Is interior wall insulation necessary?

susan3733May 20, 2009

We're building a 3500 sf home in the San Diego area, near the beach. Our climate is so temperate that we use our heat only about 2-4 weeks/year, and we don't have a/c because it would probably only be used a maximum of 5 days/yr, if that.

Our home is well-insulated on the exterior walls and roof, but our builder tells us that he did not plan to insulate the interior walls. Since we've got a fixed price contract, it would cost us extra to put insulation in the interior walls, which he thinks would cost us around $700.

I have a couple questions -

1) Is interior insulation standard and is it typically included in a standard bid for a custom (fairly high-end) home?

2) Even if the cost is not included, should we go ahead and do it as an extra? In the big picture, of course $700 is no big deal but I hate to pay for something that's not necessary.

The builder is using 5/8" drywall, which he claims is a bit nicer than regular 1/2" drywall and may help make the house feel a little more solid and quiet...don't know if that's true or not.

There aren't many adjoining interior walls where sound transmission would be a potential problem. The rooms that share a common wall are our daughters' two bedrooms and the office and guest room, and that's about it for the major rooms. The powder room shares a wall with the dining room so it might be a good idea not to hear the toilet flushing during a dinner party, but other than that, I can't think of any other areas that could pose a problem, and I don't know if insulation would make a big difference anyway...your thoughts?

All advice/feedback welcome. Thanks in advance!

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We currently have a two story home that has insulation in the interior walls and between floors. The previous owners who built the home are notorious cheapskates, yet they spent the money to buy the insulation, which they installed themselves.

The sound deadening qualities due to the interior insulation are amazing. The TV downstairs can't be heard upstairs. Sound transmission between floors is absolutely minimal.

Spend the money. It will be the best $700 you ever spent.

    Bookmark   May 20, 2009 at 6:39AM
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Insulation in interior walls will increase sound separation a little so where that is critical it should be done but it is not effective in walls with doors or shared ducts, etc. Insulation in an interior essentially reduces the sound that gets into cracks and outlet boxes on one side of the wall and must then pass laterally though the insulation to get to cracks and outlet boxes on the other side. The thickness of insulation not enough to absorb much sound but if the sound has to travel several or more feet inside the wall it is very effective. This is why wall perimeters and penetrations are so important in reducing sound transfer. Sound really likes cracks. I would just put the insulation where you are sure you need it.

5/8" GWB will help quite a bit in sound reduction. The next step is to add another layer on one side. Do that where sound reduction is really important.

The sound transfer through a floor is reduced a bit more because the insulation can be thicker and there are fewer penetrations and cracks but is it still far less effective than using resilient channels to attach the ceiling to the framing. The true cost of floor insulation must include the more expensive IC rated housings required for recessed lighting. Resilient ceiling channels are the best investment for the cost for sound reduction. They are standard for multi-family housing with stacked tenant spaces.

The toilet backing up to a dining room is potentially a major problem. You can use no-hub cast iron pipe at that level or use special sound reducing waste pipes or a double studded wall with double layer wall board with resilient channels. I would definitely recommend using a toilet that flushes fast with little noise. The only one I am aware of it the TOTO UltraMax #MS854114S

    Bookmark   May 20, 2009 at 9:55AM
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I have only bothered with interior insulation as part of a "package" including resilient channel and extra layer(s) of drywall.

Journal of Light Construction

For the same price, I've found solid core doors with as little under door clearance as possible to be much more effective.

    Bookmark   May 20, 2009 at 10:35AM
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Wow - this is SO helpful. Thanks very much for the responses.

MightyAnvil, we just passed our rough plumbing inspection yesterday so we don't have the opportunity to change the pipes in the powder room at this point, but I'll see about getting the Toto toilet in that room. I'll also see about doing another layer of drywall and resilient channel there instead of more insulation, since it sounds like that will not do much.

Worthy, thanks so much for the chart! It is great to see hard data on this so we don't have to guess at cost vs. benefit of these extras.

I should have also mentioned that our master bedroom is directly above the family room so (for those nights when our teenagers are up watching TV) maybe we should put the resilient channel between the family room ceiling and the master bedroom floor; however, we will be carpeting the master bedroom so maybe the foam and subfloor will do the trick...any thoughts on that?

It's interesting to know that the solid core doors will help. We'll be putting in solid core doors, but they are Shaker-style two-panel doors so the middle section is probably a veneer and not as thick as the rails around the edges. Don't know if this lessens the sound-deadening qualities of the door but I imagine it would...what do you think?

One more thing...I'd love to have any info about what the best clearance is for under the doors so I can discuss w/my builder. I think doors go in next week.

Thanks again for the valuable advice.


    Bookmark   May 20, 2009 at 11:28AM
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Our media room is next to the laundry room. There is fiberglass sound insulation and resilient channels, and when the doors (NOT solid core) to the two rooms are closed, you cannot hear the washer in its noisy rinse cycle in the media room. I heartily recommend resilient channels and insulation.

    Bookmark   May 20, 2009 at 8:47PM
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Green glue is another option.

Carpet and pad will help soften the noise within a room but to keep noise out the resilient channels are needed.

Paneled doors can be a problem but remember if the perimeter is not sealed they will still leak sound. Doors are undercut 5/8" or so to allow area rugs to fit under the door swing. Traditionally the large gap was solved by a raised door saddle which is in great disfavor today because of lazy flooring installers and wheelchairs. However, the saddles do work. Automatic drop seals are available at a cost.
If you are going to paint the paneled doors you could consider solid MDF paneled doors in a 1 3/4" (exterior door) thickness where the panel thickness 3/4" to 1" thick.

Here is a link that might be useful: Bolection doors

    Bookmark   May 20, 2009 at 10:50PM
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