Children's Course

Attic Insulation

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Editorial by Dr Christopher Cooper

I will focus my attention in this article on attic insulation, which in most cases, with respect to an old house, it too, is usually installed incorrectly, and the savings you think you are getting will be the opposite when heat is drawn out of your house with a resulting moisture problem.

Let’s talk science:

The science of insulation is a tricky one.  Insulation is intended to be-still the air, because in science, still air has the most effective resistance to cold and heat.  For every inch of still air the resistance equals R1.   However, if the material to be used as a way to be-still the air, a.k.a. insulation, gets wet, the reverse is true.  With respect to attic insulation, the insulation will get wet by not having the proper ventilation surrounding it:  for example, a dew point situation, when warm fuzzy insulation is slapped up against a cold surface, such as the interior side of the brick cladding or the interior side of the wooden wall sheathing, cold meets warm, and “BLAMO,” you have water droplets!  A wet or sustained moisture resident in insulation (all insulation), will now act as a heat sink, pulling the warm radiant heat to the exterior of the house, and who needs to heat the exterior on a -20 degree Celsius day in the middle of February!  By the way, do you have immense icy stalactites hanging from your filled and solid frozen eaves troughs?  If you do, guess why?  Your roof is improperly ventilated.


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Above Left: Water damaged (yellow) fibreglass insulation, note the wood rot and mould! Above Right: An early 19th century wooden wainscotting destroyed by wet mouldy cellulose insulation of the blown-in variety. The entire sill and many of the studs of this house had to be replaced.

I think we all know that when you have ice dams and you have a warm sunny day, the ice will melt and allow the water to find its way back up underneath the shingles and, voila, you have a roof leak.  As a result, now, of course, we have the following issues – with a heightened level of sustained moisture in insulation, mould, and what follows, rot, and then what follows after that, insect infestations, usually carpenter ants.

Attic insulation is not that easy.  Sadly, in every blown-in application I have seen there is never a thought to ventilation.  The usual get-in, get-it-up, get-on-to-the-next-job attitude that plagues this nation of ours, is put into play.  You must have proper ventilation in your attic.  A minimum of 1:150 is required in most old houses.  That translates to a one square foot hole in the roof for every 150 square feet of attic space.  But it doesn’t end there; you must have at least half of that ‘hole in the roof’ (0.5 square feet) in your soffits.


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Natural fibres such as sawdust, flax and hemp are becoming very attractive green alternatives to possibly dangerous man-made materials.

The Yin and Yang of Attic Insulation:

Okay, after all that sabre rattling, what is the right way to insulate an attic without causing damage and to help save energy?  Let’s first review the batt insulation that is available on the market.  For the truly green option, there is recycled denim, sheep’s wool, hemp and flax batt insulation.  These items are excellent alternatives, however, they are also very expensive, and in many cases, are very hard to source.  There are also two readily available manufactured batt insulation products: Fibreglass (a.k.a. pink, yellow or white woven glass insulation) and Roxul mineral wool insulation which is made from basalt rock and slag.  Roxul (which is the company’s name) has superior fire proofing capabilities and is the best candidate of all of the batt insulations (including the green alternatives), to recover after being saturated by water.

Both woven fibreglass and Roxul insulation have health issues.  Roxul claims “Temporary mechanical irritation of the upper respiratory tract (scratchy throat, coughing and congestion) may result from exposure to dusts and fibres in excess of applicable exposure limits.”  On the other hand, in 1991 the US Occupational Safety & Health Administration mandated cancer warning labels on products containing fibreglass that may become airborne, including fibreglass insulation.  Fibreglass also has a miserable track record for looking like shrivelled-up day-old cotton candy when it gets wet.

The previously mentioned green products are not supposed to have any issues with respect to respiratory problems, however, whatever product you use, an approved cartridge type respirator (rated for lead and asbestos) should always be used, not just a paper mask!

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What do I use?  I use Roxul batt insulation for the reasons I mentioned above.  The flat areas of the roof should be at least R40.  A layer of R20 Roxul batt insulation should be placed between the roof joists and another layer of R20 perpendicular to the first, essentially stopping the bit of thermal bridging that may happen through the wooden roof joists (see Image 1).  The insulation will need to be kept back from the soffit vents to allow ventilation. There needs to be a clear space of at least 2 ½” to 3″ of air space between the roof sheathing and the insulation.  There are several ways of maintaining this, one by just keeping the batts away from the soffit vents by cutting the batt on an angle or by installing air baffles (Raft-R-Mates, see Image 2 and 3).

Let’s Talk Vents:

I detest aluminium, as it is not character defining and is usually covering up a solid soffit area.  A vented aluminium soffit is useless if it is just a band-aid over a solid soffit with no ventilation, and usually the aluminium is hiding rot, mould and deterioration or is the root cause of the aforementioned (see Video Below).  A small readily available micro vent (see Image 4) can be found at most hardware stores which can be painted to blend into the wooden soffit and installed with a circle cutting saw on the end of a drill (see Image 5).  These micro vents also have the calculations of the area required to quantify the total soffit vent requirements (above mentioned).  The micro vents are cut into the existing soffits between the roof rafters and roof joists bay, preferably at each bay around the house, for a balanced and equal ventilation and for aesthetics (see Image 3).  The roof top vents, if existing, must be measured to see if the ventilation is adequate by using the calculation of one half of 1:150 – if more are required then more should be added.

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Well, there it is, how to effectively insulate one’s attic, without damage, and have it function properly.  Functioning properly is the key to energy savings and is the key to a healthy house.  The best thing about this is that it is a project that can easily be undertaken by the novice restorationist in a couple of weekends.


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