The best way to insulate a 3 1/2 inch wall cavity?

[grover]

This is a question not a statement.  I have 2 x 4 walls and 7/16 osb sheathing then house wrap.  My son says most builders have gone to some sort of netting stapled (I guess) to the 2 x's and then blow the insulation in the cavity.  I was thinking, and this may be really anal, to get some 1 1/2 inch-2 inch thick 4 x 8 sheets on insulating board and cutting them to 14 1/2 inches wide strips and putting that in the cavity first and then blowing in the remaining with fiberglass.  The fiberglass would give me some place to run my wiring.  I could seal the edges of the foam board with great stuff and have a fairly air tight wall.  Is the house wrap all the moisture barrier I need?

Then to the ceiling but I'll leave that to another thread.

Oh, and by the way as far as my climate, I am in the midwest (Southern most part of Indiana).  How the heck did that get to be the midwest?  Maybe in the 1800's.

[Native_NM]

"Best" is subjective and varies by region.  The method you propose could lead to issues with mold and moisture damage based on an article I read  last year.  If I remember correctly, rigid foam works best on the outside of the sheathing. 

The latest "best" method in your area would be spray foam as it seals the entire area and acts as a moisture barrier.  Complete thermal break.  I think this varies from rigid foam because the foam adheres to the sheathing 100%; rigid foam can't match that and opens up the wall cavity to mold problems.

I spent some time researching over at buildingscience.com last year and blown insulation is cost-effective and as good or better than fiberglass IF installed properly.  On that note, about 75% of all fiberglass is installed improperly.  The other 25% is also.  :grin:

They built a new home near us and it used the wet blown system which sounds similar to what you described.  It was an Energy Star home.  I live in a very dry climate.  We watched them build the house and I spent a few hours bugging the builder and the subs.  There was no outside netting.  It went up slightly damp and filled the cavity 105% on the front plane.  They used a giant scraper to scrape the excess smooth after it dried and then drywalled after.  I touched it and it was fairly firm to the touch. 

SIPS use rigid foam between two sheets of OSB, but they are applied with pressure and adhesive that gives full contact which I think replicates spray foam.  I did see a video of a SIP plant and they actually used a giant vacuum press to build the panels.     

I've been redesigning in my head for years.   Meanwhile my brother finished his place using big-box fiberglass.  If it gets cold he throws another log on the fire.  I think there is a lesson in there for me someplace.

[mgramann]

I'm in the midwest, a little further north, so I'll weigh in with what I see here in Wisconsin. 

I believe your son is talking about a "blanket" type insulation system.  With blown in, they are nice as they resist settling.

Closed cell is probably the best method-but it will definitely lighten your wallet.

Typically, 2x4 walls won't provide adequate insulation(to code around here anyway) using methods other than those above.  This is why many times 1 inch foam/r-board is used as sheathing with 2x4's.  Have you done siding yet?  It might be worth running the numbers to see what it would cost to put 1 inch foam over the sheathing, and insulate with fiberglass batts.

In our climate, housewrap is not enough of a vapor barrier.  Part of the vapor barriers purpose is to keep the moisture from inside the house out of the insulation.  If you use fiberglass batts, you can get kraft faced.  With everything else(other than closed cell foam) I would use plastic sheeting.  On my latest project I went with unfaced fiberglass batts (cheap-no settling) and a plastic vapor barrier, as it is continuous.

I hope this helps.

[MountainDon]

 1.  The reason the wet blown cellulose makes for such great wall insulation is it is blown into the studs bays AFTER all the wiring and plumbing and what not is done. That allows it to get behind stuff and do a pretty good job of air sealing.

2.  Southern IN is climate zone 4 and R13 is all that the energy code requires for a wall. That is a minimum. More is a personal choice.
http://energycode.pnl.gov/EnergyCodeReqs/?state=Indiana

3.  In climate zone 4 when installing rigid foam on the exterior of a 2x4 wall infilled with batts or other insulation that could absorb moisture, any amount of foam will suffice. In brief, the idea is to keep the inside face of the wall warmer than the dew point. Zone 1, 2 ,3 and 4 (not marine) are mild enough  1/2" will do.

....fairly air tight wall

The worry about using foam infill in the wall stud bays in cold climates (zones 5, 6, 7 +) is just that phrase "fairly tight". Can you do a 100% job of sealing all the edges of the foam sheets to the framing? Leave a little gap someplace and moisture laden air can get in there and begin to make problems as Native_NM mentioned. When the foam is applied on the exterior the seams are all taped and/or sealed with foam friendly sealants. Much easier to get that right compared to the labor of infilling cut rigid foam, IMO.

Even though any amount of foam will work on the exterior I think there could still be a possible moisture trap there. But that would need more research. The safe way would be to avoid the possibility. IMO.


4. 

...housewrap is not enough of a vapor barrier

Omit the words "enough of" and that statement will then be more correct. Housewrap is an air barrier and it is a barrier to liquid moisture. Housewrap is made with a certain amount of vapor permeability to permit water vapor to pass through.  A vapor barrier, or vapor retarder is used only on the interior in northern climates, to keep moisture from moving from the interior into the wall cavities. Not all climates should have a vapor barrier; in some climates an interior vapor barrier is a disaster.

Note from buildingscience.com ....Zone 1, Zone 2, Zone 3 and Zone 4 (except Zone 4 Marine) do not require any class of vapor retarder on the interior surface of insulation in insulated wall and floor assemblies.

Look up the buildingscience website. There is a wealth of information there on insulation and air and vapor barriers or retarders. I believe every DIY'er should research the information available there for their own climate zone. Do the homework, it is free and the advice comes from people who have spent many years studying the issues. What's good for me does not apply to folks in ND nor does it work for folks in FL.

[mgramann]

Good advice on the zone applicability Don.  Where I come from, no vapor barrier=moldy insulation   The moisture from the interior infiltrates the insulation, and builds up on the exterior sheathing in winter.

Out of curiosity Don, in what zone would you not want one?  What can happen if you do?

[MountainDon]

Zone 1, 2, 3 & 4 (not marine 4) should not have a VB or retarder according to buildingscience.  This includes the use of vinyl wallpapers. In the South (hot humid), vinyl wallpapers are a haven for mold growth.  Loads of info on buildingscience for the curious.

[MountainDon]

One of their interesting documents

http://www.buildingscience.com/documents/insights/bsi-015-top-ten-dumb-things-to-do-in-the-south

[grover]

I think the insulation/vapor barrier issue is one of the most confusing things about building.  Probably because what works in one area will not work in another.
So if I understand correctly in area 4 if I add foam board it should be on the outside of the wall over the sheathing?  No other vapor barrier other than the house wrap? 

[MountainDon]

I think the insulation/vapor barrier issue is one of the most confusing things about building.  Probably because what works in one area will not work in another.

Agreed. Insulation / air barriers / vapor barriers (or retarders) is an area that can be very confusing. It takes a lot of effort to sort out. The problem of confusion is compounded when we can find all sorts of incorrect opinions on any number of websites and especially some forums. There is no one size fits all in this area of construction. One change can affect the way another works or does not work. That is why I encourage DIY builders to read the buildingscience info with particular attention to their location. Joseph Lstiburek is a recognized expert in this field. He's been at this since the early 1980's. He has a Bachelors, a Masters and a Ph.D in various engineering disciplines.

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So if I understand correctly in area 4 if I add foam board it should be on the outside of the wall over the sheathing? 

Not exactly what I said...  IMO, adding to the exterior layer is [1] easier, [2] more effective as it covers the thermal bridge caused by the studs, [3] goes a lot ways to make an excellent air infiltration barrier.  Adding foam to the exterior also brings along extra tasks such as how to finish around windows and doors, how to properly flash those areas. And exterior foam sheet insulation lends itself to making a good rain screen wall.  Coupled with the potential for creating moisture traps inside the wall when using foam inserts in the stud bays, yes I believe exterior foam is superior to internal wall foam.

That goes for any climate zone. The major difference from climate zones 4 (marine) and higher is that the higher numbered zones have minimum foam R-value requirements, depending upon the R-value of the fiberglass or other batt insulation used inside the walls.

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No other vapor barrier other than the house wrap?

Yes, but once again housewrap should not really be labeled as a vapor barrier. And, FYI there should NEVER be more than one VB whenever they are used.

I don't want to sound like an obsessive compulsive anal retentive idiot on that, but I probably do.  Housewrap like Typar, Tyvek, old fashioned building felt, etc. etc. are not vapor barriers. They are air barriers and liquid water barriers.

From the typar website...

"Why is air porosity important in a housewrap?
    Another term for housewraps is Air Infiltration Barrier (AIB). Preventing air leakage and weather damage are the two main functions of all housewraps.
    Preventing air leakage is the key to energy efficiency. All housewraps must meet industry standards requiring a high degree of air infiltration resistance, and TYPAR exceeds these standards on both wall section and whole-house testing. In addition, tears and nail holes can greatly affect air infiltration; unlike some other housewraps, TYPAR is exceptionally resistant to tearing and puncturing, which provides extra assurance of an air tight structure.
How much moisture vapor should a housewrap pass in order to prevent condensation problems?
    Experts agree that a housewrap should pass water vapor 5 to 10 times faster than the internal vapor barrier. Since most vapor barriers measure at 2 to 4 g/m2/24 hr., a minimum value of 10 to 40 g/m2/24 hr. is sufficient. This is the range for various types of 15 pound felts that have been in use for years. TYPAR HouseWrap is 3 to 4 times greater than this minimum value range."

There is lots of misuse of the term vapor barrier. I'm not picking on you. I'm simply trying to eliminate some of the confusion. Sorry if I come across rude, but you can see the manufacturer does not call their product a vapor barrier. We should not either. That helps to build a good knowledge base and help others.

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[Squirl]

This is a question not a statement.  I have 2 x 4 walls and 7/16 osb sheathing then house wrap.  My son says most builders have gone to some sort of netting stapled (I guess) to the 2 x's and then blow the insulation in the cavity.  I was thinking, and this may be really anal, to get some 1 1/2 inch-2 inch thick 4 x 8 sheets on insulating board and cutting them to 14 1/2 inches wide strips and putting that in the cavity first and then blowing in the remaining with fiberglass.  The fiberglass would give me some place to run my wiring.  I could seal the edges of the foam board with great stuff and have a fairly air tight wall.

As others have mentioned, there are cautions against the combining of foam and fill insulation because of condensation issues.  Cutting foam and sealing it is expensive and a giant waste of time and effort.  High R value foam board runs about a $0.75 a board foot.  Spray foam kits run about $1 a board foot. 

With something like sprayfoam, if you have a large job and the walls are already uncovered hiring it out can be cheaper than DIY.  The contractors buy in bulk 55 gallon drums.  Their materials cost is so low, when you add in their labor and equipment, it can still be cheaper than doing it yourself.

I'm in a northern climate. I personally like the comfort of a lot of insulation.  I went with very thick walls for blown in.  If I had 2x4 which (which I might do for added space on an edition), I would go with spray foam or exterior foam, and maybe both.

[John Raabe]

The best way to insulate a 3 1/2" wall cavity is as Squirl suggests... fill the cavity tight with a good spray foam such as Icynene (you can search the forum for earlier discussions). At that point you have a good blanket insulating the cavities and any additional insulation should be as an interior or exterior foam sheathing. This will add to the total R-value of the wall but more importantly will insulate the conduction through the wood framing.

This is expensive but can be economically justified in colder climates where fuel costs are high.

[hpinson]

Is that sort of spray insulation resistant to rodents? Will they nest and setup housekeeping in it?

[UK4X4]

If its closed cell......its tough stuff....I image its a no entry solution for mice-

least I hope so i used it to fill all the holes in my trailer to stop them getting in !

[Ndrmyr]

When insulating the 2" x 6" infill stud-walls in my post and beam cabin, I was fortunate enough to buy 4x8 sheets of vented nail board which is a roofing product consisting of sheets of 3" closed cell foam, pine furring strips on one side glued to the foam (for an ventilation/air space and 7/16" OSB stapled to the furring strips.  Retail is $200/Sheet but at auction I averaged $20 a sheet.  After using them for roofing, I pulled the staples, knocked off the furring strips and cut them into strips on the tablesaw.  They fit so well in the stud cavities, that they had to be pounded into position with a piece of scrap wood and a hammer.  There was still room in front of the 3" foam for wiring.  It's like being in a big wooden Igloo cooler.  A 8-10 degree temperature differential isn't uncommon.  After sheathing the exterior with 3/4" and interior siding with 1/2" Oak, it still seemed like it added a lot of rigidity to the structure.  I'd do it again although the cutting was messy since the foam is electrostatic and clings to the cutter with enthusiasm.

[Don_P]

Is that sort of spray insulation resistant to rodents? Will they nest and setup housekeeping in it?

I've used open cell in houses that were not mouse tight yet, doors not in. Although we saw evidence of them probing in corners I've not seen a mouse nest in it in that type of situation. Someone mentioned above a glued up SIPS panel. The ones I've used were blown. The skins and perimeter frame were assembled and polyurethane foam was injected into the hollow cavity through ports drilled around the perimeter framing. We did have a woodpecker widen out an injection hole, excavate a nest in the foam and set up housekeeping prior to siding. I evicted her, sprayfoamed the damaged area and sided, she bored thru the siding and reoccupied. I repeated the repair and was researching my options but she gave up.

I have had carpenter ants set up housekeeping in foam in our old motorhome, but it also had moisture problems, not sure which was the chicken and which was the egg.