Attic Insulation R-Value Calculator | Depth and Bags

January comes around and you see a massive jump on your utility bill. You’ve come to realize that your attic isn’t well-insulated. The result is a drafty (and cold) set of upstairs room; and a furnace working overtime. It’s an easy fix, right? Well, kind of. You can’t just guess how much material to purchase. You has to consider things like R-values, the settling rate, and how big your attic is.

By entering the dimensions of your roof and the amount of insulation already installed, the calculator (above) does all the math for you, saving you from making costly estimation errors! It also takes the mystery out of how thick your insulation realy is when measured by hand vs. Its actual ability to resist heat flow. The average homeowner simply grabs a tape measure and measures thickness of the insulation on their roof. They may not realize that over time, the insulation will settle, reducing its effective R-value well below what was printed on the package at installation time. Both cellulose and fiberglass materials compresses under years of use.

How to Insulate Your Attic Correctly

First, measure the approximate thickness of existing insulation across various joist bays. Don’t use the shallowest area, and don’t get lulled into thinking you have as much as the deepest section, the average will give you a better idea different than either extreme. For instance, if you’ve got six inches of blown-in fiberglass, chances are you’re looking at an R-value of about 16 (at present). That’s already pretty good, but most likely isn’t up to today’s energy standards. The calculator lets you choose from whatever product you might be using, since each one has its own R-value-per-inch figure. Generally speaking, for example, cellulose is slightly denser per inch of thickness than traditional fiberglass batts, which result in a little more insulating power.

After you figure out where you start from, it’s time to determine how much you’re going to shoot for: i.e., your desired R-value. This depend entirely upon what your climate zone is. In a mild southern climate, you can likely get away with shooting for an R-30 (and still save some money and stay comfortable). But in areas of severe winter weather, such as in the North, I suspect you’ll want to at least shoot for an R-49 (or perhaps even an R-60) for best performance. Don’t worry, you don’t have to go find this information, the reference table on that page spells it out clearly by zone.

Why does this matter? Because if you underinsulate in a cold climate, you lose a lot of heat each day. It will also help you calculate what volume of new material you need to purchase. Most DIY projects go awry here. We know people who buy several bag based off the best case scenario. However, they don’t factor in the waste, obstructions like plumbing and wiring holes, or all that air space between these baffle areas and the rest of the cavity. That wasted space eats up some of your insulation budget, which is why there are fields in the calculator for both waste and baffle areas.

Remember, you can’t block off soffit vents with loose fill material; likewise you won’t be able to blow cellulose into the corners where recessed lights is located. To ventilate properly, you want to leave a channel open for air to flow in from the soffits, keeping the interior cavity dry. The other thing I’d note: if there are major leaks in the ceiling, then air sealing matters far more than installing an additional inch of insulation. Gaps around wire conduits, light fixtures and fan boots all create opportunities for heat to escape, and since heat rises, it’s getting up into the attic and leaking out of these gaps. Adding the extra insulation won’t realy matter, because whatever warmth you’re adding will just leak out any way anyhow. Patch the structural gaps first, then pile on.

If it’s time to buy supplies, tally up the bags needed for this job. Never round down. Running out of material mid-project is a waste of money, not to mention frustrating. The calculator will give you an estimate (rounded up) with a little extra for pesky areas and settling. That way there will be enough material to cover the whole attic floor, at consistent depth. Consistent coverage is better than having some thick sections and some thin sections. Thermal bridging happens in the gaps between them.

Remember: Working safely in the attic is most important. Use eye protection, long-sleeved clothing and a dust mask to avoid inhaling fiberglass or cellulose particles, which can irritate eyes and lungs. Avoid touching new material with any heating device such as non-rated-for-insulation-contact recessed lights. These things can get hot enough to cause damage or fire if covered improperly.

Adding more insulation to your attic is one of the best value improvements you can make to any house. Not only does it boost comfort, but it reduces energy costs, which means it pays for itself in the long run. Measure well (don’t cut corners here!), be sure to plug those air leaks first, and lay that stuff down nicely. That’s where the heat should of been; not floating around in the rafters!