Adding insulation to attic, no vapor barrier

[JCB]

Tony,

I believe there is also a ceiling paint that acts as a vapor barrier. I wouldn't go w/o a vapor barrier, and I've already moved old insulation and put down a vapor barrier in a previous house I owned.

A moving a couple inches to lay new vapor barrier really shouldn't be that bad of a job. A scoop shovel, a couple boards to lay across rafters and you ought to be able to finish in a day and ready to lay new insulation.

Joe

 

[JoeL4330]

Some of the replies may be confusing two thoughts...vapor barrier prevents moisture from infiltrating and condensing on (first) cold surfaces it comes in contact with ...you don't want it condensing on/in the insulation because that will degrade the insulation value; and, you don't want it condensing on the structure, because that will ultimately compromise the structure. It is this latter, structure, issue which venting would resolve. You would still have to be concerned with condensation in the insulation, which would be aggravated by adding more insulation atop the existing mass.

To just deal with the insulation-condensation issue, you could move the existing insulation aside, area by area, and put down roll/bat fiberglass that was foil faced, then repositioning the old insulation atop the bat.

Adding venting without an infiltration barrier could/would increase the draftiness in the house below...if the exiting warm, moist air infiltrated from below and exits, makeup air will come into the house from any and every crack.

And, speaking of drafts in the house, do you bring in outside air rather directly to your woodstove? If not, then any air that exits the chimney will be made up by air coming in through any cracks...the hotter you fire it up, the more draft(s) you create ...and, if you were ever able to prevent any air from coming in by sealing every crack, then your woodstove (and you) would run out of O2 ...or at the very least, your chimney wouldn't draw and your woodstove would make your house a smokehouse.

 

[Storm56]

I live in NE PA and currently own 3 buildings with blown in insulation, 2 of them even have the walls done with it. No moisture barrier in the ceiling. Attics vented very good with full length ridge vent and vented soffit. No problems. The first home I built in 1982 and owned for 24 years also had blown in insulation in the attic with no moisture barrier, no problems ever with that home either.

So I would say as long as you are vented well, forget the moisture barrier and just blow in the additional insulation over what is there now.

 

[Highbeam]

Yes, just blow in on top of what's there. Vapor barriers such as plastic are almost never installed in ceilings. Do you really think that any air is going to leak through the heavy layer of latex paint, texture, drywall paper, drywall, and more drywall paper. If so, I challenge you to put your lips to that ceiling and try and blow into your attic. Come'on guys, with a finished ceiling you don't need an additional barrier. If your ceiling is tongue and groove pine boards or some sort of tile that is not air tight then I can see the value.

Before blowing in more insulation take the time to caulk and seal all penetrations such as light fixtures and wire holes in the tops of walls. Also, be sure to install those baffles to allow soffit air to flow into the attic for ventilation.

 

[Craig Clayton]

In Canada as in the most northern states a vapour is very important. I built the very home I am still living in. It was built to a high insulation and seal rate with a air to air heat exchanger. You probably have a high inflation rate of air so the air in the house is very low in moisture. The insulation in the attic is allowing the DEW Point to be near the outside of insulation ( on the cold side ). As you add more insulation and increase the R value the DEW Point will move within the insulation. This is where in the future you will get moisture.
I would suggest that you remove the insulation and lay over and between the rafters a vapour barrier. Use a pliabable caulking to seal the laps together. There is also a red tape for that purpose. Bag the old stuff. Put in R 28-R40 friction fit Roxol or Pink Fiberglas.
Craig Clayton

 

[Reverend Blair]

Do you really think that any air is going to leak through the heavy layer of latex paint, texture, drywall paper, drywall, and more drywall paper.

Yes I do think that. I've seen it happen in real life, and it's be talked about in building science courses I've taken. It's also important to seal around any light fixtures etc..

A bit of cheat, although far from ideal, is to use oil paint. That will give you a bit of vapour barrier.

 

[Tony_S]

Hmmmm.......

More great responses!

I'm thinking there would certainly be no HARM from adding the vapor barrier, but the possibility exists for structural damage or at least a compromising of the insulation's best performance over time if NOT adding a vapor barrier.

So in the spirit of caution, there will be a vapor barrier.
Thank you all.
Tony

 

[bigtiller]

I asked my contractor in 1998 about putting a vapor barrier in the ceiling and his responce was "how is it going to get out". If the moisture can't get out, the windows will be frosted over nearly all winter long.

 

[Craig Clayton]

Tony S
To verify any info presented here check with your local building codes. I would suggest that when you reach the point that you are working in the attic you add soffit venting sleaves. They install between the roof rafter space over the support wall. You do not need them every 16"OC maybe every second rafter space.You can then install soffit vents and the idea is the air rises up through the soffit vents and into the attic. There is a ratio for attic venting, I prefer the high ratio as it lets out that hot air in the summer.
When you put in the APPROVED vapour barrier use the code grade ( not the thin mil ) do not worry about being double layer with large over laps just seal and caulk.
My house has 2 layers of CHMC APPROVED vapour barrier both sealed and caulked at laps joints or seams.
Early spring and late fall are wonderful season to spend some quality time in the attic , not too hot and not too cold.
If you have any pot lights from the ceiling below CHECK they must be approved for being surrounded by insulation. They can be boxed with plwood with clearences check.
Craig Clayton

 

[backdoor]

A call to your local insulation contractor would be a good thing. They usually have good prices and can do a much better job than you can. They furnish and install insulation for about what you can by it for. They already know the codes, and they probably will have ideas that you had not even thought of. You'll know the vapor barrier in your attic is working when condensation runs down the inside of your windows in cold weather and mold starts growing in dark places.