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Our house has vaulted ceilings, and the attic space contains three walls that are shared with living space (bedroom and living room). The attic sides of these walls have batts of insulation on them, which makes sense, but the way it's installed seems odd. The lengths of insulation are run horizontally across the studs, and the paper backing is stapled to the studs. In all cases there is a gap at the top which would let cold air in (i.e. between the insulation and the drywall), so I don't understand how this arrangement is doing much good. Is this is a reasonable installation, or should I change it?

(In addition to wanting this to be done properly if it's not already, I'm interested in reducing a low frequency hum from the whole-house fan. One of the walls seems to resonate. My hope was that insulation running vertically between the studs, right up against the wall, would help in that regard. The fan is hanging from the rafters, as shown below.)

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Your whole house fan may need to be mounted on vibration isolation mounts. That would go a long way toward lowering coupled noise into the wall section. –  Michael Karas Jun 8 at 17:18
    
Please use the editing tools for your post to add pictures rather than linking to dropbox. –  The Evil Greebo Jun 9 at 13:15
    
Check the fan blades and see if there is any dust buildup or other debris that might cause them to be unbalanced. –  Paul Jul 9 at 18:44
    
It might also be coincidental that your fan speed exactly matches the resonant frequency of one of your walls. Stiffening or adding mass to the wall, even slightly, could make a big improvement. Nailing a cross-brace to the studs of a wall or inserting some blocking could be a quick fix. –  Paul Jul 9 at 18:48
    
The bungees are a good idea too, I would not replace the chains since the cords will deteriorate eventually. I would hook them to the chain rings at a distance such that it just takes the load off the chains. –  Paul Jul 9 at 18:51

2 Answers 2

The pinkish insulation that's right up against the wall is inside the slats and presumably runs the length of the wall.

The insulation crossing in the perpendicular direction is added on top of that existing insulation and adds to the overall R-Value. Having it run across the slats is fine - desirable in fact.

Assuming the shared wall starts below the top of the insulation, having the insulation hang down a bit at the top is also not a concern. If the insulation is away from the wall where there is shared space, however, then it should be made flush - but you need to be aware of any possible outside air venting.

What I don't really like here is the chains holding the fan in mid air. Why is that fan there? Is it supposed to be drawing air for an air exchanger? If so, it should be coming from OUTSIDE (not your dust/particle filled attic). If not, then all its doing is introducing cold air into your system. And the chains are certainly going to encourage noise transfer. I'd replace those with bungee cords or something else that will dampen the vibrations.

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It's a whole house fan that pull air from the house into the attic. The chains and that method of installation were were provided by the manufacturer (Airscape). –  LVB Jun 9 at 22:48
    
Well that fan is fine then, but I still would swap them out with bungee cords. Chains are rigid materials that will transfer sound just like a tight string between two tin cans. Something with flex like bungee cords will dampen the sound the same way a shock absorbers bounce dampens bumps in the road. –  The Evil Greebo Jun 15 at 12:20

You're pretty much right. Having potential airflow between the insulation and the drywall does hamper the effectiveness of the insulation. Likewise, having the fiberglass exposed allows more airflow through it, which also impacts its effectiveness.

Now, the air in attics tends to be relatively still (except when that attic fan of yours is running, of course), so neither factor renders the insulation completely useless (after all, we regularly use loose fill insulation in attics), but it does mean the actual R value of the assembly is quite a bit lower than the rating on the fiberglass batts.

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