[AKoester]

I'm redoing a bedroom ceiling that once was a garage. It has 2x6 trusses. It has R-11 faced fiberglas with the paper to the inside of the room,but is in poor shape. Q#1-- can I put a sheet of 8 mil plastic on the ceiling before drywalling and leave the paper vapor as is?
Q#2-- The height of the truses at the high point is 36" how full can I fill it with blown in insulation?

[manhattan42]

Q#1-- can I put a sheet of 8 mil plastic on the ceiling before drywalling and leave the paper vapor as is?

No. You are creating a double-vapor barrier situation and gain no benefit by doing so.

Q#2-- The height of the truses at the high point is 36" how full can I fill it with blown in insulation?

The amount of insulation you need depends upon where you live. For most parts if the US, that amount is at least R-30 or about 9" of insulation.

Any deeper amount will be a waste of money and not return in energy savings what it will cost to install.

So in your case, either remove the fiberglass insulation completely, then fill will with enough blown insulation to give you about a 9" settled depth, or spray enough on top of the existing fiberglass to give you your R-30 desired value.

PS: You use rafter vents in the eaves and rafter spaces to keep the air flowing between vented eaves and roof vents above.

[hkstroud]

Have to disagree with Manhatten, you can add plastic as long as you put it on the same side as the existing paper barrier. However, unless the existing paper barrier is almost no existent, I wouldn't bother. Condensation occurs when the warm and cold air masses mix. The moisture barrier prevents the mixing, it is not necessary to be air tight. You only need to prevent the volumes of air from mixing.

An insulation contractor can probably blow about 10" of loose insulation from one central point but I doubt that you would want to pay the cost he would have to charge for one room.

Make an open area in about every third bay by removing the some existing bat insulation, dump your bagged loose insulation on top of the bat and spread it around using a broom.

About 10" of loose insulation on top of the existing bat should give you about an R50.

[21boat]

Hi Manhattan, hsktround is correct and also per code your correct

Lets do simple basic science. The REASON a vapor barrier is put on the Warm side to begin with is that spot is where the moisture air temperature are basically the same.

To GET condensation you need two things.
1. Moisture
2. Rapid change in temperature.

Other then that the moisture in the air stays suspended. (Explains rain) For Example. A poorly insulated attic will have frost on the roofing nails and its not always because of ventilation in the attic or barrier on insul. When the insul WITH vapor barrier is under rated for that Geo the HEAT loss from the floor below is quick and will start to condensate/moisture in and on top of the attics insulation. Like dew on the grass having the right amount of moisture and the speed it changes from hot to cold Its all about the Speed in the changing temperature with moisture in that air

For example a Styrofoam cooler is obviously a VAPOR barrier. It holds water and yet the out side will condensate. If that cooler had MORE insul for a slower time to go from cold to cold no condensation will form on the out side Or if enough air flow was pointed at that cooler to evperate the froming condensatioon again no Mostuire on cooler.

Lets look at the double vapor barriers. Drywall with PAINT on it is also a % vapor barrier. That said we have what. 4 partial vapor barriers. 1 on insul. 1 on back of drywall, 1, on face of drywall. 1 on paint. Latex paint isn't 100% permeable.

In theory if mil plastic was added to OPs ceiling and tucked taped 100% letting NO moisture through it the paper barrier on insul is a moot point. The problem arises when a bad tape job and moisture from the room escapes above the plastic and hits the paper barrier especially if a cavity is there.

OP if it were me I would simply take the time to peel of the paper from that insul and tuck tape your plastic OR do the partials that's bad and tuck tape the overlap where good paper meets plastic

[ballengerb1]

Got to say this thread was a very good discusion and gave me several things to think about. Read the Oregon code, never thought you should/could skip a vapor barrier on a ceiling with an attic above, think I'll still put them in but good read. Another thing to keep in mind for all of us is even a 6 mil visqueen barrier is not perfect at keeping all diffusion from happening. Think about how many punctures are made just with the drywall screws or nails let alone all of the picture hanger nails driven into many wall. I'll stick with sing barrier but will keep doing ceiling abrriers attic or not. Nice discussion guys, all good.

[21boat]

All due respect to you both, but hkstroud and 21boat are both wrong and show a gross misunderstanding of "vapor barriers", dew points, moisture drive, and Energy Codes by their comment. for simplicity of discussion, drywall, latex paint, unfaced open cell foam insulations and plastic sheeting 4 mils or less are NOT "Vapor Retarders"....They are "Vapor Permeable Membranes". Here's why, and Secondly: "DEW POINT".

Hi Manhattan, Sorry I have a gross misunderstanding of basic general science.

Or is it that basic science is what set the Bar/measurements to get to a CODE of what's acceptable or not.

The words barrier and retarder at the base level are synonymous especially since a Plastic 6 or 8 mill gets punctured as Bal pointed out so well. Show me a retarder or barrier that isn't a physical object for transference or not of moisture. The closest I can come up with is an Air curtain for a commercial door opening or is air a Physical barrier.

Anything that becomes a retarder I would think would have some kind of Barrier needing to be there. Any barrier I would think weather It wants to or is a retarder at some level.

In cased you miss it I also mentioned dew points "grass" which states in general have different ones which obviously explains different Codes for different states...

At least I see you didn't correct me on my advice to OP to remove some of the bad sections of the kraft paper and use plastic in those spots only. Hope I got something right...

[manhattan42]

The problem is that the moisture dynamic in buildings is difficult to understand... let alone control.

Do a Google search for "Building Sciences Coproration" and you will find links on vapor barriers and moisture that will keep you reading for months.

And by the way, Lstrubek of the Building Science Corporations is one of the major advisors to the US DOE and the ICC and his work is incorporated into the Energy Codes...

So don't think Energy Codes are outdated or that they haven't caught up with the Science. They have and are right there on the cutting edge of the science...

But here is an exceptional link discussing how even trapped moisture between vapor barriers or even constant exposure to excess water vapor can rust away fasteners in ceilings, causing them to collapse (among other things):

Shelter Ecology - Article: Moisture Control in Buildings

(Just had a major plaster ceiling collapse in a local Muncipal Building in my region from this exact phenomenon... Thank God the building was not occupied at the time the plaster gave way or their would have been deaths and major injuries.)

But the main point is that a double-vapor barrier prevents drying to either side once liquid water forms between them... and drying is one of the main concerns with moisture control in a building.

[manhattan42]

"EXCEPTIONALLY TIGHT CONSTRUCTION"

One of the major concerns in homes built today is that they have become 'too tight'.

In our quest to make homes more energy efficient and weather proof, we have sealed up the envelope and made the buildings so tight, that there is no longer a normal 'air exchange' that used to occur in buildings of older construction.

This 'air exchange' provided some relief from excess water vapor in homes, allowed drying when water entered building cavities, provided ventilation air, and allowed combustion air for solid and gas burning appliances to be readily taken from the inside of the building.

No more.

Placing vapor retarders on ceilings in new homes that are also otherwise well sealed with fuel burning appliances creates a condition known as 'exceptionally tight construction'...

The result is that if the building become so tight, one cannot use interior air for combustion air on most fuel burning appliances, and these appliances must then only be 'direct venting' type... taking combustion air from the exterior only

Placing a vapor barrier on the ceiling below an attic is a major contributor to making a structure 'exceptionally tight', and why it is a good idea to not use one on the ceiling of an attic, when the attic is well ventilated, and when the rest of the structure has been well sealed with fuel burning appliances used.

[21boat]

One of the major concerns in homes built today is that they have become 'too tight'. We have sealed up the envelope and made the buildings so tight, that there is no longer a normal 'air exchange' that used to occur in buildings of older construction.

Yep, got that one in 1976. Customer had a walk in fireplace in a very old home and for some reasons the fireplace started smoking and didn't before. The irony here is it was the Science teacher from my school.

I started the fireplace and yes it smoked. I said I will be back in a minute while it burned. Came back and it started to burn better. He asked what did I do. I said Opened up some windows in other rooms. See the walk in fireplace needs so much air flow to burn properly. Your drafty 1930 farm house leaks air but helps the fireplace burn properly. I saw you had replacement windows put in and that was enough to tighten up the house so the fireplace flue was starving for airflow. IE Smoke

Yep could get into static pressure in buildings and how that effects its airflow and moisture and in turn its insul properties