Attic and Walls insulation nuances


Old 03-02-08, 08:48 AM
Thread Starter
Join Date: Jul 2004
Location: USA
Posts: 78
Lightbulb Attic and Walls insulation nuances

After reading multiple great posts on this site I decided to insulate attic and walls with cellulose.

Property info: house 50+ years old, state MA.

Attic questions:

1) Currently have fiberglass in attic about R22, need R50, is it ok to blow cellulose above fiberglass?

2) I have central AC that has several A/C vents in the ceiling. Should I cover them in attic with Foil Insulation ( to keep cellulose off the A/C hoses?

3) should I isolate side walls in attic from the cellulose with felt paper or plastic, or fiberglass, or it's ok cellulose to touch the wood walls?

Wall questions:

4) it's usually said that this work should be done by professional. What can I screw up doing that myself? Note: I used to do most things my self, but certainly is not professional

5) walls have old fiberglass. Will cellulose fill up the entire wall just fine? Is technique required?

Other questions:

6) I have a space behind closet and wall/roof on the second floor. I can see in the space that the area between the 2nd floor and 1st floor ceiling if empty and cold. Is it ok to fill it up with cellulose along the wall? I will have to drill holes in the floor like in walls for that.

7) can I fill up garage walls attached to the house with cellulose? Not sure if they are totally empty though. I can see mesh in some open spaces where stucco fell out.

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Old 03-02-08, 10:57 AM
Join Date: Feb 2008
Posts: 82
Cellulose in walls

Cellulose works great it walls due to the fact that it is very fine and fills every cavity. We used to do insulation and used both fiberglass and cellulose. Rental blowers do a pretty good job with cellulose but sometimes not so good with fiberglass. We used truck mounted 18 hp commercial blowers that would do it well. Get the most professional one you can find.

Be sure to keep the insulation away from air vents at the eves. You can buy dams that are sized to 16" & 24" rafter placement. Dam around chimney and gas appliance vents. Many can lights should also have dams around them to avoid excessive heat build up.

You need two holes in a standard 8 ft wall stud cavity to blow in insulation, one about 18 " from the floor and the other about one ft from the ceiling. Blow the bottom whole first until the blower shows resistance. Then move to the top hole and do the same thing.

Make sure that your wall board is well attached because you can get quite a bit of pressure on the wall. A few additional screws or nails can save you from pulling off the drywall, shaving down the insulation and re-installing it.

Watch out for firestops and other restrictions that would keep you from filling a cavity. Also watch out for holes such as those made by plumbing or hvac. This might save you from blowing your kitchen cabinet full.

If you are blowing insulation from the inside or outside patch the holes properly. If you drill holes from the outside in many types of siding and plug the holes the plugs often show over time due to expansion and contraction. If you are fortunate enough to have a removable siding like shingles or some lap siding you can remove the siding at the hole level, drill the holes, plug them and then reattach the siding or shingles over the plugs.

Blown wall insulation is very effective in not only insulating the wall but stopping air infiltration. It is wise to have a vapor barrier on the inside of the wall but this is usually formed by painting.

If you have any questions contact your local building department. Insulating your ceiling offers a good opportunity to check to see if your attic venting is adequate.

Be sure to wear a protective mask and eye wear when you are blowing insulation. Cellulose is more dusty than fiberglass but fiberglass probably represents more of a health risk.

Old 03-02-08, 08:35 PM
Thread Starter
Join Date: Jul 2004
Location: USA
Posts: 78
many thanks for your great post.

I am still hesitating whether get to walls from inside or outside as I have wallpaper that probably have to be totally replaced in entire house after the work. Also it's not easy to remove wallpaper (did not check yet) unless I put new one above.
Penetration from outside has also a problem. I guess it takes enormous amount of time to remove wood shingles, replace with new and paint.
It seems that contractors would prefer getting from inside that takes less work.

It would be great if somebody could also reply to 6).

Old 03-03-08, 10:33 AM
Join Date: Sep 2005
Location: ohio
Posts: 195
OK this is going to be a long post. Let me start by saying that I just finished insulating the walls, side attics, and garage ceiling of my 50+ yr larger cape with cellulose all diy and done from the inside. The walls did already have 1-2 inch batts in them which which didnt pose a large problem but needed some techniques to do it right. Im amazed at the difference in my house and think everyone should do it so i'll try to give as much detail as possible in answering your questions.
First off before you do anything think about airsealing your attic space correctly from the living area below. Get a few cans of sprayfoam and caulk and even pink xps foamboard in neccessary to seal up any gaps you find or whats called attic bypasses-places where air can come up thru the living area and get around the insulation. Common ones are attic hatches/doors, chimney chases, plumbing chases recessed light cans, gable end walls open to the attic space and the kneewall/floor junction. Its really important for attic moisture control as well as energy savings. Here is a link to give you a primer:
If you need more links for airsealing ive got some.
Now to answer your questions:
1. Yes its fine to blow over the fiberglass. If you have soffit vents dont cover them up. Use the stryrofoam baffles sold at lowes or HD and staple them up to keep the flow open. Dont cover recessed light cans unless they are IC (insulation contact) rated. Keep 2 inches away from any chimney flue which should already be airsealed and have flashing before you blow. If the end walls of your house (gable end) are open to the attic you need to cap them off with 2 inch foamboard and sprayfoam. This is a big bypasa area. Again do all your airsealing before you blow and you will be good to go.
2. Ductwork in the attic can just be totally covered with the cellulose. Make sure there are no leaks in the seams anywhere--use foil tape. Pay attention to where the duct meets the actual vent and seal it with foil tape if needed--not duct tape.
3. Are you talking about the gable ends of the attic here? If so its ok for the cellulose to touch it. You dont need to do anything here except to make sure that the wall cavity from below is capped off so air wont leak here. If there is no top plate cut pieces of 2 inch pink foamboard and fit it in each space and then seal around it with sprayfoam.
4. This stuff isnt rocket science but you can mess things up too if you dont read up enough before hand. The really common mistakes seem to be covering up soffit vents, incorrect sealing of bypasses, and incorrect amounts of cellulose blown into walls for real coverage and then issues of settling. You might think about doing all your drilling first before you do the walls as it takes a while. I used a 2 inch hole saw when I drilled from the inside and saved all my plugs so patching was easier. Also be sure to probe each hole for blocking etc. If you have return air just using a stud space be SURE not to fill that up with cellulose and mark it off. One good book ive used is "Insulate and Weatherize" by taunton books. It was like a bible so i'd recommend buying it.

5. You need to figure out how thick the insulation is here. If your batts are R13 Its going to be difficult and might not be worth your while. If its old stuff it might be like mine; 1 to 2 inches thick. Drill a couple of small holes and insert a plastic stick to try to measure the thickness or take off an elec. outlet cover and see if you can tell.
Regardless if you do it from the inside or outside you will to blow your walls at a density of 3.5 lbs cellulose per cubic ft in each stud bay to achieve proper airsealing and to prevent settling of the blow in, to do this you will need something called a fill tube to do it right. The tube is just a long 10 ft or so thin flexible plastic tube that you will attach to the normal nozzle and larger tube from the blower. I used 2 sizes of tube for my walls. Each 10 ft long with 1 1/2 OD and 1 1/4 OD.They can be gotten here : If you just use the wall nozzle commonly given out the coverage and density of insulation in your walls will be off.
Estimating the density is easy. Calculate the volume of the stud bay or other void you want to fill. Multiply that volume by 3.5 and thats how many pounds of cellulose you need to blow in there. Divide the weight of 1 bag of cellulose by how many you need for the void and you will know how many bags you need for each wall. It isnt exact since you have batts in there and will have windows, fireblocking etc but counting the bags blown works generally.
Insulating the walls with a fill tube isnt really hard to do but theres a few techniques to master. Ive found a couple of really good tutorials online that i completely read through about dense packing sidewalls from outside and in using the fill tube. I suggest you read and then re-read them.

6. Here it sounds like you have a floored side attic with a kneewall. This is a source of huge amounts of leakage. If the floor joist bays are open they will let conditioned air escape from the 1st floor and allow cold air in there. If you have access to it block off each space with 2 inch pink foamboard snugly fit and then use sprayfoam to seal around it. If you cant get to it cause theres a subfloor there, you can drill holes in the floor of the side attic or even the 1st floor ceiling and use the fill tube to dense pack it. You need to carefully insulate this space and do it differently depending on if you use this space for storage or not. If no storage is needed insulate the back of the kneewall with R19 batts and the floor joists with dense pack cellulose. Be sure to airseal any penetrations of the kneewalls as well as the top plate where it meets the sloped ceiling and the bottom plate. The back of the kneewall should be covered after putting in the fiberglass with either tyvek housewrap with the seams taped or 1 inch foamboard sheets nailed to the studs and sealed all around with sprayfoam to keep out air and prevent the windwash (remember that fiberglass wont stop air) from lowering the R value of the insulation.
If this area is to be used for extra storage then you need to define the thermal boundry at the slope and put your air barrier there and also still seal the floor. A couple of links I found talk alot about these kneewalls and how to fix them.

7. Yes if there is space you can fill them up. But if its metal mesh I would bet its gonna be hard on your drill bit to get thru there. Be sure to insulate only the walls next to a heated space. Insulation wont do anything to heat/cool an area--it just keeps more of the heat/cool in thats already there.

OK I hope that helps out some. If I left out some info keep asking but do read all those links. They are good and will save you tons of time.
Old 03-09-08, 09:38 AM
Thread Starter
Join Date: Jul 2004
Location: USA
Posts: 78
Thanks d00bs,
I feel more confident now even though underestimated the work as just attic preparation may take a month for me :-)

Old 03-30-08, 11:28 AM
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Join Date: Jul 2004
Location: USA
Posts: 78
could you tell me (give full description) what exact tube should I buy. Looking for 1" 1/4 ID
Old 03-30-08, 02:37 PM
Join Date: Sep 2005
Location: ohio
Posts: 195
i got the ST-125 tube they have here:
I bought 11 feet of it and also did get the smaller ID one as well (st-100) just in case i ran into problems with smaller areas. i was right and needed the smaller one too but only for 2 or 3 areas in the whole house.
Old 03-30-08, 04:25 PM
Thread Starter
Join Date: Jul 2004
Location: USA
Posts: 78
Exclamation Attic vents insulation

One more question. I have several a/c vents connected to a/c located in the attic. Each a/c vent that goes down to the ceiling of the second floor looks as metal box that is connected to a/c via plastic duct something like this

I filled the gaps between the boxes and the attic floor with foam, but I believe that boxes themselves produce big leak as having small holes and also is good source for heat transfer.
I was thinking to cover them with plastic or foil sheeting, but do not know what is better and how. I could not find good recommendations on web except this document that is also confusing

Any ideas?

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