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Old 11-10-2011, 11:59 AM   #1
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Blown in insulation question?


Hello, first, just wanted to say thanks for all the advise I`ve recieved from this site, its been a big help.

Home is located in Hamililton Ontario, GTA

Ive rented an Atticat blow in machine for this weekend and have a simple question that nobody can give me a straight answer to. Ive called Owens Corning, spoke to the boys at Home Depot, and searched the net.

My home is 20 years old and I would like to top up the blown in that exists; R25. I will top up to R50. The attic has ventilation that is sufficient, plywood over 9 trusses extending into soffit area. There is no mold or rot anywhere, just had the roof shingled 2 years ago.

All spaces where top plate and roof truss meet has batt insulation to prevent the blown in from going into the soffit. Current level of blown in does not touch the inside deck of the roof.

When I blow in the new insulation it will touch the plywood on the inside of the roof deck. Some say this is ok, others say I need to install more baffels. I really dont want to change the ventilaton ratio as eveything is fine. I will not be covering any existing baffels. Do I need to prevent the blown in from touching the roof deck or not? Would appreciate any response as I will be blowing it in 2 days.
Thanks, El Gringo.

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Old 11-10-2011, 12:18 PM   #2
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Blown in insulation question?


There should be baffles to not allow any insulation into the soffitt area on the exterior. The Fiberglass needs to be removed, and replaced with the proper foam baffles. This is something common that is beat up over and over on Holmes on Homes & Holmes Inspections. You can look at the info at buildingscience.com which will pretty much explain everything.

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Old 11-10-2011, 11:20 PM   #3
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Blown in insulation question?


Welcome to the forums!


I wouldn't let it touch the roof deck for a thermal bridge/sink. It will also get wind-washed so close to the roof deck= where the hot air rises and flows along the underside. This is the baffles purpose. Leave a 1" gap between ends of baffles if more than one is required to let condensation out, per manufacturers installation instructions; http://www.bergerbuildingproducts.co...sAccuvent.html These are the same price as the foam ones except you also need to purchase the "Windblock" that is built-in to the plastic ones, to protect the edge of insulation over the top plate wall framing. They don't rip on exposed roofing nails either. Insulation installed close to the intake vents gets wind-washed.

Cover the batts with a housewrap to prevent air infiltration or expect up to 40% R-value loss. 60% loss with ill-fitting batts= gaps width/lengthwise, pp.5: http://www.aecb.net/PDFs/Impact_of_thermal_bypass.pdf

Convective loops are inherent with low density batts and blown-in fiberglass insulation, be certain of your density: http://infrared-energy.com/files/Spe...onProblems.pdf

C.l.’s, thermal bridges, air pressures, wind-washing: pp. 42-62: http://www.buildingscienceconsulting...Measure_Up.pdf

I sure hope you air-seal the attic before the new installation….. more good reading on f.g. I assembled earlier: The "biggest loser" in fiberglass insulation....

You also may want to compare your 12" of attic-cat density (0.53) to the insulation that failed the test, almost identical..... http://www.r-prollc.com/Documents/At...Fact_Sheet.pdf

You need to cover the f.g with a "blanket" or 2" of cellulose to stop the inherent convective loops: http://www.homeenergy.org/show/article/year/1992/id/895


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