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Johngandersonjr 01-02-2013 08:18 PM

Attic vapor barrier
I have an older house. Built in 1964.

I am thinking about removing the insulation in my attic and replacing with blown cellulose. I currently have this black paper wrapped wool insulation that is about 1.5 inches think along with r-19 fiberglass batts.

If I remove all the insulation and replace with blown cellulose do I need to install a vapor barrier? If so how do I accomplish this? What do I use????

I live in Wisconsin and am thinking about r-60.

Thanks in advance for any input!


Windows on Wash 01-02-2013 10:37 PM

Where is the home located?

Be sure to incorporate air sealing as part of any holistic approach to improve the comfort and efficiency of the home.

Johngandersonjr 01-02-2013 10:39 PM

Northeast Wisconsin. Right now we have about 18 inches of snow on the ground.

Windows on Wash 01-02-2013 10:58 PM

How is the attic vented and is it?

The reality is that if the attic floor is properly sealed and air tight, the diffused moisture that makes it into the attic (extremely slight as compared to bulk air movement) should be more than handled by proper ventilation.

Johngandersonjr 01-03-2013 08:05 AM

The attic is Vented OK.

I have 3 quite large gable vents and I cut in a ridge vent when I put my new roof on.

The house originally had plywood soffit and was covered with aluminum for looks. No holes where cut in the plywood soffit for venting even though they installed vented soffit. The eaves are stuffed with that 1.5 inch wool insulation I was talking about earlier.

When I install the new blown insulation I plan on removing all the insulation and removing the soffit and drilling holes through the plywood to create a better vent.

The attic is roughly 2100 sq ft.

Windows on Wash 01-03-2013 11:49 PM

Vent the eaves and you should be in much better shape with that eave venting and the combined ridge venting.

Get the attic floor sealed up well and blow it to and R-50. Big difference maker.

Maintenance 6 01-04-2013 07:56 AM

If you have a ridge vent, then you really need eave vents. The idea is to create a chimney effect. Warm moisture laden air exits the ridge vent and is replaced by cooler air entering at the eaves. This allows air flow under the roof surface where any condensation is likely to occur. There is an opinion that ridge vents should not be used with gable vents as they cause a short circuit in the air flow. The air enters the gable vents and is immediately exhausted through the ridge vent, allowing stagnant air to accumulate throughout the rest of the attic space. Gable vents alone work by allowing air to enter one vent and leave through the other, creating a cross flow that siphons air from lower portions of the space. The warmer air rises to the peak where it is evacuated by the cross flow. If you have several coats of paint on the ceiling below, you likely already have a vapor barrier created.

Johngandersonjr 01-04-2013 11:52 AM

Thanks for the detail.

I will pull the soffit down this spring and drill holes in the plywood above and replace soffit. I will also remove all insulation, air seal and replace with blown cellulose.

Do you know how I figure out how many holes I need to drill in the eaves to get the correct amount of air flow?

Windows on Wash 01-05-2013 07:47 AM

Give us a square footage of the attic floor, how many bays there are in the eaves, and what type of ridge vent you have. Typical rule of thumb is 1 square foot of ventilation for every 150 square feet of attic floor.

Gary in WA 01-05-2013 04:31 PM

If going for 1/150; a good baffled ridge vent is 18 NFVA per foot so 1/2 that for each side of a gabled roof = 9 per foot. The punched metal soffit has around 4.5 NFVA per foot so your soffit would need to be at least 2' deep. And the holes/slots have to remain clean as they clog easily due to the passing humid air depositing moisture there, which collects dirt. The build-up around the openings restrict the passive air-flow effectively.
Replacing with R-60 sounds as if the attic is tall enough (with intake/exhaust vents height difference to promote the stack effect) that you may get by with 1/300 as per minimum code; (though 50/50 is good, best is for slightly more intakes than exhaust)

In your cold location, ice dams are problematic, with enough insulation, "B" will work (though code is minimum, like a "D" in school); Are there signs of moisture damage or mold on the wood framing?

R-60 of cellulose is heavy, possibly overloading the drywall ceiling. How thick is your drywall (at access hatch) and how far apart is the framing (ceiling joists). No vapor retarder;


Johngandersonjr 02-02-2013 11:06 AM


Sorry for not getting back sooner......

I've attached a picture.

This is what I have for soffit venting.

the vented panels are every 48".

Gary in WA 02-03-2013 08:51 PM

If the soffit is 2' deep, you need those double 4" (vented), every other one- the length of the house. 2 vents (8"), one solid (4"), 2 vents, one solid, etc.


Johngandersonjr 02-03-2013 08:55 PM

Looks like I have work ahead of me.

I'm assuming I have to take the gutters down?

Should I just remove all the plywood soffit when I take the aluminum down?

Gary in WA 02-04-2013 11:26 PM

Well, your picture wouldn't load again for me... Problem on my end. The gutter usually doesn't require removal if the soffit panels are held in at there ends in a J-bead or similar. You need to decide on the final venting to remove enough for the style, continuous with a double louver is best (-3" wide), right next to the fascia board for optimum air-flow pressure. Many other members can direct you on aluminum removal better than I...


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