So I had an addition build on my house and am planning the insulation. The rest of my house has rigid foam on the outside under the siding but the builder said they use house wrap now and did not add this under the siding on the new section. Since it is a new building I want to go as efficient as possible. I found that R15 batts will cost almost $700 to put into the space. I is actually cheaper to go with R19 and shim the walls with 2x2 wood to add the cavity. Just for giggles I did the math on how much it would cost to use the R30 rolls and cut them to length. The cost for just the rolls is more than half of every other type I have calculated. I can add my own vapor barrier. The cost per board of 2x4 is also only a little more than 2x2s. It would still be cheaper to go R30 than r15.

This leaves a problem though. Two 2x4s only leave 7 inch wall cavities. This is unless I leave a gap between the 2x4s. I considered putting strips of 1inch rigid foam to prevent thermal bridging. This will still give me a total of 8 inches and that is it. So, my question is how R value will I realistically lose by pushing a 9 inch bat down to 8. I know it will physically fit because the damn batts usually roll out to about 6-7inch thich when you unroll them and sometimes less. Would the insulation loss justify just going with R15 or R19? Thanks.

House wrap is not a substitute for rigid foam. Is the new addition just 2X4 framing, then? Where are you located?

Yes very aware of the house wrap providing nothing other than a vapor barrier. 2x4 framing and located in Virginia so it meets code. However, since I dont want to pull drywall down later I would like to get it right the first time. Oh and I wish we had used a radiant barrier under the siding too. The existing house not only has 1 inch rigid foam but rigid foam with a radiant barrier. Seems kind of odd for late 1980s construction but it is nice. The radiant barrier does play hell with the cell phone reception out here too. I have a sprint airave for the inside of my house and since they left the existing insulation in the walls between the addition the thing does zero for cell reception in my addition. Also, worst case scenario the existing house only has R9 between the studs. Unfortunately, I found the same insulation under the attic stairs which were over the main staircase to the house. I had to rip out the stairs to put in R30 and reshim the existing framing to allow space for the R30. The stairway is no longer a hot or cold experience to walk up and down. 

Found some info on some people using a similar design. http://www.carb-swa.com/articles/homepage/R30_walls_final.pdf

Very encouraging to see that it works and the windows actually look pretty good too.

Also watched some youtube videos and saw some people retrofitting an old house by building on the outside and then filling from the top. This is not an option for me but I would be more than happy to build out inside the room with 2x4s, install the sheetrock, and then blow in from the attic.... would be very cool and easy.

Posted By kogashuko on 20 Jan 2013 11:37 PM
Yes very aware of the house wrap providing nothing other than a vapor barrier. 2x4 framing and located in Virginia so it meets code. However, since I dont want to pull drywall down later I would like to get it right the first time. Oh and I wish we had used a radiant barrier under the siding too. The existing house not only has 1 inch rigid foam but rigid foam with a radiant barrier. Seems kind of odd for late 1980s construction but it is nice. The radiant barrier does play hell with the cell phone reception out here too. I have a sprint airave for the inside of my house and since they left the existing insulation in the walls between the addition the thing does zero for cell reception in my addition. Also, worst case scenario the existing house only has R9 between the studs. Unfortunately, I found the same insulation under the attic stairs which were over the main staircase to the house. I had to rip out the stairs to put in R30 and reshim the existing framing to allow space for the R30. The stairway is no longer a hot or cold experience to walk up and down. 

House wrap is highly vapor-permable, and absolutely NOT a vapor barrier.  If properly detailed (that's a big "if"), it can be made into an AIR barrier.

Radiant barrier is nearly worthless under siding. It requires real air gaps on both sides of the material to achieve any significant performance, and then only at the temperature extremes.  But the inch of rigid foam makes a 2x4 wall perform as well as a 2x6 wall.  If there is at least 3/4" of nothing but air between the radiant barrier and the siding it may be giving you somewhat better than an R1 average improvement in performance but not much more, whereas the foam is giving you pretty much it's rated R/inch. (~R4 if EPS, ~R6 if polyiso.)

It's usually possible to "dense-pack" fiber insulation over the thin "econobatts" without opening up the wall.  This is done by drilling a ~2.5" hole in either side of the wall at each bay and snaking in the blowing tube to the top/bottom of the bay before blowing. In doing so it compresses the batts a bit (to a higher R/inch density) and adds R value, but it also dramatically improves air-tightness.  This is usually a very worthwhile retrofit in a VA climate.


Back to the original question, compressing an R30 batt into a 7.25" space delivers R25 at center-cavity, so if you took it all the way down to 7.0" you'd  be looking at ~R24.  But the thermal bridging of the studs cuts that down substantially to no more than R19-R20. http://numsum.com/spreadsheet/show/21111 (<< those are Owens-Corning's numbers- Certainteed has published similar tables with identical numbers.)

Any fiber-insulated cavity that is not fully filled will suffer severe performance loss issues at the temperature exteremes due to parasitic convection currents within the cavity and through/around the batt. It's far better to compression-fit a fatter low-mid-density batt into the space than it is to only partially fill the cavity.

Going to build out a second interior frame and dense pack fiber. Thanks for the help.