Q: We live in Kingston, NY and cathedral ceilinged the main living space of our 1925 raised ranch. The rafters are 2×6’s. we used R-21 fiberglass batts (5 1/2 ” thick) and 1″ pine tongue and groove boards. The roof is an unvented shingle roof. Is there an affordable solution to prevent humidity build up? – Kate
A: In cold climates, unvented roof cavities can lead to moisture problems. If moist air gets into the roof cavity, the moisture can condense on the underside of the cold roof sheathing and damage the sheathing or roof framing. Homes with high interior moisture levels are at risk, especially if there is are easy route into the roof cavity – for example, through recessed lighting fixtures in the ceiling.
Many contactors use closed-cell foam for unvented ceilings, because the foam, if properly installed, blocks moist air from entering the roof structure. Fiberglass batts, however, will allow moist air to easily pass through and reach the sheathing.
The best solution is to create an effective air barrier on the interior face of the roof structure. However, this is nearly impossible to do with tongue-and-groove boards.
Ideally, you would have installed an air-barrier material prior to installing the wood finish. One of the new “smart” vapor retarders, such as MemBrain or Intello would be best as these one-way membranes prevent air and moisture from entering the roof cavity, but allow the roof cavity to dry to the interior of the building if moisture builds up. That is, they allow moisture to pass one-way into the living space.
Another option would be standard drywall, carefully sealed at seams and edges to block airflow. You should paint the drywall with two coats of latex paint to provide a Class III vapor-retarder primer with a perm rating of 1.0 to 10. You never want to use foil, plastic, or any impermeable material on the underside of an unvented roof as the structure cannot dry out if it gets wet from interior moisture or a roof leak.
Also, keep household moisture levels under control in the winter – no more than 50% RH. Sources of excess moisture may include wet basements, uncovered dirt in a crawlspace, firewood drying in the basement, and kitchens and baths without exhaust vents. Small, tight houses with many occupants and lots of plants will tend to have much higher moisture levels than large, leaky houses with fewer occupants and a lone cactus.
If you’re seeing moisture condensing on the interior of insulated-glass windows, your moisture levels are too high. Or you can take the more scientific approach and buy a humidity gauge. If moisture levels are too high, first try to reduce the sources. If that doesn’t do the trick, increase ventilation levels. A whole-house ventilation system may be necessary in a small, very tight home. – Steve Bliss, BuildingAdvisor.com
Read more on Cathedral Ceilings:
Combining Spray Foam & Fiber Insulation
Insulating Cathedral Ceilings with Spray Foam
Insulating with Foam Board
Preventing Condensation in Cathedral Ceilings
Sealing Unvented Cathedral Ceilings
Marc says
How To Prevent Condensation on Ridge Beam in Attic?
We are in a similar situation. In Westchester County NY. Put an addition on the house last year, new bedroom with cathedral ceiling with a structural ridge beam.
In the finished space the ceiling is covered with drywall and then a wood façade (faux beam style). Then it ducks into an attic where it the triple LVL is exposed.
No roof venting at all and closed-cell spray foam in all the joist bays, etc. The new attic abuts a third-floor attic bedroom (original to the house from 1922). This is where the issue lies: on hot summer days I am seeing condensation dripping through/clinging to the underside of the triple LVL ridge in the new attic space. No visible moisture anywhere else. Only on the ridge beam.
Contractor and insulation installer think that air is leaking/seeping into the new attic from the old finished attic (poorly insulated, air conditioned, leaky in all directions). It’s causing condensation to form possibly above the ridge and then seeping down through. One of the 3 x LVL is separating from the other 2 by as much as 1/4″ and you can see the nails/bolts inside looking a bit rusty…! They are suggesting we open the ceiling on the old attic side and spray new closed cell everywhere we can, particularly in the bay where the new ridge touches the old roof. And then they want to encase the 3 x LVL ridge beam in the new attic with a layer of closed cell. What do you think of this?
buildingadvisor says
I’m having a difficult time following your description of the new insulated space (addition), new attic, attic where the triple LVL is exposed, and old third floor bedroom.
Perhaps some photos and/or sketch would help.
I’m assuming this is a cold weather problem — correct?
Some questions:
Does the addition have both a sloped (cathedral) section and a flat section, leaving a small triangular “attic” space at the top?
Does cathedral ceiling insulation go all the way to the ridge? If not, how is the flat section of ceiling insulated?
Or if the cathedral ceiling runs all the way to the ridge, is the triple LVL exposed in the addition or finished with other materials?
Please describe the “new attic” where you are seeing the condensation. How is it insulated and ventilated?
In general condensation forms when warm, moist air hits a cold surface that is below the “dew point” of the air. A familiar example is a the droplets that form on the outside of a cold beer can on a hot day.
If this portion of the roof is not insulated, the beam and roof sheathing will be cold in winter. If warm, moist air from the household is leaking into this space, you will get condensation as you describe. This is the moist likely source of the moist air, although in some cases it gets into the attic directly from the basement or crawlspace through a “chase” for plumbing, wiring, chimney or flue.
Without a clearer description of the various spaces, I can’t say whether your contractor’s approach will work. My suggestion is to hire a building energy expert to figure out the cause of the problem and suggest a solution. A significant a amount of moist air is leaking into the space. Finding and blocking those leaks is more likely the most cost-effective solution. While foaming over the beams may prevent condensation on the beams, you will still have problems with other cold surfaces such as the underside of the roof sheathing.
Marc says
See my responses to your questions, plus attached pics.
Does the addition have both a sloped (cathedral) section and a flat section, leaving a small triangular “attic” space at the top?
RESPONSE: The cathedral leaves no small triangle. The ceiling joists go from the walls all the way to the structural ridge. If there’s any space above that ridge it would only be 1-3inches where the sheathing extends slightly past the top of the ridge.
RESPONSE: The closed cell is in every joist bay all the way to the ridge beam though i doubt that the spray made it above the ridge. I’d need to ask the contractor.
Or if the cathedral ceiling runs all the way to the ridge, is the triple LVL exposed in the addition or finished with other materials?
RESPONSE: In the finished space it is clad first in drywall and then has a decorative wood beam (faux beam) around that.
Please describe the “new attic” where you are seeing the condensation. How is it insulated and ventilated?
RESPONSE: The ’new’ attic has closed cell spray foam applied in every joist bay from the soffit to the ridge beam. No ventilation.
In general condensation forms when warm, moist air hits a cold surface that is below the “dew point” of the air. A familiar example is a the droplets that form on the outside of a cold beer can on a hot day.
RESPONSE: Yup, familiar with this. Fair to say it works the other way too yes? If the surface is hot/warm/humid and cold air hits it, condensation forms as well?
If this portion of the roof is not insulated, the beam and roof sheathing will be cold in winter. If warm, moist air from the household is leaking into this space, you will get condensation as you describe. This is the moist likely source of the moist air, although in some cases it gets into the attic directly from the basement or crawlspace through a “chase” for plumbing, wiring, chimney or flue.
RESPONSE: We think that humid air (and at times cold air from the AC) is leaking into the ’new attic’ or possibly into the small pocket above the ridge beam (or both) from the old and poorly insulated ‘old’ attic/3rd floor bedroom which the addition abutts.
Without a clearer description of the various spaces, I can’t say whether your contractor’s approach will work. My suggestion is to hire a building energy expert to figure out the cause of the problem and suggest a solution. A significant a amount of moist air is leaking into the space. Finding and blocking those leaks is more likely the most cost-effective solution. While foaming over the beams may prevent condensation on the beams, you will still have problems with other cold surfaces such as the underside of the roof sheathing.
RESPONSE: I agree that a building energy expert will be best equipped to actually field test and identify specific locations where we have leakage. Right now, between the contractor, the roofer, the insulation contractor and myself (and now you) we are doing our best hypothesis based on research.
buildingadvisor says
Images posted for Marc:



Thanks for the pics and additional information. I wish I had an easy answer for you, but have to say I am stumped.
As for the laws of condensation working in reverse, they don’t. If cold air hits a warm surface, you will not get condensation. The surface must be below the “dew point” of the air, which you can look up on a chart. It’s possible that the air-conditioning system is cooling off portions of the beam making them colder than the ambient air – but that’s a long shot.
Some thoughts that occur to me:
– Is it possible that the gap in the LVL beam allowed warm, moist air to reach the small triangular space above the beam, which caused condensation and frost (frozen condensation) to collect in that space and then drip down when the weather warmed?
– Another avenue to explore is whether there are leaks in supply or return ducting of the air handler which is causing very moist air to be expelled into the attic.
– Leaks in the ductwork and also either pressurize or depressurize the attic, which can increase air leakage in the space, possibly introducing moist air into the space.
– The moist air can come from the basement or crawlspace directly into the attic via framing “chases,” which are framed openings for plumbing, electrical, chimneys or flues. These can lead to significant moisture problems in attics.
– Is there possibly a roof leak at the ridge. This is not common, unless there is a ridge vent without the proper baffles to stop windblown rain.
The idea of foaming around the beam may stop the problem, but I would be reluctant to do that without figuring out the source of the moisture.
Marc says
Thanks for the follow up and additional info Steve. Wondering if you could take a look at the screenshot I attached showing the architects elevation with roof spec [POSTED ABOVE]. The area I circled in red above the structural ridge – that tiny triangle – should that have closed cell foam applied like the rest of the underside of the roof deck? How would they have even done that? What are the implications if they didn’t?
buildingadvisor says
With Ice & Water Shield over the entire roof, I doubt that a roof leak is the source of the water.
And since this is a warm weather problem, condensation within the tiny triangle does not make sense.
Should they have foamed the little triangle? Not a typical spec and, as you point out, no easy way to do this. Could drill holes from above before roofing is installed and spray open-cell foam or another product with good flow characteristic.
At this point, it makes more sense to spray around the exposed portions of the beam (since leakage from above does not appear to be a moisture source). This will keep attic air from reaching the roof sheathing any time of year and keep the surface facing the attic warm enough to prevent condensation.
One issue not considered is the quality of the workmanship. Most LVL beams are built on-site from 3 or 4 separate LVLs lagged or bolted together. Perhaps the gap you see was always there, letting moist air reach colder areas of the roof (in winter).
Also, spray foam that is not mixed properly can shrink away from the rafters and other roof components leaving leaks. There have been a number of lawsuits regarding poor quality spray foam installations, although these mainly focus on alleged health issues.