Insulation, Radiant and Vapor Barriers - How to insulate under a bump out?

Searched and haven't found anything quite like this problem.

My house has bump outs in the kitchen and dining room. Ever since we moved in, we noticed that there seems to be a LOT of cold air coming in through the electric outlets and that dishes stored in the cabinets above the bumpout are cold.

Now that I'm finishing the basement, while runing the vent for the bathroom, I realized that the joists run over the foundation and the bump outs are built on top of those cantilevered joists. AND THERE IS NO INSULATION UNDER THERE. There IS about a foots' worth of regular R13 folded in half and just stuffed above the foundation in between the joists.

So, what would be the proper way to insulate under there? Unfaced? kraft faced (which way should the kraft face?) Two layers of kraft faced with kraft on top and bottom? Before the drywall goes up, I can reach in there to stuff the insulation all the way in. My main concern is the best way to vapor barrier it.

I'd really like to insulate before it gets too cold here in Northern VA.

Thanks,
Tom

Tom: The vapor barrier always faces the living space. If it requires more than one layer of insulation, the additional insulation should be unfaced. Since you can get to it from the inside, you may have it easy. Quite often, we have to dismantel the undersides and reinsulate where the original contractor failed to do so.

Besides a vapor barrier i think a main concern should be a proper air barrier. If you dont stop the cold air from flowing in there no amount of fiberglass will work, and you will have the same problem as befdre minus some money and hard work. FG doesnt do anything to stop air it just filters it and gets a lower R value as a result. If you can reach in all the way you would be doing yourself a world of good to empty a few cans of spray foam and seal up any and all gaps around all the joists. Even if you dont think theres much of a gap still spray any joint of 2 pieces of wood you find in there cause air can get in.
Now if you cant create an effective air barrier I would just fill the entire area with dense pack cellulose to keep out the air plus to insulate. If its packed in tight enough then you most likely wont need a VB there.

doobs is right. Fiberglass does nothing for stopping air or even slowing it down. You will want to first fill up or block the air leaks. Then blow in some dense packed cellulose. It has a higher R-value that fiberglass and will stop any additional air movement.

Thanks fellas!

I was hoping to not have to go the trouble of blowing in insulation. I'm not sure I can reach all the way to the back with a can of foam. I can reach far enough to stuff fiberglass back there though.

Would caulking form the outside have the same effect as foam on the inside?

Tom

Would caulking form the outside have the same effect as foam on the inside?

Tom
Probably not but im not there to look at it!
Why dont you caulk it now (should be done anyways) and see if you can get the air infiltration down before you insulate. Like I said I doubt only doing it from outside will work, foam inside and/or cellulose will solve your problem, rolling out fiberglass while cheap and quick, will not.
good luck

Thanks.

i'll take a closer look at it and see what can be done wither inside or outside.

tom

Tom, I have a similar situation in my house where I have a 3 foot bumpout in the back of the house with the kitchen cabinets and sink being over the bumpout. I have pondered the same question you have.

While I can access it from my basement, it is too deep to seal the gaps up with foam. An alternative would be to try to access the area from the exterior but this would involve removing whatever is covering the joists, plywood in my case. Regardless, I don't think you will get much out of sealing with foam. Assume it is reasonably plugged up if the construction is decent. Remember, you still have a subfloor under your kitchen.

That foot of insulation was probably placed there to stop warmer basement that rises and escapes/is cooled by the bumpout. Remember that heat rises and the sill plate area and upper portion of the basement wall is very important in the heat transfer equation.

I have two thoughts. Fill the joist cavity with unfaced (remember faced is flammable and has to be covered) fiberglass to slow down conductive heat loss from your kitchen and prevent losses from your basement.

Also, is your exterior kitchen wall insulated? I suspect that contributes significantly to the cold cabinets.

John

Fill the joist cavity with unfaced (remember faced is flammable and has to be covered) fiberglass to slow down conductive heat loss from your kitchen and prevent losses from your basement.

That was actually my first thought. I wasn't sure if a barrier would help or not.

Also, is your exterior kitchen wall insulated? I suspect that contributes significantly to the cold cabinets.


I have my doubts, but it is basically a pretty solid house. I haven't had cause to open a wall and check. I'm hoping that the cold cabinets are more a result of cold air coming up from the bumpout.

I'll check it out further this weekend when can get out there in daylight. Also, for the cost of a roll of unfaced FG, it's probably worth it before I close in the basement.

Thanks for your input!
Tom

Just a quick update....

I used a can of the (minimally) expanding foam from the outside to at least seal up the worst of the gaps. I bought some insulation, just haven't had time to move all the stuff to get to the area from the inside.

Last night temps dropped into the upper 20 - lower 30's with winds 25-35. Noticed this morning that the cabinets were not NEAR as cold as they had been in previous winters.

Thanks for the tips! Now to move all that "stuff" (that's the nice word for it) and get the insulation in there......

Tom