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Difference Between Blown Cellulose and Fiberglass Blown Insulati

Posted by jln1980 (My Page) on
Wed, May 23, 12 at 8:22

Is there a difference between blown Cellulose and High Density Fiberglass Blown Insulation? Is one better then the other?


Follow-Up Postings:

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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

In the grand scheme, theres not much of a difference but to energy and building science nerds there is.

There are many decisions that are related to this question that are much more important. Where are you intending to install it; walls, flat/vaulted roof, below grade? What kind of airtightness levels are you aiming for on your blower door test? Are you planning on doing a layer of insulated sheathing?

For purely green intentions its tough to beat cellulose although personally, I wouldnt do it in below grade foundation walls. I think there are Indoor Air Qulaity concerns popping up about the flame retardants in Cellulose.

The new fiberglass systems like spider can achieve similar energy performance as cellulose so for strictly finaincial intentions its best to get pricing for your area as things can vary throughout the country.


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

Thanks for the reply. It's for our ext walls which at 2x6 24 inch on center. There is not a layer of insulated sheathing.

I would have to ask the builder your other questions. I was thinking more from a money and energy saving point of view if one product was better.


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

Cellulose 'behaves' better in the presence of moisture vapor.

It can absorb vapor and release it without going through the liquid state better than fiberglass.

Fiberglass forms ice crystals that then melt and evaporate, wetting the insulation.

For freezing weather and no vapor barrier cellulose is liable to have fewer issues.


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

Climate is important and energy codes will soon be requiring insulated sheathing (if stick framing) depending on where you live.

Some areas are very competitive but in general cellulose is cheaper. Be sure the builder doesnt mean high density fiberglass BATTS which doesnt sound to be the case.

2x6 24" OC is promising especially if smartly framed but traditional 16" 2x4 could actually perform better with a decent layer of insulated sheathing. I would educate yourself on blower door testing and push your builder to get the tightest results possible. This is almost always the cheapest path to saving money on energy bills.

Here is a link that might be useful: Explanation of Blower Door Test with Pics


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

I agree with you Brickeyee that vapor MIGHT pose more of a problem in some rare situations but I think bulk water (from leaks) is much more likely and I think that FG performs better in theses instances because it dries faster. Dang, we are getting overly technical already!


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

I agree with SH. Cellulose has a good moisture buffering effect, however that is not always a goo thing. For example, loose blown in an attic space- if you have a roof leak, the cellulose will absorb it and hold it. It then spreads throughout the insulation, finally getting to the drywall layer. At that point, it may be tough to know exactly where the source of the leak is. It also means your leak could have been happening for a long period of time and you do not know about it. Fiberglass tends to not hold water as much, thus the leaks show up sooner. Same concept in the walls.
Newer fiberglass insulation products, if blown to proper density, will have similar performance of cellulose. Previously cellulose preformed better at reducing air movement through it, not so much the case anymore. Note I said reduce...no insulation (besides expanding foams) stops air movement. Never let a contractor tell you insulating it will make it air tight. I have heard this laughable claim many times.
Dense blown fiberglass will also have a slightly better r per inch, at roughly r4-4.2.
Around here, the price of cellulose and blown fiberglass is the same. The main reason is, cellulose as product is slightly less, however the labor to install it is higher due to far more bags required, its more dusty, etc. This is netting and blowing. On a 2x6 wall I would assume the contractor would install cellulose wet blown, thus no netting required as it would still be for fiberglass.
From a green side of things, cellulose is the way to go.

But as stated above, the type of insulation really does not matter if you have air moving through it. Air sealing is the first priority, insulation second.


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

I am at a complete loss with spray foam/blown insulation/caulk/wall paneling/foam panels/etc... I want a home that is sealed up tight, energy efficient, etc. Can someone give me a rundown of what I should use to get the best r values/air tight seals? Some say use house wrap, some no. Basically from foundation up what should I ask for/use? Run down of best products for a house in the midwest for an air tight/water tight home. Thank you so much!


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

midwest- can you be more specific? Midwest has 2 main climate zones, 5 and 6. While they are similar, 6 has more IECC requirements. I design for 6 all the time, even though some of my projects may be in zone 5. I sit directly on the line between the 2.
Either way, house wraps are NOT, repeat NOT an air barrier to provide a tight home. Depending on the climate 5 or 6, the easiest way to get a tight, energy efficient is to use typical framing with an exterior air barrier plane, and insulating sheathing. The climate zone will decide how thick the exterior foam sheathing is.
My go to wall structure is as follows: air tight drywall on the interior, NO vapor barrier/plastic sheet, blown fiberglass insulation, 2x6 framing (24" o.c.) with rubber gasketing to the subflooring, Huber ZIP sheathing caulked to top and bottom plates, seams taped, 2" XPS insulating sheathing (2 layers of 1" with the seams offset and overlapping), 3/4" vertical strapping drainage plane, siding of your choice. The exterior thickness of insulation will adjust in thickness up or down based on your climate zone to eliminate condensation on your sheathing/within your wall.
Another cost effective, highly efficient wall type that I also like to use is the double stud wall. I have found it has a similar price as the one above, but has a higher r value, but it is a thicker wall and you lose interior floor space.
Either solution, caulking and air sealing is your first priority. Insulating second. There is a ton of info out there. I would suggest you check out Green Building Advisor website for a ton of great information. Also check out the Build Science website for wall assemblies, pros and cons, etc.


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

"if you have a roof leak, the cellulose will absorb it and hold it."

Allowing failures to influence choices that affect day-to-day characteristics is not a very rational method.

unless you have a perfect vapor barrier, vapor IS going to be moving in and out of the insulation.

If the dew point is inside the insulation, fiberglass gets wet and fails.
Cellulose will often just absorb the water as vapor before it condenses to a liquid.

If you also have a freeze zone in the insulation layer, fiberglass fares even less well.

If you have very minor leaks the cellulose may be able to cope with them with NO real damage (it will absorb and release with possibly not even matting if the weight of water is small).

Any insulation method will make finding the source of liquid leaks from the exterior more difficult.

Even insulation like closed cell will allow the water to travel a significant distance before being detected.

Water can run many feet in the inside of roof decking or rafters.


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

We went with spray foam closed cell insulation on the outside walls to seal the envelope of the house. It is the only insulation approved by FEMA for flood zones as it does not absorb water. It also requires no vapor barrier. It also adds to the structural rigidity of the house and we've found it to really make the house quiet.

Only thing is, if you go with it, make sure you have all the plumbing and electrical in all the exterior walls exactly right as you won't want to change it in the future. It is a little better that way than open cell as with open cell the full 6" cavity needs to be filled, but with closed cell, it does not to achieve the proper R value. But it still fills around the electrical outlets and pipes.

Also, you might look into insulating under the foundation floor. We did that and our basement which is finished space maintains a comfortable 63 degrees all winter long with no heat on at all. (We are in CT, zone 5).


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

lzerarc, we are currently in Minnesota and our basement is cold in the winter (and in the summer basically from the AC) that is not really usable without layers of clothes on or a space heater going. We are looking to build in either Nebraska, Iowa or Minnesota and I want a warm basement!!!


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

"we are currently in Minnesota and our basement is cold in the winter (and in the summer basically from the AC) that is not really usable without layers of clothes on or a space heater going. "

It sounds like many basements that are not actually inside the controlled environment living space of the house.

No insulation on the walls would be a clue, or even insulation on the basement ceiling to the living space.

Basement walls need insulation down to well past the frost depth to be usable as living space, and often even under the slab.

The earth is a very large heat sink compared at the temperatures we typically want in living space.


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

Dreambuilder- I would highly recommend considering ICF as a basement choice in zone 5 and 6. It gives you a continuous r value of 24-28 depending on the form, from footer to your first floor structure. It creates a thermal break between the basement slab as well. I would also install 1.5-2" of XPS foam under your slab, more if you want infloor radiant heat.
The other option is exterior foam (2" XPS min) on the exterior of a poured wall. Then you fur out the interior walls and insulate those as well. I much prefer ICF for basements as its warmer and dryer for about the same cost as the items above.


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RE: Difference Between Blown Cellulose and Fiberglass Blown Insu

We built into a southern facing slope and the basement was designed to be full walkout and finished. We used closed cell insulation and passive solar design. We also insulated the foundation floor. As a result our lower level maintains a temp of 63 degrees all winter. On sunny days it warms up to 67 just from the sun.,.we haven't heated the space yet...or cooled it in the summer for that matter....


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