|Understanding your attic|
Almost all attic areas are now insulated with any one of numerous types of blown in insulation. There maybe even layers of different types of insulations.
Big Block Inspections recommends a minimum of R-50 insulation in the attic. (Big Block is as of composing this document, constructing a 4300 sq.ft. Big Block designed home and the attic insulation is min R-65 and exterior walls are min R-35.
I think of insulation as money in the bank, it never stops making you money, 7 days/week, 24 hrs./day (possibly a poor analogy for what banks pay in intrest, although I am sure, you all understand the comparison))
The attic although may be insulated to a proper value, still requires ventilation originating from soffits (roof overhang) into the attic (through pink chutes placed above exterior wall) and flowing through roof with either roof vents or roof ridge venting. (I am partial to whirly bird type venting as a few inches of snow covers roof type vents and may block air circulation and resulting non-exit of moisture)
The attic using a typical build requires ventilation, as insulation does not prevent all heat loss,(remember in winter, attic is cold and in summer attic is hot) plus there is movement of moisture from interior of home to exterior in attic.
Since insulation is not 100% a preventer of heat travel, one must consider what causes ice damming on a roof.
Hmmm! You say, I just added 600mm of blown insulation to my attic and still have ice damming plus possibly areas of dampness where insulation touches roof sheathing.
Why you ask? Lack of VENTILATION from soffit area to attic. Remember insulation is not 100% efficient, heat travels through the insulation searching for a cooler temperature, the attic area and roof is the cool area during cool seasons. The area of roof above exterior wall is in close proximity to heat loss from interior of house therefore causes snow to melt.
The insulation blown into area where roof meets wall acts as a media for heat travel.
If you look in your attic you should see (usually) pink or black coloured foam/plastic chutes placed between roof sheathing and top of insulation.
These chutes allow ventilation between soffits and attic and which convects heat loss through insulation into attic instead of to roof sheathing.
These vents in the opinion of Big Block inspections should be installed in every rafter or truss space. (The 11 new homes that I have constructed, never have ice damming, as I install these pink vents continuously along all exterior attic walls.) One thing to remember and I see so often, if you have ice damming, do not jam more insulation into area above exterior wall, as the more you compact the insulation the easier for heat travel. This area must be ventilated to transfer heat to the attic area and out to atmosphere.
The cooler the air the less moisture it will contain, the warmer the air, the more moisture it will contain.
With this thought in mind, cool air travels from the soffit, is warmed as it passes through the area where exterior wall meets roof line (remember insulation is not 100%efficient) and since is being warmed, has the ability to hold moisture.
Think about this, if there is a continuous vent between where roof meets exterior wall, the moisture and heat will rise or convect (pulling air from soffits) into the attic and out the roof vents.
I see most homes have very few of these vents and area are plugged with insulation, this every time I witness ice damming.
Winter inspections, especially with many freeze thaw cycles, are very informing when inspecting a home.
Now the effects of summer heat!
The attic is now heated by the sun and the attic temperature is higher than the interior of the house. What occurs? Remember temperature travels from area of higher temperature to area of cooler temperature. The hotter attic temperature is, will create conducting through the insulation, plus solid parts of ceiling including all wood and drywall and heating the interior of the house.
The soffit to attic vents now move the heated air out of attic as hot air rises.
Ahaa the opposite of winter, and the attic air is still removing heated air and moisture.
** NOTE**-There has been a lot of explaining, just remember install a pink plastic vent (these are very cheap in price) between each joist or truss, minimum 400 mm of insulation and minimum 2 whirly bird vents on roof of average home. And don't forget to insulate your attic hatch and seal with foam tape.
David Palmer is the owner of Big Block Home Inspections and is an expert in home inspections in the Barrie Area. He has built, renovated and inspected structures for the last 45 years.