Another Attic Insulation Question & Help

16 years ago

Just purchased a 1951 brick rambler in the northern Virginia area. It is about 1,500 sq ft up and another 1,500 sq ft down in a fully finished basement. Need some help in deciding the route to take on adding insulation keeping in mind I want to do the work myself.

The upstairs attic with 1x6 inch flooring over the entire space is completely floored and usable as attic storage. Pulling up some of the flooring after noticing some cold rooms, I found that all of the attic flooring is sitting on 2x8 inch ceiling joists 16 inches on center above downstairs ceiling plaster/some drywall. The sad thing is that there is only about 2 or 3 inches of blown insulation between most of the joists. Over quite a few places in the house, mostly over new bathrooms and bedroom additions, there is ZERO insulation at all.

So there is some blown at no more than 3 inches in height and some areas with nothing between downstairs and attic other than drywall.

I would like to be able to keep most of the attic as storage. This is one of the real nice things about this house. But I want to add insulation and can't decide on which R-factor to go with when I get started.

Ripping up the floor in sections is easy enough and I was hoping to do the attic in sections. Add insulation and then place the flooring back in place and move on to another section. This will allow me not to have to empty out the attic too.

So, after pulling floor boards and opening up to the 8 inch high joists on 16 inch center, I know I could easily add unfaced R-19 which is about 6.25 inches thick. That would bring the insulation right to the top of the joists where there is some blown insulation. Other areas where there is none, it would be below about an 1 1/2.

But, I would really like to get the R30 value between the joists and then put the flooring back on but the unfaced R30 is about 9 1/2 inches thick. I put a piece in today to check it out and you can get the flooring to settle down on to the insulation and the joists but is this advisable? Would the compressed R30 wreak havoc on the plaster and drywall ceilings below? I know that insulation works best without the compression but will I be better off with less insulation (R19) and not always filling the joists completely or better of compression the R30 just a bit into the joists?

Batt fiberglass would be easiest since I am doing this in sections but am I just going in the wrong direction and really should be looking at blowing cellulose with a rental unit a couple of different times?

8 inches to the top of the joists of cellulose would be a rating of R29.60 if the R value of 3.70 is correct.

Any thoughts and I really appreciate the advice and help here.

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Comments (4)

zippyhvac
16 years ago
Why not option #3 - pull the boards yourself and have a crew blow in cellulose? You'll get the full r-value, and save yourself some labor. A pro crew can joist-walk to do the install, they do it all day.

For my previous insulation projects, buying batts and laying them myself was about the same or slightly higher cost than hiring someone to blow-in cellulose. Might be worth looking into.

Cellulose is a better barrier to air movement than fiberglass, so you'll get slightly better performance with cellulose than glass, and you'll be able to go to the tippy-top of the joists without compromising the r-value.
energy_rater_la
16 years ago
in the areas that you want to keep for storage
you could do this:
toenail 2x6's across the ceiling joists
(I use screws rather than nails for toenailing
and decking ...less banging = less damage to ceiling..)
this will raise the area to be insulated without
compressing the insulation.put R-19 in joist bay
and another batt (unfaced) in the area created by
toenailed 2x6's.

then you could blow the rest of the unfloored
attic.

personally I would look for air leakage sites and even get
a blower door and duct test. Seal all leakage sites
then insulate. once you insulate..you won't be able to address the infiltration as easily.

when holes that allow air leakage are sealed, then the insultion can perform. with air moving through the insulation, it robs the insulating values.

personally I don't care for cellulose. I find that the fine
dust it makes finds its way into the living space below via
recessed lights, poorly sealed vent fans, small cracks and
holes.

best of luck.
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david_cary
16 years ago
I'm unclear on the flooring myself - I guess there are beams going the opposite direction of the joists. Ok - I can picture it.

What about your r-19 grouped with some foam? I am imaging 1 inch of foam on the plaster/drywall sprayed in to seal it. It also adds some R-value and thickness. Then if you wanted some more r-value, you could do 1 inch rigid foam (or 2 inches) on top of the joists. This would help with thermal coupling of the joists.

Getting to a magical R-value is not the only concern. Sealing properly is worth a lot. That being said, you would be at r-27.

Spray foam is about a dollar per sq foot at an inch thickness. So you are looking at $1500 - but you might use less because of the joists. The rigid foam is about 1/3 of that so you would need $500 for that. Not cheap but 30% tax credit would count.

I think you would be fine from a vapor barrier standpoint. 1 inch of rigid foam does not count as a barrier but 3 inches would. I would want some independent confirmation on the vapor barrier question.
jackcal
Original Author
16 years ago
Thanks for the input. I think after pulling floorboards and checking what I have up in the attic, I have decided to blow insulation to the top of all the 2x8 joists and then put the flooring back in place after sealing all air leaks. I am pretty confident that will put me just below R30 and if I need more, I can lay R30 fiberglass on the floor but I think I will be pretty good to go with the blown insulation. I should be able to handle the job myself...

Again, thanks and if any one has any further advice or suggestions, please have at it.