Preventing ceiling drywall sag when insulating

[WVBrady]

Hi Guys: I have a ceiling with 1/2 inch drywall that I want to blow some cellulose insulation into (onto?). I have 2x4 trusses on two foot centers with 3 1/2 inches of fiberglass insulation that I would like to increase to R30 by blowing cellulose insulation on top of the existing insulation. The problem is that drywall will not take the extra weight of the insulation without sagging. I can see a small amount of sag already if I shine a flashlight along the ceiling. The trusses make it too awkward to try to run fiberglas matts across the bottom chord of the trusses.

Because of the low clearance to the peak, it would be difficult to get anything very large up there, such as boards. I have thought of two possibilities. One would be to cut thin paneling into strips and just lay it accross the bottom chords of the trusses. They would be flexible enough that I could get them into place (I think!). The other would be to staple some sort of netting to the trusses to support the insulation, such as this:

http://www.teksupply.com/farm/supplies/prod1;;424c9158_TC2638.html

Anyone ever done anything like this? Any other ideas?

TIA, Brady

 

[mhoffm911]

Is the dryall ceiling already finished? I'm thinking I would just put in more drywall screws, say every 8-12 inches. I did blown-in on top of my drywall and did not have any sags.

 

[Junkman]

A 24" span for the drywall cieling is more of a gap than the drywall can support without any additional weight. Usually when you have the trusses on 24" centers, a 1" x 2" furring strip is run across the trusses at either 12" or 16" intervals, and the drywall ceiling is screwed to that. If you don't care about the estetics of the ceiling, you could add the 1" x 2" strapping to the underside of the ceiling. If you don't do something to support the drywall, you might find that the drywall will fall down with the added weight of the insulation.

 

[mrsleeve]

If you have trusses on 2 foot centers you have to use 5/8 board, 1/2 will not support itself for long over that distance let alone with insulation blown in. I used to hang an finish board for a living.

it would be a PIA but you could run cross brace toe nailed in between the trusses every 16 inches and then re screw the board to that, I would also suggest that if you go that route use some good construction glue too. The netting will only take some of the load from the new insulation, the paneling is about you best idea.

 

[bochnak]

You probably want some of these:

http://www.lowes.com/lowes/lkn?action=productDetail&productId=12324-41075-12324&lpage=none

I just went through this with my kitchen last week. I had to use 1/2" again since I had to blend it with existing ceiling. I stapled in batts of R30 before drywall, so sagging should not be an issue. I'll blow in enough for R-19 later.

Unfortunately, Lowe's only carried 16" pieces when I was there.

 

[Bib Overalls]

The proper way would be a second layer of 1/2" board glued to the existing ceiling and screwed to the studs.

Our house was built with ceiling joists and rafters on 2' centers. We used 5/8 OSB for the roof and 5/8 board for the ceilings. Lots of insulation. No sag.

 

[Notch1988]

Quick question... would 5/8 drywall be strong enough for 24" centers with blown in?

 

[5wndwcpe]

The proper way would be a second layer of 1/2" board glued to the existing ceiling and screwed to the studs.

Our house was built with ceiling joists and rafters on 2' centers. We used 5/8 OSB for the roof and 5/8 board for the ceilings. Lots of insulation. No sag.

That would be my call.

 

[oldgoat]

They used 5/8 on my ceilings in the garage and I understood that that is rec. because of the sagging issue. I tried using the wire things from Lowe's but didn't have much luck with them and if using blown insulation I don't see much use there. I'd think about laying chicken wire across the top of the rafters and then blowing it in.

 

[Junkman]

Are you going to be using the blown in cellulose or the blown fiberglass? I believe, but am not certain, the fiberglass weighs less per cubic foot of material. Either way, you will need to do something to keep the old wallboard from sagging. I am not certain that at this point, that adding an additional 1/2" layer of sheet rock is going to solve the issue. You will also need to know as to how much loading the roof trusses are designed for. If you add another 3/8 or 1/2 inch of sheet-rock to the existing, and then add additional insulation, you might overload the roof system, causing other problems. I would check with your local lumber yard since they should know the rating of the rafters if you give them all the dimensions and design factors. If you know who manufactured the original trusses, that would make it even easier. They would know all the design parameters.

 

[mrsleeve]

Quick question... would 5/8 drywall be strong enough for 24" centers with blown in?

yes it would

 

[WVBrady]

Wow, thanks for all the responses, guys. Unfortunately, the ceiling is already finished. I am not certain that it is 1/2 inch; it could be 5/8. The only thing that I am sure of is that it is sagged a tiny bit. I never noticed it until I went to check for sag with the lights turned off and shined a flashlight parallel to the ceiling.

I was planning on blowing in cellulose because of what I had read about fiberglass having a lower effective R-value because of convection currents at low temperatures and because of the skin irritation caused by fiberglass. It is true that the cellulose is a little heavier, but I think that I need some sort of support regardless of what I use.

I like the idea of the chicken wire. I think the netting might tend to sag a little with time, whereas the chicken wire will not. I will probably use a combination of chicken wire and the strips of wood, depending on which will be easier to work into a particular spot.

Thanks again for such a great forum.

Brady

 

[IDASHO]

More than likely it is 5/8.

If so, you are fine.

 

[Kevin54]

Post a pic of your ceiling. It sounds to me like the drywall is running in the wrong direction. The 8' run of drywall should span 5 ceiling joist/trusses. If they ran it longways only spanning 3 joist, that is why it looks like it is sagging. 5/8" is more than adequate to support batts plus blow in without having to put anything else up. The only other thing would be that whoever finished off the drywall did not feather it out far enough. To tell that, hols a straightedge crossways to the seams and see if you have gaps.

Kevin

 

[drbill]

Your ceiling was probably never straight in the first place. Either the finish job isn't good or the trusses were not built or installed level.
I'm glad I'm ignorant : because my 1/2" drywall installed on 2' on center truesses is as straight as the day I put it up 4 years ago. Heck my 50 year old house has 1/2" drywall ceilings and 12" of blown in insulation and it hasn't moved either, but it's 16" on center.

 

[6768rogues]

Use 5/8 or high strength 1/2. They cost about the same here.

 

[jeepinjz]

I used 1/2 on mine never have a had problem in 12 years