Installing Attic Insulation and Attic Air Sealing


Key Considerations: 
The attic, along with the basement or crawlspace, is one of the first places you should be looking for improving your envelope.  It may be tempting to look at windows first because you feel the drafts there but a tight attic floor will usually return more in savings and comfort than new windows. 

In the winter, warm air will migrate to the higher parts of your house because warm air is less dense than cold air.  This is caused by natural convection, also called the stack effect.  This means that on average, there is a higher difference in temperature across your ceiling than across your walls.  In the summer, the average temperature difference across your ceilings is also higher than across your walls because radiant energy from the sun has a more direct line of sight to your roof than most of your walls.   When you have a higher difference in temperature that you have to maintain then you need more R-value to do it.  Simply put, attics always need more R-value than walls.

Now back to the stack effect.  Warm air that has floated to the top of the house will leave if there are any holes to allow it.  In the past, when energy was cheap, we used the stack effect to ventilate the house.  It was a good idea to let warm air billow out the top of the house and let outside air replace it in the lower parts of the house.  That is why many older homes have gable vents, to allow warm air to escape as part of the ventilation system.   Experience taught us to let the house “breathe” and the system worked pretty well as long as we used our heating system to provide the energy to keep the stack effect flowing.  In the 1970’s though some of the mavericks decided that it was wasteful to allow so much of our heat escape to the outside and they installed air an vapor barrier systems to stop the excessive heat loss.  This also reduced the stack effect which was ventilating the house, their house stopped “breathing”.   Many of those who did not make special provisions for the reduced stack effect started to have poor indoor air quality, high humidity, condensation, and all the problems that come with it.  We’ve learned from those mistakes and now we know that we have to create an air and vapor tight lid in our attics if we want to reduce heating costs AND we have to replace the stack effect with some other means of ventilation, usually mechanical ventilation. 

Possible Downsides:

  • Poor energy efficiency (big time)
  • Drafts throughout the house
  • Condensation and frost in the attic
  • Mold, mildew, or rot in the attic
  • Ice dams
  • Wet insulation and reduced R-value

Recommendations:

Air seal all the penetrations into your attic: light fixtures, hatches, tops of interior walls, chimney chases, plumbing chases, bathroom and dryer vents, etc. Can foam or kit foam is the tool for this.  

Insulate to at least an R-38 value.

Reduce the number of radiant heat sources (chimneys, ducts, furnaces) in your attic or install insulative jackets around them. Not all insulation is rated for direct contact to chimneys, see Insulation Types to find the correct ones.

Install a vapor barrier on the warm side if the insulation.  If installing more than one layer of insulation, make sure that you don’t have a vapor barrier between layers, it can become a surface that moisture will condense on.  If you are retrofitting and existing attic and can’t install a continuous vapor barrier do all you can to air seal the attic to reduce the amount of warm air that enters the cold attic and then paint the ceiling below the attic with a vapor barrier paint. 

If you have a complicated attic or roofline consider “cathedralizing” your attic with an airtight insulation system.  In effect, you will be changing your cold, vented attic into indoor space but it is a better practice to have the building shell in alignment with your thermal and air barrier.  Insulation-Guide recommends that you still vent the roof above the insulation with continuous vent chutes form the soffits to the ridge vent but you don’t have to as long as the insulation system is air tight.  Open and closed cell spray foam is a great system for air tight attics but it can be expensive.  Rigid foam with foam sealant and many other insulation systems in combination with a vapor barrier and  mudded sheetrock can also work as long as the installer is careful and fastidious.  Make sure you have a local code official review your attic design before proceeding, some insulation systems are in hot (excuse the pun) debate right now regarding the need for a fire resistive coating. Among them, spray foam, rigid foam, and paper-faced fiberglass. 

1. Insulation in the attic floor should not touch the under side of the roof at the eves.  If it does there is potential for blocking the ventilation path and condensation on the roof wetting the insulation.  Use vent chutes or baffles so that outside air can stay above the insulation layer.  Make sure the insulation is installed so that cold air from the soffits cannot get under the insulation.

2. Recessed lights, also called can lights, provide open pathways into your attic and when they are on can actively pump heat into your attic.  For this reason, Insulation-Guide discourages the use of recessed lights in attic floors or cathedral slopes.  If you have them, make sure they are rated for direct contact with insulation (IC rated) and install an air tight layer of insulation all around them.  If they are not IC rated, you will have to build fire resistant boxes that maintain a minimum of 3" all the way around the lights in order to insulate over them.  Consult your local code official for details about insulating around recessed lights.  Finally, if you have recessed lights in your attic or cathedral slope replace the bulbs with compact fluorescent bulbs (CFLs) which run much cooler and pump less heat into the attic.

3. The top plate of your interior walls can be a source of heat loss into your attic.  Most often interior walls are empty so the air inside these walls will migrate to the attic if there is a pathway for it.  Lift up a few insulation batts, if there is a dark band of dust on the batt where an interior wall happens to be you'll know you are losing heat this way.  Air seal the tops of your interior walls with foam.

4. Ductwork, AC units, furnaces, and chimneys all radiate heat to the outdoors that you could otherwise be using to heat your house.  The best approach is to insulate your attic in the plane of the roof with an airtight insulation system.   If you can't do this you need a figure out a way to reduce that heat loss as much as possible by insulating the object you have in the attic.  Chimneys can only have certain kinds of insulation applied directly to them like rock wool.  Beyond poor efficiency, having heat sources in an otherwise cold attic can lead to ice dams.

5. Dryer vents, plumbing stacks, and bathroom vent fans must exhaust all the way to the outside, never just into the attic.  An additional moisture load in the attic can lead to condensation and frost, wet insulation and rot. Rigid pipe like PVC or ABS is preferred over flex duct.  Be careful where you locate those vent dampers too,  sometimes having a dryer damper directly below roof eave is the same effect as having the dryer vent right into the attic because the warm moist air goes right into the soffit vent opening and into your attic.  Unfortunately, bathroom and dryer vents loose warm air to the outdoors when they are off, gravity dampers rarely shut tight enough to prevent this and mechanical dampers are complicated to install for vent openings this small.

6. You should consider insulating in the plane of the roof with an airtight insulation system if you have any of the following conditions:

  • if you have a lots of heat sources in your attic (see # 4 above)
  • if you have a complex roofline with many changes in ceiling plane from attic to cathedral slopes
  • if you have many hips and valleys
  • if you have skylights or dormers

Vented attics work best in simple "A" framed attics where there is plenty of air intakes and exhausts and air flow is unobstructed.  If you don't have a simple attic then you probably don't have enough outdoor air flow to properly vent the attic.  In that case, "cathedralize" your attic by insulating in the plane of the roof with an airtight insulation system.  Polyurethane foam is particularly good at this application, see the recommendations above.

7. The vapor barrier in a vented attic should be on the warm side of the insulation.  Make sure that if you add more insulation to your existing attic insulation that the layers on top are vapor permeable.  You don't want to have condensate trapped between insulation layers.

8.  The attic hatch should have tall jamb extensions so the insulation can be installed at full thickness right up to the sides of  the hatch opening.  Glued or screw rigid insulation to the hatch door with an R-value at least equal to the rest of the attic.  The hatch frame should be air sealed to the ceiling and the vapor barrier that it penetrates. Finally, the hatch opening should be weather-stripped and have a latch that cinches the hatch tightly to the weather-strip when closed.

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