Is there any point in adding insulation to just part of the attic?

Question

Background: One room in our house is consistently 3-4 degrees colder. I’ve gotten quotes from a few insulation companies, who recommended insulating the basement rim joists and adding insulation to the attic. The attic currently has 4-6 inches of blown-in cellulose insulation, which they recommended topping up to 15". (We’re in northern Illinois, where this matches the recommendation of R49.) One of the companies also said something about the tops of the walls not being sealed? We went ahead with the rim joist insulation, and also recaulked the windows, but haven’t noticed a tremendous difference. Increasing the attic insulation was outside our budget for now, but we hope to go ahead with this next year.

Here is my question: if I were to add cellulose insulation myself to cover just the attic above that cold room to 15", would that make any difference to heat retention in that room, or just be a waste of time without doing the whole attic? Likewise, I know the pros would also air seal the whole attic, which I wouldn’t be doing — would adding more insulation not make much difference without also air sealing?

Also, is this a reasonable thing to do myself? I have gone up there before and walked around on the joists to address a different issue and am comfortable doing so. It seems simple enough — open up some bags of cellulose, spread it around with a rake - but I wonder if it’s a much bigger undertaking than I’m assuming, or if there’s potential for me to do more harm than good. (e.g., I know moisture is a huge issue - do I have to worry about like, if sweat drips off my face into the insulation?)

Answer 1

If you double the amount (thickness) of insulation over part of the ceiling, using the same type of insulation as is already there, you will cut the heat loss through that part of the ceiling by half. This is just basic physics.

Now whether that will improve the comfort of that room depends on where the heat is being loss. If the bulk of the heat loss is through windows (even good ones are notorious energy leaks) or through air leaks, then doubling the attic insulation may not improve things much.

And could you expand a bit more on your comment "One of the companies also said something about the tops of the walls not being sealed." All walls should have top caps on them the same size as you wall studs (2x4 or 2x6) and so should be sealed, except for wire or pipe penetrations.

I'm not a big fan of cellulose insulation. It's one advantage is being to blow it into the far reaches of the attic. But I prefer un-faced 6" fiberglass batts or rolls or Rockwool batts. Both of these can be pushed around using a rake or stick so you really don't need full body access to every nook and cranny of the attic.

And finally, you don't have to worry about a bit of moisture from your face getting on the insulation. It will evaporate in due time.

Answer 2

Air sealing should generally be the first step. Among other things, it's a royal pain to do air sealing after multiple feet of insulation have been piled up. But it also tends to be the single biggest energy-use improvement for work/cost invested.

I don't know where you are, but in my area the utilities offer a free service which will send a contractor to your house to evaluate what most needs to be done (ideally including testing the air seal with a blower door, though only the first guy I brought in actually went that far) and give you a quote on a discounted price for the most important parts via the utility and local government. It's a serious discount; admittedly it was a decade ago but I had air-sealing done at foundation and attic, basement ceiling insulated (which arguably was unnecessary) and something like three feet (before settling) of insulation blown into the attic, all for about $600 out of my own pocket. Plus being left with some advice for additional things I could consider (ways to improve the tightness of my windows) and some consumer energy-saving devices (a batch of energy-saving light bulbs, a power strip which turns off all the outlets when the master device is turned off, water-saving shower heads). Definitely worth checking if a similar service is offered in your area, either by utility or local authorities.

As to what the best thing to do with your room in particular... I'd ask their advice. You need to figure out whether the problem is not enough heat coming in or too much heat going out; for the latter you need to figure out whether the problem is insulation or air leakage or needing storm windows or...