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Is my plan good for insulating and air sealing 1956 home attic


Old 04-29-14, 04:05 PM
Thread Starter
Join Date: Sep 2013
Location: USA
Posts: 14
Is my plan good for insulating and air sealing 1956 home attic

Hi all

Within the next week I will be air sealing and insulating my attic. In a previous post, I was actually going to reside my home and then reinsulate my walls. However, this fell through as the cost for residing would have been too high. Nevertheless, the insulation of my walls got me thinking about insulation in other areas of my house, and after much research, I realized that the attic is a high priority location.

Currently in my attic I have a very thin layer of some sort of blown in insulation that is covered by a sheet of fiberglass insulation. The top of the insulation is even with the joists, which are 2x6, and according to my calculations, this gives the insulation about an R-value of 15-19. My zip code is 53220, and therefore the recommendation for me is to upgrade to an R-value of about 49-60. My plan is to add additional R-38 insulation, but I have numerous questions.

1. When I air seal, I plan to:
Caulk all partition wall top plates where they meet the drywall
Caulk all lighting boxes KOs and gaps around the drywall with high heat fire caulk
Fill pipe and wire penetrations with fire block spray foam or fire caulk where appropriate
Seal my kitchen exhaust vent duct work with aluminum foil tape
Install rafter baffles and provide blocking to prevent insulation from blocking the soffit ventilation
Seal the attic hatch door
Seal around the chimney with sheet metal and fire caulk
I may also remove all receptacles and switches (on both interior and exterior walls) and caulk/foam gaps between the drywall and box.
Am I missing any areas?

2. There is no vapor barrier in my attic. However, I am aware that there is some debate over the necessity of the vapor barrier (especially if the ceiling is painted and air sealing is completed in the attic). I have read from some previous posts that the only real way to apply a vapor barrier would be to rip out the ceiling and install plastic, but I will not take this approach. Yet, given my cold climate location, I am unsure if this is going to be sufficient. Could anyone provide their opinion?

3. I was originally going to install additional R-38 fiberglass batts in the attic, but I have read that blown in cellulose has numerous advantages over the batts. Since I have a lot of existing FG batt insulation in the attic already, is it okay to (after I air seal) place the FG batts back in the joist bays and then blow cellulose over the FG batts?

4. I am unsure if I have enough attic ventilation. Currently on my soffits I have a total of 3 vents; two on the sides of the house, and 1 in the back. All are about 1 square foot each. This gives me a total of about 3 square feet for an approximately 800 square foot attic. I do not know if this is enough, especially since I have been reading about NFVA and how the dimensions affect the airflow. On my hip roof, I have 2 vents, each measure about 1 square foot, although I do not know how the NFVA would even be calculated from this.
Although I have read that you need about 1 square foot NFVA for every 300 square feet of attic space (which I have), my concern for attic ventilation comes from this winter when I was in my attic and I noticed that the nail tips were all frosted. Pretty much every nail had frost on it. I believe this has been occurring for years, but fortunately I have not noticed any water damage. From my understanding, this means that there is air leaking into the attic and a result of the condensation of the rising vapors on the cold nails.
I am unsure if more ventilation is necessary, especially after I air seal, so I would appreciate an opinion regarding about whether or not I should add more attic vents before I begin air sealing and insulation, or just wait for winter again.

5. There is currently about a 10’x10’ section that has scrap wood floor boards attached to the ceiling joists that is used to support storage. Underneath is fiberglass insulation (about R-15) installed flush with the joists. Can I just remove the items that are stored on the wood floor boards and just blow in cellulose on top of the boards? My reason for not wanting to remove the boards is because I may want to store stuff in the attic in the future, and would just brush the insulation over (I understand this is not optimal, but it is a compromise for storage).

Thanks in advance, and if necessary I can run outside or up in the attic and take photos by tomorrow.
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Old 04-29-14, 06:49 PM
Join Date: Oct 2008
Location: New England
Posts: 9,776
Air sealing and paint is probably good enough for the vapor barrier, especially if you have sufficient attic ventilation.

Yes, cellulose over fg will work fine. It also helps to air seal the fg. Read up on Roxul batts as they fit snug together and are very dense. 2 six inch layers with staggered seams and trimmed at the roof angle to possibly eliminate the baffles.

Ideally you would like venting into every rafter cavity, but with a hip roof even that doesn't give the best air flow. If you use 70% for the NFA it should be close.

When the ceiling is well air sealed and you have a continuous vapor barrier then you use the 1/300. Anything less, then you move towards 1/150. It would take too long to explain, but if you knew where those numbers came from you would laugh or cry. Since you can always cut in more from the outside at a later date, you can wait and see how the air sealing works out.

Boards are better than plywood as any air leakage or moisture that comes up from below can still continue into the attic and be vented out. Hopefully there are no major air paths below that 10x10 and if not you should be fine.

The link below may help you find more air leaks, they call them thermal bypasses.

Old 04-29-14, 07:43 PM
Thread Starter
Join Date: Sep 2013
Location: USA
Posts: 14
Thank you Bud for your quick response! Much of my knowledge about attic air sealing and insulation was actually obtained from your old posts

I just looked up Roxul batts and unfortunately the cost would be too high. I priced it out and to obtain a value of R-38 would cost a little over $2/square foot. Blown in cellulose on the other hand will only cost me about $0.30/square foot for a value of R-38. Even with the cost of baffles the cellulose would still be cheaper, but thanks for the recommendation.

After I air seal and insulate, I will keep an eye on the attic for anything strange. Unfortunately, I will not know about any frost on the roofing nail tips until next winter, but I will be sure to post back if an issue arises.

I have also downloaded the PDF and will read it before I begin.

I just have two more questions:

1. It would cost me about $130 more to remove all the old insulation and replace it with an equivalent R-value of blown in cellulose. Do you think there are any advantages to this, or would it just be wasted effort and money?

2. What do you specifically mean by “If you use 70% for the NFA it should be close”? Do you just mean that since I have at least 70% of the recommend NFVA for my attic (I would need about 2.67square feet, and 70% of that is approximately 1.8 square feet vent needed, and I have >3 square feet) I may be fine?

Thanks again!
Old 04-30-14, 03:28 AM
Join Date: Oct 2008
Location: New England
Posts: 9,776
As long as the existing insulation is not contaminated by rodents or so deteriorated as to not function at all, covering it is fine. It will compress a bit and require a little extra cellulose to get the total desired depth. Cellulose over existing fg is a standard practice.

The 70% NFA applies to the existing vent area. If you have 2 one ft² openings it would be only 1.4 ft² of Net Free Area in total. If you want to dig, search for a similar new vent and see what the mfg lists for NFA. Unfortunately, no one polices the mfg claims, such as ridge vents that claim 100% NFA. Your concern is a combination of total area and its location. But, if moisture cannot get into the attic, you may get by with less.


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