Hello,

I'm looking for suggestions for best practices with regards to insulation. I could better convey my questions with drawings and pictures but I don't see a method to include them, so I will do my best with words.My wife and I are removing a shed roof sunroom and replacing it with a larger reconfigured room. I have three areas of insulation concerns. The roof, walls and the floor. Location Northeast. Size 16x37.

Roof; 1.5/12 pitch. 5/8 Advan-tech sheathing. SSR metal roof. 11 7/8 TGI230 16" OC joists. 16' span. vaulted ceiling. sheetrock interior.

Walls; 2x6. 1" foil faced foam board over sheathing. vinyl siding. Sheetrock interior.

Floor; combination of 9.5 and 11 7/8 TGI 230 16" OC joists. (9.5's supported by old foundation). 3/4 T&G Advan-Tech. Inaccessible frost line foundation crawl space. shale ground covered with 6 mil vapor barrier. 2" foam board exterior foundation walls to sill.

Question; I want to use a combination of closed cell spray foam(to limit air infiltration) and fiberglass between joists and studs. I have done a lot of reading on this subject and haven't been able to come to a conclusion on what would be best to do. Only lots of different ways.

Can anyone suggest how and where the laminations of insulation, vapor barrier and air space should be applied and in what order for the situations I've described? Additional clarification info drawings and pictures available upon request.

Thank you
Ken Bignoli

You might be over-engineering your addition, and you may easily end up with more than one vapor retareder, which is a fundamental error. There should be one and only one vapor retarder, and in your climate, it should be located on the winter warm side of the wall assembly. Same with the roof/ceiling and the floor.

I would lose the foil face insulation over the sheathing, and lose the spray foam, because both could form a partial or complete vapor retarder. House wrap reduces infiltration while allowing the wall to breathe. Pay close attention to sealing around doors and windows. Those are the best ways to reduce infiltration.

Richard,

Thanks for your time and reply. Not to argue with you but just to discuss this as I understand it a vapor barrier is intended to stop warm moist air from condensing on cold surfaces. If there is no air movement then what is the mechanism for the saturated air to travel to an undesirable location? What would be the downside of not allowing a wall to breath if there if there is no air movement? The room we are tearing down had sheetrock walls, faced insulation, no sheathing and 1" foam faced boards (taped) on the exterior and vinyl siding. There were no signs damage in those walls and they were about 15 years old. I want to improve upon what we had since at this point all options are open. Although foam is expensive I see it as one of the best ways to accomplish what is intended to happen when insulating a house. Effectively stopping air flow (more so than any other method) and providing significant R-value to stop heat flow. Any additional comments would be appreciated.

thank you,
Ken Bignoli

There hasn't been a construction method yet which can stop all air movement, at least none that the average homeowner can afford. What can happen with two vapor retarders is that moisture is trapped between the two, and can lead to mold or structural decay or both.

An example: I saw a roof/ceiling assembly which had fiberglass insulation, with its vapor retarder on the winter warm side, as it should be. Above the insulation, someone had placed those polystyrene foam baffles that are intended to keep an air space open in tight places such as eaves, but here they were placed all the way up the roof. Even with an open joint every 18 inches or so, the baffles formed at least a partial second vapor retarder. The result was insulation that was dripping wet, simply soaked with water. That is what you want to avoid.

It is an absolute fundamental in the design of wall, floor or roof assemblies: one, and only one, vapor retarder, on the winter warm side of the assembly.

Violate this rule at your own risk.

Richard is right, one vapor retarder only. Many problems with moisture could have been avoided it this simple rule was followed.

As far as insulating the rest, I usually tell people in this situation to use the existing house as a guide - if it has fiberglass batting, then use that, same for the ceiling - if it blown in cellulose then use that for the new addition. This is assuming that it would meet your local building codes. It is a big waste of money to spray foam the walls if the rest of the house doesn't have it on the walls - you will accomplish very little. You mentioned that you have foil faced on the outside. I agree with Richard in that this may cause problems if your vapor barrier is on the cold side of the wall.

As for the floor - I've unfortunately had to do a few of these and I firmly believe that closed cell spray foam is the way to go. This is one area where you need very durable insulation and nothing compares in durability than spray foam - not even rigid foam. Mice seem to love to make nests in rigid foam.

Richard your comment on letting the house breath surprised me. I'm not aware on any situation where we can let a house breath anymore, albeit I'd love to do that in some cases but the need for heat efficiency has superceded that I guess. On a side note, since the building "scientists" has started tinkering with vapor barriers, mold in houses has dramatically increased. What's considered good building practices one year may prove to be wrong 5 years down the road. Progress right?

House wrap is precisely designed to resist infiltration while letting a house breathe. It does allow moisture to migrate through it and out of the wall. If it didn't do that, it would be a second vapor retarder, wouldn't it? That's what I was referring to when I said "let a house breathe"...in the old days, that meant lots of infiltration, but not any more. Maybe I should have said "let a wall breathe".

Jay and Richard,
Thanks to you both for replying. After reading a bit more and speaking to other professionals I believe there is no practical way for me to apply science here. The air outside is extremely variable and the inside air although limited in range is variable too. So instead of worrying about where the dewpoint of the inside air will be and in what material it will fall in I think I'm going to foam the entire structure. No air space, no fibrous insulating material, no vapor barrier, no house wrap, no exterior foam. I can't see any downside to foam other than cost and I'm not sure of this yet since I haven't crunched the numbers.

Everyone I talk to and all I read says one vapor retarder only. My original construction as mentioned before was sheetrock, faced fiberglass insulation, 1" foil faced foam on the exterior of the framing and vinly siding. So I had two vapor barriers, one on the faced insulation and the other the 1" foam board. So if this is a problem than there must be some other variable here to explain why after 15 years that there is no sign of moisture damage.

The only apparent damage to the original construction was due to mice. This time I will insure that they have to chew through at least 1 1/2" of wood to get in.

So now, why not foam?

Thank you,
Ken Bignoli

quote:

Originally posted by Richard Hetzel:
House wrap is precisely designed to resist infiltration while letting a house breathe. It does allow moisture to migrate through it and out of the wall. If it didn't do that, it would be a second vapor retarder, wouldn't it? That's what I was referring to when I said "let a house breathe"...in the old days, that meant lots of infiltration, but not any more. Maybe I should have said "let a wall breathe".

Gotcha, I guess I assumed everyones definition of "letting a house breath" was the same as mine, which is no vapor barrier - which means air is free to move in and out of the interior of the building, which is obviously against code. That's why I aaid it surprised me that you said it.

quote:

So I had two vapor barriers, one on the faced insulation and the other the 1" foam board. So if this is a problem than there must be some other variable here to explain why after 15 years that there is no sign of moisture damage.

The only apparent damage to the original construction was due to mice. This time I will insure that they have to chew through at least 1 1/2" of wood to get in.

So now, why not foam?

Faced insulation is not a reliable vapor barrier, the older kraft face got brittle over time and lost all characteristics of any kind of vapor barrier, that and you got lucky....

Why not foam? Other than cost, nothing. I'm an advocate of closed cell foam if applied properly, we use it all the time. Just make sure that it is thick enough to meet the perm rating for a vapor barrier. I believe that is only an 1 1/2" or maybe 2" but better check. Another good practice while installing it is to apply an inch or so on the first pass and the next day, come back and do the rest. The reason we like to do this is because sometimes when this stuff dries, it will shrink a little causing a hairline gap between the foam and the stud it's next to. Doing it in a 2 step process will fill in this gap on the second pass.

I would definitely check with your local building inspector about doing a foam roof, seems like everyone has different requirements for spray foam roofs nowadays.