Home Insulation, How Much do I need & Where?

Think of the heat in your home as if it were water behind a dam, and that insulation levels were the concrete wall of the dam. Would you choose the height of that concrete wall randomly, or would want an even level of concrete to hold back as much water as possible? You certainly wouldn't want an extremely high wall on one side and lower on another; the relationship between heat movement and insulation is a bit like that.

The cold hard facts about heat loss:

  • Yes, a bit more heat is lost through the ceiling than the walls, but not nearly as much as much as you might think. The rate of heat loss from any surface of your home depends on the difference in temperature, and it just isn't that different at the ceiling compared to the walls. And on a related topic, another debunked urban legend that stuck in my head most my life is that 90% of body heat is lost through your head. It just ain't so, for your head or your roof.
  • You won't gain heat from an un-insulated basement floor in winter. Heat moves from warm to cold seeking equilibrium, and the ground in Canada is roughly 4-7° Celsius when you go down 5 or more feet. So unless you keep your basement colder than say 7° C, you will lose heat to the ground, not gain it. And if it IS that cold down there, you should probably insulate the floor above, or it will rob heat from upstairs to keep feeding into the ground, not to mention give you chilly feet in the process.
  • If you have in-floor radiant heat in the basement and only 2 inches of insulation (which is pretty standard), you'll be losing most of your heat down there and not the attic as you might suspect. That is a perfect example of misplaced resources and unbalanced insulation if you have a fluffy R60 blanket in the ceiling but a poorly insulated slab floor.
  • Air leaks are responsible for a surprisingly large amount of heat loss, so plan your air barrier well in the design phase and be very meticulous with air sealing when working onsite.
  • High-quality windows on the south can gain more heat in the day than they lose at night, giving you a net heat-gain. Windows on all other sides always result in a net heat-loss, so keep that in mind, buy good ones and don't make them overly large for no reason.  
  •  Surpassing the insulation requirements of building codes might save you money not only in the future, but immediately. The purchase and installation of greater levels of insulation will increase building costs and your mortgage payment every month, but that can often be offset entirely when you are rewarded with lower utility bills. Meaning, your monthly expenditures for a better house may be about the same at first, but would be much cheaper once your mortgage is paid. 

There is a tendency to beef up the insulation of a building envelope somewhat randomly, and in an unbalanced way. While it is true that the more insulation you put in any area will slow heat loss in that spot, the question remains, could that insulation be of better use elsewhere? That answer is often yes.

Energy modeling can help determine how much heat would be lost from each surface of a home; armed with that knowledge you can apply your insulation dollars in a way that they give you the most comfort and the best return on investment. If you're building new and have enlisted the help of a designer or architect, ask how they arrived at their conclusions as to how much insulation to put in different locations, and if energy modelling was involved.

Choosing the right windows:

Another case of balancing your thermal envelope is in terms of window quality. Having an extremely well-insulated wall is great, but if you blew the budget on insulation and had to put in low-quality windows, they become the weak link in the chain and you can effectively undo all your own efforts to reduce heat loss.

So in the case of a super-insulated wall with cheap windows, the loss of thermal performance of that entire wall section (windows included) may have been a more expensive option than reducing the insulation level slightly and applying that money to a much higher quality window.  

In Conclusion, How much Insulation is best?

Balancing insulation is about respecting the laws of diminishing returns. Example - If I have a wall with just 2 inches of insulation and I add another 2 inches, I will certainly get my money out of it. If I have a wall with 30 inches of insulation and I add 2 inches, I may never get my money out of it.

The amount of additional heat saved by that relatively paltry addition becomes so small that the energy required for manufacturing and the money required to purchase it may exceed any savings over the entire life of a house. So there is balanced insulation in a nutshell - spread it evenly and in the right amounts for the right areas and you will get the most out of your investment.

To compare different types of foam insulation boards see here, from the EcoHome Green Building Guides