without having to walk on the joists.
- I have some cellulose insulation in place, which I plan to keep. I could cover it with fibreglass without problem, but when it comes time for another upgrade, for best effect I would have to continue with fibreglass, which could be difficult for reasons stated above.
- Cellulose insulation is fairly dense and solid. For this reason it does not sit well on top of fibreglass, as the weight would squash the batts and reduce their R value.
- Because fibreglass is such a light weight material, it suffers from a “wind effect”. If there is a draft above the fibreglass, such as might occur in a well-ventilated attic, the insulating value of the top two or three inches is reduced. Covering the insulation with a semi-permeable material, such as house wrap, which allows moisture to pass one way, but not the other, would solve this problem, but why go to that trouble and extra expense?
Regardless of the insulation used, it only works properly when the insulation is dry. Try lifting a hot dish from the oven without burning yourself! This is because the heat travels much faster through the moisture in the cloth than it does through the air it replaces.
Three factors help keep the attic insulation dry:
- A water tight roof. If the roof leaks, you’ll have soaking wet insulation which, if it becomes heavy enough, will crash down through the ceiling into the room below.
- The attic needs to be properly ventilated, so it can get rid of excess moisture. The Ontario Building Code states that for a typical attic, the total ventilation area must be at least 1/300thof the attic area, and should be distributed as evenly as possible between the top and bottom of the attic. As an example, a 1000 square foot attic needs at least three and one third square feet of ventilation area.
- In order to keep moisture from the house itself from penetrating the insulation, there must be a vapour barrier. The vapour barrier (typically 6 mil polyethylene) goes on the warm side of the insulation, behind the drywall, and it must be completely sealed, without tears or holes. In the case of a ceiling, it would go above the drywall, between the drywall and the joists and insulation.
There is one more factor to consider for keeping your house comfortable. When it comes time to replace the shingles, don’t even think of using black or dark coloured asphalt shingles – use the lightest colour available. Black is the most effective colour for absorbing the sun’s heat. During the height of summer, the attic of a black roof can be as much as 40° hotter than that of a light coloured roof, and this will reflect significantly on your comfort and air-conditioning bill!
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