# Mixing fiberglass and cellulose?



## Easy Livin’ 3000 (Feb 2, 2017)

My old house has blown in fiberglass insulation in the attic between the joists. I was going to replace it with cellulose, then I started wondering if, instead of trashing all the fiberglass, I figured out a way to sandwich what's there with a layer of cellulose on top and on bottom.  The joist bays are 8" deep, and currently covered with 1"x 6" tongue and groove boards with no vapor barrier.  The attic is used for storage.

Two questions:
1. Any way to use the existing blown in fiberglass with new cellulose?
2. Should I put a vapor barrier on top of the joists before I put the tongue and groove back on over the insulation?


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## EatenByLimestone (Feb 2, 2017)

The vapor barrier goes between the conditioned space and the insulation.  You'd want it below the insulation.  Rather than remove all the insulation I'd just add new to the top and make sure the ceilings below were air sealed.


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## Easy Livin’ 3000 (Feb 2, 2017)

EatenByLimestone said:


> The vapor barrier goes between the conditioned space and the insulation.  You'd want it below the insulation.  Rather than remove all the insulation I'd just add new to the top and make sure the ceilings below were air sealed.


Well, that makes things more complicated, as the 2nd floor ceiling is plaster and lath that I really don't want to disturb.  Also, the fiberglass is already filling the joist cavity, no space on top without removing some. I guess my point of removing the fiberglass and replacing it with cellulose was to create an air barrier so I wouldn't need to rip out the plaster and lath on the ceiling.  I do plan to add perimeter knee retaining walls that I'll fill with cellulose on top of what is there while still retaining attic storage space in the middle.


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## EatenByLimestone (Feb 3, 2017)

Air sealing doesn't have to be complicated.  Remove the light fixture in the ceiling and seal the holes around it that air can get through.

If I remember correctly,fiberglass has an r factor of 3.2, cellulose 3.5.  I wouldn't go through the trouble of removing it.  Building up the attic by adding 2x and filling the new bays will work too.


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## EatenByLimestone (Feb 3, 2017)

But if you are adding Cellulose regardless I'd dig into the blown fiberglass where the light fixtures are and seal from the top.  You'll only be disturbing a couple inches above the lights.  Then blow new on top.


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## Circus (Feb 3, 2017)

Installing a vapor barrier above the insulation will probably cause water stains on your ceiling. No vapor barrier is better than one incorrectly installed.
If you don't have water stains now, adding a vapor barrier, even correctly, won't do much anyway.

Adding insulation to a 2000 sq ft ceiling from r30 to r60 might save you $60 a year.

Adding insulation to a 2000 sq ft ceiling from r2 to r30 might save you $1800 a year.


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## Easy Livin’ 3000 (Feb 3, 2017)

Circus said:


> Installing a vapor barrier above the insulation will probably cause water stains on your ceiling. No vapor barrier is better than one incorrectly installed.
> If you don't have water stains now, adding a vapor barrier, even correctly, won't do much anyway.
> 
> Adding insulation to a 2000 sq ft ceiling from r30 to r60 might save you $60 a year.
> ...



Water stains, ha! This old house is so racked with peeling paint, cracked plaster, etc., I'd love to notice a water stain.  My favourite is the bedroom where they painted over the wallpaper, a lovely dirty pink color.

I'm more concerned with the warm air moving unimpeded through the blown fiberglass insulated ceiling into the uninsulated attic.


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## Easy Livin’ 3000 (Feb 3, 2017)

EatenByLimestone said:


> But if you are adding Cellulose regardless I'd dig into the blown fiberglass where the light fixtures are and seal from the top.  You'll only be disturbing a couple inches above the lights.  Then blow new on top.


I like this idea. Very practical.


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## woodgeek (Feb 4, 2017)

@EatenByLimestone nailed this one.

As @Circus said, improvements to existing insulation can have lousy payback.  If you are losing a lot of heat to an apparently insulated attic, its not 'convection' in the FG (which is overblown as a problem) its good old fashioned conditioned air leaking into the attic through the insulation.

In my case, I had an older house with ~14" total of FG (old, blown between joists) AND crosswise batts on top.  I thought I was covered.  And then I noticed that my attic was always as warm as the house, and the snow melted off the roof darn fast.  When I airsealed my attic floor (over a few weekends, over a couple years) I dropped my heating bill 30%, without adding ANY new insulation!

1. Airsealing between the house and attic is the priority.  You need an air (not vapor) barrier which will be your plaster...this is a good air barrier except where it meets the framing, and small penetration like light fixtures....

You need to take up the T&G decking (hard) and dig under the FG (easy) in locations around the perimeter of the house and over interior walls.  You want to see if there is a wide open path (or a long narrow crack, or holes) between the framing below and the attic air space.  If there is, you need to seal all that.... 

Open stud bays, you can stuff 16" hunks of (new) FG batts into kitchen garbage bags, fold them double and stuff them in.  The FG holds it in, the bag blocks air motion.  If the house is framed balloon style, the whole perimeter might need this treatment.

Interior walls often have a top plate, and the plaster might not be done up to the top plate, creating an air gap the length of the wall, often on both sides.  You can seal this with caulk or foam.  If you need to tackle a 100' of such a crack or more, buy a powered caulking gun and the big pro tubes, or a foam gun.

You might have a plumbing chase, a couple square feet in cross section tying your house into the attic, with a little FG batt or sheet good sitting on top.  That you just cut some rigid foam sheet like XPS or polyiso to size, and caulk/foam in place.

And so on.  Google is your friend.

A company would do this for you for a few thousand $, less if you removed the decking first.  Payback would be good, and they would estimate it for you.

2.  When the above is done, and you want to add insulation, you will have to reduce the area that is decked out (which I assume is for storage).  In the decked smaller area, I shoveled the loose FG into the larger area and spread it out, then stuffed new FG batts into the joist space.  I bought thicker batts and compressed them slightly into the space.  They cost the same per sq ft, will have slightly higher R-value, and will take longer to collapse vertically due to age.  And there is no convection in dense batts.  You can use a different insulation here, but loose cellulose would be a poor choice, since it will settle under the boards.

In the larger, undecked area, I had a company just blow cellulose over the top to a total depth of 14" over the joist tops.  They made a little foamboard dam around my decked area, and blew it in in no time.  This would DIYable if you like...cellulose blow is easy. 

Now all of my attic floor is airsealed and most of it is >R-50, and a small decked portion is ~R-20.

So, if you are uncertain, you can
1. look at snow on your roof
2. put a wireless thermometer in your attic, and measure overnight/predawn temps veruss outside temps.
3. buy a cheap IR thermometer, and scan it around your plank attic floor and look for hotspots.

If the snow melts fast, you have giant icicles, your attic runs 10-20°F warmer than outdoors, you can see warmspots over interior or perimeter walls, then the above plans will save you a bundle on your heating bill and will greatly increase house comfort year round and reduce draftiness.


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## Easy Livin’ 3000 (Feb 4, 2017)

woodgeek said:


> @EatenByLimestone nailed this one.
> 
> As @Circus said, improvements to existing insulation can have lousy payback.  If you are losing a lot of heat to an apparently insulated attic, its not 'convection' in the FG (which is overblown as a problem) its good old fashioned conditioned air leaking into the attic through the insulation.
> 
> ...



Great advice, now I have an improved plan.  I'll be doing all the work myself, just need to be methodical about it, like it sounds like you were.  

I won't be reducing our heating bill, our little insert provides 95% of our heat. But it is very drafty, I want to improve the comfort level. 

Half the house is 18" thick rubble stone walls.  The other half, poorly insulated stick frame built much later. I think that most of the problem on the stone side occurs where the frame was built on the interior, to support the lath and plaster. There are lots of irregular gaps between the original stone walls and the frame, nothing impeding airflow there.  I'll use your FG in trash bags idea to plug those up.

For the perimeter knee walls, should I leave the boards off and pile more insulation on top, or put the boards back and pile the insulation on top of the boards?  I'm leaning toward leaving them off and making it contiguous insulation.

One more thing, what internet sites do you recommend for more reading?


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## woodgeek (Feb 4, 2017)

The classics:

https://www.energystar.gov/ia/partners/publications/pubdocs/DIY_Guide_2016.pdf

http://www.greenbuildingadvisor.com/blogs/dept/musings/air-sealing-attic

Get good PPE:  Skip the paper masks and get a $20 valved respirator and goggles, much more comfortable too.  I didn't go the tyvek suit route, just changed clothes and showered immediately.

My buddy with stone/rubble wall house had surprisingly low energy bills (compared to mid 20th century stick construction).  I think the R-value, while not great was effective due to good airsealing.  YMMV.


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## Easy Livin’ 3000 (Feb 4, 2017)

woodgeek said:


> The classics:
> 
> https://www.energystar.gov/ia/partners/publications/pubdocs/DIY_Guide_2016.pdf
> 
> ...


Thanks woodgeek, more good advice, I've been fumbling with the paper masks for far too long.


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## woodgeek (Feb 4, 2017)

They're also handy on Halloween.


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## hickoryhoarder (Feb 9, 2017)

In my current house I saved fiberglass in the attic that was still good and blew in cellulose over it. Have noticed no disadvantages.  I try to seal each house perfectly, so that no air is getting into the attic through ceilings, walls, attic hatch, etc.  You can also do a vapor barrier by painting the ceiling with a vapor barrier primer or paint. 

When I did an uninsulated attic I painted the vapor barrier in the rooms, put down 8 inch kraft faced fiberglass, paper down of course, then put the attic floor back down on top the joists.  That worked extremely well in that house. 

Your climate, amount of attic ventilation, etc. all enter in.  To me, the two most important things are air tightness between house and attic; and insulating the entire attic floor, not leaving gaps because you didn't want to custom cut insulation.  If you have stairs to the attic, I weatherstrip the door to that bedroom, as it's an outside door, and then at the top I build a hinged door of plywood (part of that top is a nailed in piece of plywood, then a large hinged door). I weatherstrip that plywood and door as tight as possible.  This works very well in practice.

My houses have been built between 1930 and 1959.  Typical heating bills have been $450/year, central Indiana.  Total investment dollars per house have been between $300 and $3000 (mostly wall insulation by a contractor).  The basic principles of sealing and insulating work out real well, if applied, and it's pretty quick work, except in low attics and crawlspaces.


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## Easy Livin’ 3000 (Feb 9, 2017)

hickoryhoarder said:


> In my current house I saved fiberglass in the attic that was still good and blew in cellulose over it. Have noticed no disadvantages.  I try to seal each house perfectly, so that no air is getting into the attic through ceilings, walls, attic hatch, etc.  You can also do a vapor barrier by painting the ceiling with a vapor barrier primer or paint.
> 
> When I did an uninsulated attic I painted the vapor barrier in the rooms, put down 8 inch kraft faced fiberglass, paper down of course, then put the attic floor back down on top the joists.  That worked extremely well in that house.
> 
> ...


Thanks Hickory.  Real nice tips. I have about 10 high gain projects in mind, hopefully before next winter. I really liked your ideas about the attic door. I'm thinking instead of the plywood at the top, I'll try panels of polyisocynurate (?) insulation.  Easy to work with, and easy to move around and store when the cold is over.


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## hickoryhoarder (Feb 9, 2017)

ED 3000 said:


> Thanks Hickory.  Real nice tips. I have about 10 high gain projects in mind, hopefully before next winter. I really liked your ideas about the attic door. I'm thinking instead of the plywood at the top, I'll try panels of polyisocynurate (?) insulation.  Easy to work with, and easy to move around and store when the cold is over.



Many people use Styrofoam or polyiso in these situations.  The disadvantage is that this hatch is not heavy enough to seal well, to press against weatherstripping.  I glue Styrofoam to the attic side of the plywood sheets.  The foam board is of course much easier to construct.  You just set it up there.  But the door thing took me maybe an hour or two, including hinges, handles, a hook (raise it up and hook it to a post to sit open), etc.


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