Just looking for thoughts about sealing an attic that has HVAC equipment in it by using a netting product and dense packing cellulose in the rafters. I have seen this before, although I usually see it combined with foam on the roof deck.  Home is in southeastern Virginia, climate zone 4 and we traditionally do not insulate between the rafters, just the ceiling. The home in question has 85 can lights and we are looking at a cost effective way to eliminate the air flow through these bad boys.



Views: 1766

Reply to This

Replies to This Discussion

Hey Brad,

My two cents,

1. Build some "hats" around the cans. look up can light tophats on the net. You can manufacture these 'hats' with drywall or sheet metal. I think the clearance from the cans is 3". Get a good system and fabricate. That is a lot of lights.


2. Seal the hell out of every duct joint that you you can get your hands on with mastic, including plenums.


3. wrap all duct work including returns, supply and plenums with FSK fiberglass...R-19..tape well.


4. plug all penetrations, bypasses in lid from attic.


5. blow loose fill cellulose or fiberglass to a minimum R-38.


If you blow rafters with netting your thermal boundary not be aligned (insulation + air barrier). You could pack into a plastic "netting" and possibly incur moisture liability.


there are my two cents....in this economy they are probably only worth one anyway.


please let me know what you come up with, as I am learning and would love to know what your solution is.








The can lights are in the attic floor, you are asking about insulating the rafters, correct?
Yes, another weatherization company in my area has done this for a client I did an audit for.  I do not see the advantage in dense packing the rafters.
What size are the rafters? With cellulose insulation you are going to need to install baffles for ventilation of the roof deck, so take that into account as you figure the max R value you can achieve that way, and compare it with what you could do with loose fill on the attic floor. But, the wild card is the mechanical equipment and all those can lights, and it may be that overall higher efficiency can be had by turning this into a conditioned attic by insulating the roof. I believe it is more common to condition an attic by spray-foaming the roof, but it probably varies regionally.

At $4300, you could replace all of those lights with these:


They tend to leak very little and throw out a decent amount of light for LEDs.

And, over time, they would pay for themselves (not sure what that would be though, you do the math :-)

85 recessed lights seems a bit excessive... They might consider mudding over half of them and using task lights instead... sheesh.

It was a ton of lights.  Turns out the dense pack did not work.  Test in 2755cfm50, test out 4500cfm50

Me thinks something is wrong there

Me also thinks something is wrong there with your in and out tests. Baseline fluctuating from wind? That's a huge difference in readings. And with the dense pack, I would expect the test out to actually be lower rather than higher.

Brad, did you/they change anything with those lights, are they 85 leakier holes now?  And what did you do with the ventilation?



I am not involved in the project at this point.  I did the initial audit only and not the test out.  The WX contractor dense packed the rafters and did no other air sealing.  The homeowner wanted to expand the home into the attic and that is one reason that they dense packed.  So, the test out number is a bit misleading because they added about 1400 sq feet of "conditioned" space.  I have not been back to the house since they did this and my original concern was that it was not the best option for the homeowner.


Sounds like this is done, but for anyone interested, there is a much more cost effective solution to air seal all of these cans. Everyone talks about building a "box" over each can light...very labor intensive and on sloped roofs, very difficult access in many situations. There is a retrofit air tight trim that may be installed from the bottom. It is cone shape and has the gasket on the top where it clips into the socket, and the rim just gets caulked to the drywall. Simple. They are only $5 a piece and take 5  minutes to install. They are for 6" cans only and will not fit in a low profile can. Here is a link www.affordablequalitylighting.com/docs/indoor/recessed/6in120vstand... . That is the only place I have ever found these and I have done probably 300 lights and never built a drywall box in the attic. Combine them with a good quality R30 fluorescent and you can have a big savings package.

Craig, thanks much for that link to the retrofit trims! I've been looking for something like that for quite a while.


Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network.

Latest Activity

Alfie Davis posted a blog post

Welding: The Soul of Modern Industry

Welding is one of the most valued of all existing industrial processes out there, since it is the…See More
5 hours ago
Profile IconDenise Cooper, Maren Cooke, Steven Antonini and 1 more joined Home Energy Pros
Crosbey Archery joined Allison A. Bailes III's group


HVAC design, Manuals J, S, T, & D, Duct leakage, Air flow, ENERGY STAR new home requirements,…See More
Crosbey Archery commented on Ottawa Furnace Filters's blog post Some Useful Information about Furnace Air Filters
"This post is really helpful especially to those homeowners who are still confused on what to…"
Diane Chojnowski posted videos
Diane Chojnowski posted a discussion

Incentives & financing options for home energy upgrades

Energy.gov has a useful database of incentives and financing options for home energy efficiency…See More
George Kopf commented on Home Energy Magazine's blog post My Fall Into The Smart Home, Lazy Life
"Matt, Another way of looking at this is that by freeing up my time/brain power, I can give more…"
Chris Laumer-Giddens's blog post was featured

Sparky Doubled the Air Leakage in this Home!

Sparky Doubled the Air Leakage in this HomeThis is the home.This is the hole that Sparky…See More

© 2015   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service