7 Inches Of R-38 Really Necessary? - Insulation - DIY Chatroom Home Improvement Forum

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Old 10-04-2012, 06:47 PM   #1
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7 inches of R-38 really necessary?

Hi all,
I had a roofer/attic guy checked out my house and he said I have an inch or so of insulation in the attic. He recommended at least 7inches to make it R40. Right now, we're at R30. I'm wandering if this is really necessary and will I see a big difference in climate control? I know my daughter's room in the second is always hot in summer and cold in winter. Her room doesn't face the sun. The guy the extra insulation and a turbine would help. Is he feeding me a line of crap? Thanks in advance!


Last edited by need2speed75; 10-04-2012 at 08:29 PM.
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Old 10-04-2012, 09:10 PM   #2
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I just insulated the ceiling above our garage with R38. It was not previously insulated, but I installed a slit system AC unit so I can be comfortable in my woodshop! :-) While at it, we also increased the insulation over one bedroom and bathroom on the same end of the house as the garage.

The insulation company said in our area R38 has become the standard. No argument from me.
Amazingly, it has helped reduce sound coming in from outside of the house.


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Old 10-05-2012, 07:06 AM   #3
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R-30 to R-40 is not going to be that big a deal.

Going from R-10 to R-38 is a big deal.

Keep in mind though, if you have blow in fiberglass, it is only working at about 50% of its intended R-Value.
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Old 10-05-2012, 07:17 AM   #4
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For a better ansewer go back and add where it is you live in your profile.

You also need to concider what type venting you have, if the attic has been air sealed (filling any holes where wiring and plumbing were run through the top plates, light fixture and ceiling fan boxes)
Really need to make sure the soffit vents are not covered with insulation and in most cases a vidge vent works best.
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Old 10-05-2012, 08:42 PM   #5
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May be trying to get the extra work. The highly rated company in IL that came out to quote me wanted to install a few hundred dollar attic fan, but had zero experience or knowledge of air sealing or ventilating the attic. They just blow the insulation in for $2k. Yikes!

Get a few more quotes and see how they compare. Another tip is: look up a few home inspectors and see what they recommend. A good inspector will know the minimum values and methods recommended for your area.
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Old 10-05-2012, 08:56 PM   #6
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If you already have R-30 in insulating value, it's certainly more than one inch. (It could be one inch of fiber glass fluff plus several inches of somethine else underneath.)

If it is really just one inch of insulation over the ceiling panels or plaster then the R-value is certainly much less than R-30.

Fiber glass batts intended to provide R-40 of insulating value will actually provide much less if squashed to less than the intended thickness.

It is okay for insulation in the ceiling to extend above the tops of the joists. If you wanted to add an attic floor at a later date, you would need to shim the tops of the joists so the floor boards or panels don't squash down the insulation.


The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.

Last edited by AllanJ; 10-05-2012 at 09:06 PM.
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