Added cellulose insulation to attic last weekend

[Razl]

Last spring, I had my builder add the code min. of fiberglass insulation to in my attic (R-30), so I could go back later and bring it up to R-49 for much less than he was charging. I knew I wanted to use cellulose to top it off because of its superior barrier-like coverage and its r-value gets better in the cold, where fiberglass can leave all kinds of gaps and loses its stated R-value quickly in cold weather.

Well this weekend, my brother was in town, so we headed off to HD and bought 35 bags of cellulose and got the rental blower for free (20 bags min) for 24hrs. We got the small unit so we bring it up in the attic. The 100ft hose was long enough that we could have left the blower in the truck, but thought communication would be so much easier if we were together. Anyways we took turns being the feeder guy and blower guy.

Overall, it was real easy to do and took us about 4-5 hr to blow the 35 bags. It gets blowing pretty strong so make sure to use a good face mask/breather. I had those disposable paper surgeon type masks and they were terrible. We ended up tying a T-shirt around our face that worked 100% better.

A few observations for those considering it:

1. Add reference lines on the framing so you get even thickness. Once things get going you’ll have no idea how thick you’re going

2. Start from the farthest point and work your way back. Common sense, I know.

3. Build a perimeter wall around area you want to prevent the loose cellulose from getting into undesired areas later (attic stair opening, air handler, soffit baffles, etc. I just used some of the existing fiberglass batts.

I got a quote of $1400 from a local installer to do this but we were able to do it in a half a day for just under $300 (not including the tax credit). I figure the payback will be about 2 years, but the additional winter comfort was immediate, especially the masterbed over the garage. I was short a few bags, but overall am very satisfied with the results and cost. Feel free to PM with any questions too.

 

[Johnny]

Thanks for this info, it's good to know for my project next spring.

 

[benjamming]

Was the original blown fiberglass or batts? I assume blown? How did the fiberglass look? How much had it settled?
So is this a wet-applied cellulose process? Isn't the fiberglass difficult to tell exactly how much is blown in since air really 'fluffs' it up?

 

[Pablo]

What is the cellulose treated with?

I kinda knew blown cellulose has a better R value than blown fiberglass, but why? Nature of the material? Size? I don't get the "iberglass can leave all kinds of gaps and loses its stated R-value quickly in cold weather." part - Why?

 

[Shannow]

Pablo,
I can only assume that the fibreglass is batts rather than blown. THen, there's gaps all around the place.

When I had my skillion roof blown a few years ago, I anted up for the cellulose/wool, as it hold ist shape better, and stays fluffy longer.

It's treated with something like Borax to discourage vermin and fire.

 

[Razl]

Johnny - it's the most bang for your buck insulation.

Benj - The R30 was fiberglass batts, not blown. Adding the cellulose on top of the fiberglass will cause some compression, but that doesn't matter as long as you reach your target final height. For me, it was about 16 inches; the original batts was 9.5" providing an R-30, then the 6.5" of cellulose provided an additional R-22. With some settling, I should be just above the ideal R-49 for my region. The nice thing about cellulose is that fluffing or compressing it does not affect its per inch R-value. Fiberglass on the other hand is adversely affected when you change its engineered thickness. The stuff I used is just fluffed with air. The cellulose used in walls is done by a pro has water added to it so it sticks to the vertical walls like a giant spitball.

Pabs --the cellulose (mostly recycled newspaper) is treated with borates for the reasons Shannow stated. As for the superior R-value of cellulose vs fiberglass, I’n not sure. . My guess has to do with the molecular structures. Less dense materials are better insulators (bigger spaces between the atoms slows down the transfer of energy, heat in this case). I think fiberglass is made up of mostly solid plastic like strands so it needs lots of air space between the fibers to be a good insulator, which explains why it always fluffed with lots of air. The final cotton candy-like structure gives it an R-3.2 per inch. Remove the air by compressing the fiberglass, and the R-value severely diminishes. So an R-13 batt in a typical 3.5" wall may net less than an R-10 due to compression (many installers don't take the time to do it correctly. This is where you get the gaps and compression I was referencing). Flatten it to an inch (like when a drain pipe or wiring is in the way an your lucky if its an R-2. Cellulose is less dense and can probably trap pockets of air within its rigid cell wall, so it doesn't rely on air gaps to give it its heat resistance. Fluff it, compress it and you'll still get about 3.5 R-value per inch.

As for fiberglass losing R-value in extreme cold, I dunno? Studies show that the greater the differential temp, the greater the r-value loss. For example R-19 of fiberglass will give you R-19 at a 20F differential (ie. 70F inside home, 50F outside). But the same 70F house in weather conditions like 10F (60F diff), the fiberglass would be perform like an R-12. Most homes here have R-13 in the walls, so the same scenario would give you roughly an R-8 wall. In real world applications you'd get even less because you'd need to factor in large losses do to air movement, which can be significant in exterior walls. Cellulose is packed denser so less air swirling goes on.