Expert: Bruce E. Johnson - 7/20/2011

Question
QUESTION: Hi Bruce,

I've attached a pic, please let me know if you didn't receive. It shows exactly what I'm trying to write.

I have porches that have been become additions to the main house. Their are no walls separating the porches from the main house. From the interior, the main living space now flows into the porch additions.

The main house has a large attic with R43 fiberglass insulation.

I'm now remolding the porch additions. Currently the porch roof is:
metal roof, then felt, then OSB, then 2x4, then fiberglass insulation R13, then drywall (interior ceiling). It is NOT vented. The previous owners did it. Existed for 7 years, Oregon, very wet, only freezes a couple times in  winter.

1. I see no way to vent this! There is no space to vent. I'd have to leave out the insulation to vent it which is a no way can do.
2. One neighbor said ventilation is not necessary is a closed space with no dead air space. The best guy at Lowes (but no expert) said you ALWAYS need ventilation.
3. My roof case is just like a house wall; filled with insulation and no ventilation. The guy at Lowes said a wall is different. ?

If ventilation is 100% necessary, the only solution I see is changing the 2x4 (rafters) into 2x6. Use R13 insulation which will leave 2" air gap. All is a problem to do. The ceilings are already to low with 2x4 rafters.

Thank you very much for any help!

ANSWER: Hi Doug, I build unvented roof systems all the time.  As long as the space is sealed from the outside you should be fine.  The conventional wisdom has been changing on the subject of attic and roof insulation.  Closed, unvented systems are becoming more the norm now as opposed to vented systems that promote air and subsequently moisture infiltration.  An attic space that has the ceiling insulated but the rafters uninsulated is a different story, that needs to be vented to allow the heat building up to escape.  But in your case, you are correct there is no difference between your wall cavity and the roof cavity.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: Thanks for the quick reply!

If I build the porch-room roof just like a wall: Metal roof, felt, OSB, studs/rafters, fiberglass insulation, ?????barrier????, drywall(ceiling). Do you think I should put a breathable barrier (Tyvek) or a plastic barrier just before I put up the drywall? The guy at Lowes (who was wrong from the start) said if you close that space off he'd "put up a plastic barrier to make sure no moisture came down on the drywall." Then I thought Tyvek would be better since its supposed to allow air but not moisture.
So: no barrier; Tyvek, or plastic? just before I put up the drywall?

A tip for you: Tyvek original has been pulled from the stores in the Northwest. Replaced with Tyvek Rain Gutter ribbed paper. I have a good friend who has Tyvek original on his house. During the winter he started remolding his house from the inside. Pulled off all drywall. Could see the OSB from the inside of house. He noticed where he installed Tyvek Original he could see water condensation on the inside of the house walls on the OSB. But there was NO condensation on the inside walls that he had NOT rebuilt yet. Those outside walls still had the old felt. This is NOT new. We have been reading stories about this in our local news for several years that new expensive houses... occupants were "feeling" sick. Insurance investigators tracked it to mold in the walls. Now Tyvek original has been pulled from our stores.

ANSWER: Hi Doug, in this instance the felt paper on the roof is the vapor barrier..My take on the vapor barrier thing is "moisture needs to be stopped before it penetrates the wall or roof cavity so any vapor barrier on the inside of a wall or ceiling will actually trap moisture inside the wall cavity.  Your best defense against moisture is from the outside in.  Scrap the idea of house wrap on the inside before drywall..I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: Bruce,

Attached is a pic of the porch-room ceiling drywall. See those stain patterns on the underside of the drywall? Are those moisture marks? The whole ceiling shows NO MOLD or rot. Talking to neighbors, I believe this was built at least 12 years ago maybe 16 years ago? I forgot to wipe my hand across this stain, I can tomorrow. The previous owners were heavy heavy smokers and this was their favorite room. Unless you say this is definite evidence of moisture, your original evaluation of "okay to build no vent ceiling" is what I'm going to do. The insulation showed NO signs of the paper being rotted.

Do you think it's worth the high expense of using rigid/hard insulation 3" foam, the kind that won't absorb water? This site says to use blown cellulose on a no vent ceiling.
http://www.buildingscience.com/documents/reports/rr-1006-ba-high-r-roofs-case-st   (look under "Unvented Cathedralized ceiling".)

Answer
Hi again doug.. Yeah that doesn't look like moisture related staining and your insulation looks fine..drywall that is getting wet will be crumbly and show signs of deterioration..tobacco smoke or any smoke will penetrate drywall especially if it wasn't painted properly..if you don't want to tear all the drywall down you can paint it with a sealing primer like Killz..sincerely Bruce Johnson