Ceiling question - two options a db question??

[JohnGardner]

Guys,

Here's a quick sketch of my live rooms existing ceilings construction. please note: I CANNOT REMOVE THE EXISTING PLASTERBOARD.

I have built inside out walls up to this existing plasterboard with rubber on top and now I need to upgrade this ceiling. (see my project post)

I have two choices:

(1) John recomended I attach a furring channel to these 70x35mm ceiling battens straight through the existing plasterboard and then run two new lasers of 5/8th board from this channel with it's 50mm cavity stuffed with rigid insulation. This option saves me alot off headroom and is pretty easy to do.

(2) I run new joists off my new walls with two new layers of board and insulation. I lose alot of headroom with this option in an already small room.

How much difference in DB can I really expect between the two options - I know option two is better but really how much better??

Anyone want to take a stab at a rough STC/NRC of the two?

Thanks

JohnG

[AVare]

I am certain that I am missing something because I have never had any incidence of not agreeing and understanding John's recommendations. In this case both are. Hopfully John will clear up my (mis)understanding. The following is based on my knowledge, and open for improvement.

In this case EITHER method will give worse results than just doulbling up the plasterboard. Page 4, figure 3 in

CTU25 shows the effect of having an internal layer of plasterboard.

Resilient metal channels between two layers of 15.9-mm
fire-rated gypsum board in the ceiling give this construction an
STC of only 38. As shown in Table 4, removing the internal layer of
gypsum board gives an STC of 52 for an OSB subfloor. Putting
both layers below the resilient metal channels gives an STC of 55.

An improvement of 14 dB in STC by not having an internal layer! Bear in mind this is with RC.

IR-766 has some more detailed data, incudling TL curves.

Andre

[JohnGardner]

Hi Andre,

Thanks for that - interesting reading.

Just a thought however, are these results based on the fact that there is a sealed subfloor almost directlly above the existing bottom layer of plasterboard making it basically the middle leaf in a three leaf system once the hat channel is added?

I am slightly different in that I have a large angled open roof cavity/attic that is only really weather sealed above my existing plasterboard / ceiling layer. I am guessing that John discounted this as a leaf because it is not air tight and used my existing plasterboard as mass one, the furring channel (2") as the airgap with insulation and the new double layer of plasterboard as mass two. The M-A-M principle.

The other option is just adding two new layers of plasterboard directlly to the existing layer already up there giving me 36mm of plasterboard in total but that seems like it would be less effective as its only one leaf if I discount the roofing iron again.

Any comments or help to make the best of this situation would be appreciated.

Please bear in mind I can't remove the existing plasterboard and I can't get into the ceiling cavity.

Thanks

JohnG

[AVare]

Just a thought however, are these results based on the fact that there is a sealed subfloor almost directlly above the existing bottom layer of plasterboard making it basically the middle leaf in a three leaf system once the hat channel is added?

I don't understand what you mean by "based on the fact".

I am slightly different in that I have a large angled open roof cavity/attic that is only really weather sealed above my existing plasterboard / ceiling layer. I am guessing that John discounted this as a leaf because it is not air tight...

Quite possible and a likely explanation.

What we are writing about here reminds of an experience Paul wrote about here and in his diary on Studiotips. Making a long story short, while he was moving a door around in one room with loud sound in an adjacent room, he noticed that when he moved the door close to the wall, the sound level increased! Obviously there was no air tight cavity in this case, just the door held close to the wall.

Maybe Paul will be so kind as to join in here with his experience.

Obviously this is a key thing to get clearly understood. 18 points difference in STC is big!

Andre

[JohnGardner]

Hi Andre,

What I mean is that all those tests in that paper seem to be based around construction of a ceiling with an existing floor directlly above it. Both attached to the same joists (multi storey dwelling).

There are not any specific tests I can find for a ceiling of a single storey dwelling with only an attic / air space and roof above the joists.

I am no expert at all but I would have thought that a large airspace, building paper and roofing iron could not really be considered a leaf - and if it is it's effect would be not worth getting to excited about anyway.

As always any one elses input would be great.

Thanks

JohnG

[AVare]

There are not any specific tests I can find for a ceiling of a single storey dwelling with only an attic / air space and roof above the joists.

I am no expert at all but I would have thought that a large airspace, building paper and roofing iron could not really be considered a leaf - and if it is it's effect would be not worth getting to excited about anyway.

Code: Select all

Teh large airspace will reduce the MSM frequency, not the coupling.  The airtightness, or lack of, would affect the coupling.  How much is noit clearly known  Brian R has ben doing some interesting testing and peoting STudiotips about the affect of holes in partitiions.  HE also has interesting resuls on double/triple quadruple leaf.  Read to see how important avaoiding multiple leafs is!

What is the average depth og the airspace?  Meaning from plasterboard to inside of roof?  Possibly the MSM will be so low as to not be importatnt any more.

Andre

[JohnGardner]

Thanks Andre,

Average airspace would go from about one foot at the side of the room to 5-6 feet in the middle.
What is MSM frequency?
Thanks
JohnG