- Liam Butters
Weekend project: insulate your attic hatch
That tiny little access panel to your attic actually punches above it's weight when it comes to how heat is transferred from your living space into your attic. Frequently attic hatches aren't properly insulated, and the consequence of this lack of insulation is surprising,
The good news is that they're easy to insulate. The bad news is that you've got basically no excuse to avoid doing so.
When it comes to how heat is transferred from inside the home to the attic, we're concerned with the area of the attic. We're also concerned with how much insulation is up there, the spacing of the roof trusses, the depth and width of the bottom chord of those trusses, and some stuff about seasonal air films that will bore you to tears.
The value we want to calculate is the attic's composite R-value. Think of R-value like a measure of how hard it is for heat to move through something - the higher the R-value of a material, the harder it is for that heat to move through that material. The harder it is for that heat to move through the material, the less heat is transferred. In more technical terms, R-value is a measure of how well a material resists the conductive flow of heat per unit of area, with imperial units of: (sqft x *F x hours)/BTU.
When it comes to calculating R-value, it's easy when we're just got a stack of materials. In this case, where the cross-section of that stack is the same no matter where you take the cross-section, we simply add up the R-values "in series" (electrically inclined folks' ears just perked up).
Things get way messier when things look different across that stack of materials depending on where you take the cut. Imagine your attic. If you take the cut in one direction, you're just looking at insulation and some drywall. Take it in another direction and you've got a bit of the attic hatch, some trusses, insulation, and drywall. In this case we need to add up some of the R-values "in parallel", and others in series.
For the calculation of composite R-values we use this formula (formally referred to by ASHRAE as the "isothermal planes method")
Skipping to an example, let's say we've got:
2x4 truss roof construction
Trusses spaced on 24" centres running on the short axis of the house
R28 fibreglass attic batts
A 2ft x 2ft uninsulated attic hatch constructed of one piece of 1/2" drywall
1/2" drywall ceiling finish
(Note: for convenience, I'm using National Resources Canada's (NRCan) HOT2000 energy modelling software for this calculation, but the formula HOT2000 is using can be found here).
The result? The composite R-value of the above detailed attic (ie. no insulation on the attic hatch) is R25.5.
What if we add R28 worth of insulation to the attic hatch so the entire attic is insulated to R28? The composite R-value increases to R27 - a 5.8% increase.
Now 5.8% isn't huge - you aren't going to notice 5.8% less insulation in your attic on your heating bills. But you'll be paying for that difference. And you'll be paying for it 24 hours a day, 365 days a year. Not really worth it when you consider that insulating that attic hatch will cost you ~$25 and take about 1 hour.
How to do it? The easiest approach is using rigid foam board insulation and some glue like this. You can do it with batt insulation, but it's a bit trickier and the batts have a bad habit of getting pushed off the hatch after a while.
Oh, and while you're up there don't forget to add weatherstripping between the attic hatch and the ceiling!