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Old 01-15-2012, 08:04 AM   #1
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Insulating your attic

Insulating an Attic in an Existing House

Unless your home was constructed with special attention to energy efficiency, adding insulation will reduce your utility bills and make the home more comfortable. The attic is the first place people consider to add insulation since it is the easiest to do. Wall insulation will yield more energy savings, but look to the next blog to read more on wall insulation.
Although adding insulation can be a DIY project I recommend using a profession insulation installer that is certified by BPI or RESNET. A qualified home energy auditor will include an insulation check as a routine part of an energy audit. This should be your first step in attempting adding attic insulation.
Existing insulation

An evaluation of existing insulation is the first step. Determining the R-Value is calculated by the quality of installation, condition, depth, and type of insulation when considered together. You climate zone determines the amount of insulation that will yield the most energy savings. In New England area served by my company Home Doctor of America we recommend R49 in attics. R38 is the minimum by code. Most attics in our area have R11-R19 now. If new insulation is installed over the old, the R-value of the existing is slightly reduced. Add an additional one inch of the new insulation to compensate for this loss.
Air Sealing

Air leakage, or infiltration, occurs when outside air enters a house uncontrollably through cracks and openings. Properly air sealing such cracks and openings in your home can significantly reduce heating and cooling costs, improve building durability, and create a healthier indoor environment. Exfiltration is when your conditioned air is leaving the building through cracks in the building shell. The two will always be equal. One cubic foot of air infiltration will cause one cubic foot of air exfiltration.
Air sealing is important, not only because drafts are uncomfortable, but also because air leaks carry both moisture and energy, usually in the direction you don't want. For example, air leaks can carry hot humid outdoor air into your house in the summer, or can carry warm moist air from a bathroom into the attic in the winter.
Before air sealing, you should first do the following:
  • Detect air leaks
  • Assess your ventilation needs for indoor air quality.

  • Along both sides of the interior wall top plates
  • gaps around electrical outlets, switch boxes, and recessed fixtures
  • gaps behind recessed cabinets, and furred or false ceilings such as kitchen or bathroom soffits
  • gaps around attic access hatches and pull-down stairs
  • behind bath tubs and shower stall units
  • through floor cavities of finished attics adjacent to unconditioned attic spaces
  • utility chaseways for ducts
  • plumbing and electrical wiring penetrations.

These leaks between the living space and other parts of the house are often much greater than the obvious leaks around windows and doors. Since many of these leakage paths are driven by the tendency for warm air to rise and cool air to fall, the attic is often the best place to stop them. It's important to stop these leaks before adding attic insulation because the insulation may hide them and make them less accessible. Usually, the attic insulation itself will not stop these leaks and you won't save as much as you expect because of the air flowing through or around the insulation.

Ventilation and Moisture Control

It is unwise to rely on air leakage for ventilation because it can't be controlled. During cold or windy weather, too much air may enter the house. When it's warmer and less windy, not enough air may enter. Air infiltration also can contribute to problems with moisture control. Moldy and dusty air can enter a leaky house through such areas as attics or foundations. This air in the house could cause health problems.
When moist air touches a cold surface, some of the moisture may leave the air and condense, or become liquid. If moisture condenses inside a wall, or in your attic, you will not be able to see the water, but it can cause a number of problems. Adding insulation can either cause or cure a moisture problem. When you insulate a wall, you change the temperature inside the wall. That can mean that a surface inside the wall, such as the sheathing behind your siding, will be much colder in the winter than it was before you insulated. This cold surface could become a place where water vapor traveling through the wall condenses and leads to trouble. The same thing can happen within your attic. For more information about controlled ventilation, see the ventilation pages on our web site here->

Adding Insulation

Note that air sealing alone can't replace the need for proper insulation throughout your home, which is needed to reduce heat flow.
Prior to adding any insulation inspect and address these issues.
  • the electrical insulation on your wiring is degraded,
  • the wires are overloaded, or
  • knob and tube wiring was used (often found in homes built before 1940)
  • All junction boxes must have a cover plate on them. They should have some marker added so their location is still known after covering with new insulation. Code does allow these to be buried in insulation.
  • Ventilation fans are ducted through the roof, not into the attic or soffit
  • Baffles are installed to prevent closing off the soffit vents
  • Isolation boxes are installed over fans or recessed lights that are not IC type.
  • IC cans still need to be air sealed. Use metal tape over the small holes on this can
When you stack new insulation on top of existing attic insulation, the existing insulation is compressed a small amount. This will slightly decrease the R-value of the existing insulation. This effect is most important if the new insulation is more dense than the old insulation. You can compensate for this stacking effect and achieve the desired total R-value by adding about one extra inch of insulation if the old insulation is fiber glass, or about 1/2 inch if the old insulation is rock wool or cellulose.


A homeís attic access, such as an attic hatch, pull-down stairs, or knee-wall door, often goes uninsulated, representing one of the biggest deficiencies in the thermal barrier between the
attic and conditioned space. This gap in the attic insulation increases heat loss in winter and heat gain in summer, and makes indoor living areas uncomfortable. Pre-made access tents are available in various R-value ratings. It is best to get one closely matching the R-
value your energy auditor has recommended.
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Old 02-29-2012, 07:44 AM   #2
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Nice information. Is it ok to add batts over blown in insulation? My situation is that I currently have blown in up to the height of my ceiling joists. I would like to add more insulation by running batts perpendicualr to the ceiling joists.
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Old 03-01-2012, 05:56 PM   #3
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Originally Posted by rakuz66 View Post
Nice information. Is it ok to add batts over blown in insulation? My situation is that I currently have blown in up to the height of my ceiling joists. I would like to add more insulation by running batts perpendicualr to the ceiling joists.
Probably OK. You'll get a quicker answer if you post this question in the insulation forum.
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Old 06-09-2013, 09:06 AM   #4
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Thanks for sharing this information. Very useful for home insulation.
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Old 06-09-2013, 10:48 AM   #5
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Most attics in our area have R11-R19 now.
I have over R-100 in my attic, can never have too much there!
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Old 07-07-2013, 08:49 PM   #6
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How you insulate your attic depends on whether you want a "cold" or "warm" roof space. A cold roof requires insulation at joist level to stop heat escaping through the unused roof space. A warm roof is insulated between and under the rafters of the roof itself. The recommended depth for insulation has been increased recently, so you may have to increase the depths of the joists or rafters if you want to create usable platforms of storage space.
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air sealing , insulation , moisture control , ventilation

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