My experience from working in the energy conservation field for a number of years is that you can get voids if there are a fair amount of hangups/obstructions in the walls (like protruding nails, plumbing and wiring that runs horizontally, and sometimes the rough backside of a plastered wall). This occurs mainly if you are using the blown-in type of insulation (the foamed-in-place stuff tends to flow around obstructions). However, good installers have a sense for this and can usually tell if they're not getting enough into a particular wall section.

Since you mention new electrical, I am assuming that means you don't have old wiring in place in any of the exterior walls? The reason I raise that is because I seem to recall that old knob-and-tube wiring should not be surrounded by insulation due to possible fire hazard.

If it were me, I would ask the lower-priced guys how they deal with possible voids and hangups. If they gave plausible answers, I would ask them for references, and I would also check with the State Labor Dept (as I would with any contractor) to see if there were any complaints against them, and to check that they were licensed and bonded. If that all checked out, I'd go for them. You could even consider getting an energy auditor with an IR camera to come out afterwards, as that would be a lot cheaper than $4000. You could also check with your local or State governmental agencies, or your utility. Sometimes they will do that service cheap or even free.

Thanks, kudzu9. Sounds like good advice. The low tech guys talked about measuring and "fishing" the cavities to make sure they've gotten around the blocking common in old lathe and plaster dwellings. The wiring is being taken care of as you suggested.

Do you have an opinion about foam vs borate treated cellulose? I haven't gotten any bids from foam contractors(don't know of any in the Bay Area) but is it better in some sense?

F

The low tech guys sound reasonable in that they are open about this issue and have a way of dealing with it. I can't give you a slamdunk answer on whether foam or cellulose is better. Here are a few things to think about:

Foam:
-You may get some odor from offgassing, depending on type of foam and how well it is mixed.

-Foam can sometimes shrink inside the cavity after it has cured, so you can lose some of its insulating and draft stopping capacity.

-Foam is harder to fish wires through if you ever have to add any more wiring of any kind.

-Foam is going to be more expensive, maybe twice the cost.

Cellulose:

-May tend to hang up more, so distribution may not be uniform in all cavities.

-Cellulose can settle some over time, so, like foam, it can lose some of its conservation properties.

-If you have any leaks or vapor transmission issues, cellulose can take on water.

In considering either foam or treated cellulose, you should ask about fire retardant properties.

One other thing:
The magazine "Fine Homebuilding" has an excellent reputation for thoroughness and has done several articles in recent years concerning retrofit insulation. Below is a link to one of them. You might want to do a search on their site for "insulation" or "cellulose insulation" or "foam insulation." You can either get the articles that are interesting at the library, or do the 14 day free trial for access to their archives.

Here is a link that might be useful: Cellulose insulation

Chemicals in cellulose can corrode all metal they come in contact with and moisture can permanently reduce the thermal efficiency, plus leave voids. I only use cellulose in attics.

Here is a link that might be useful: Cellulose Drawbacks