Hi all - I recently saw a contractor installing baffles in an attic that didn't have soffit vents - they had gable vents. Do others see this? And are there real benefits to this, maybe by keeping the insulation from touching the underside of the roof to protect it from extreme temps or condensation? Do others think the time/costs are worth it? 

Tags: baffles, roof, vents

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Yes  insulation should never touch the sheathing unless it is meant to & designed as a hot roof system. The other item is future proofing - they may close off gables &/or install soffit venting later or even add inline / intake/ eave whatever they wish to call it venting later

Thank you!

Sean -  I'm curious as to why you say the insulation should never touch the sheathing?  That's one I haven't heard before.  Insulation in walls touch the sheathing.  I've never liked soffit venting, and certainly baffles are critical when soffit venting is there to reduce wind-washing problems but that whole process reduces the R-value at the top of the exterior walls.  Is there something I'm missing about insulation contacting the roof deck?

Ever seen insulation frozen to a roof deck? The problem at least further up north & shoot even other areas down south is moist air flowing through the insulation & then hitting the condensing surface. If said insulation is touching the surface then not only do you have more of a reduction to the R Value but you are allowing for it to build up leading to rot & other damage. With the gap you have that escape valve for it to disperse.

But wait what about flat roofs - most don't have insulation all the way up & for the rest, well obviously it was the bad roof install

Yes this can happen in walls to but for the most part the bulk of any moisture / vapor flow especially during the winter is going to be heading up - not straight out through the walls  

The whole rationale for venting a roof is to provide a path for moisture to escape. The standards date back to the 1930s when fiberglass was beginning to be popular as insulation. Air moves through the fiberglass and if the roof deck is below the dewpoint, the moisture will condense. Once it condenses, it will have a difficult time evaporating and escaping the rafter bay if there is no air ventilation path. A vent chute combined with a soffit and ridge (or gable) vent provides that path. I do not see how the R-value is reduced if the insulation is touching the roof deck.

The problem of condensation is usually confined to roofs in northern heating zones, since they lose heat quickly by radiational cooling at night. Exterior wall sheathing doesn't reach the dewpoint unless there is very little insulation or the interior RH is high.

Any air movement through the soffit/chute/ridge venting is either by wind or heated air rising. If the roof slope is low, there won't be much of the latter. If there is no soffit or ridge venting, the moisture will have a harder time escaping, and will depend on diffusion to escape. Of course it all depends on how much condensation there is.

Wet insulation = less R Value & if in direct contact with sheathing it can get soaked (it still can with a gap if it drops on it - but that means you have an even bigger issue) As you said it all depends on just how much water vapor / condensation there is

The problem never occurs in wall sheathing... I think there are a few that would disagree / show pictures but one main reason why you see more issues higher up is because cold dry air comes in generally lower while hot moist air goes up & out - stack effect, etc...

I didn't say never. Read my qualifying remarks regarding walls.,

The question I have is - Where did the moisture/ vapor come from?
If you have a moisture problem in the attic the solution is not just to add a baffle so the insulation doesn't touch the sheathing.
A better question is why do you have warm moist air in your cold (dryer air) vented (outside) attic.
Rule #1 - Never try to control energy movement - we manage it
Rule # 2 - Always go to the source to resolve the problem
Rule #3 - Block pathways - Air seal bypasses from condition space to unconditioned spaces.

If you don't have a roof leak where is the moisture coming from?
High occupancy, bathrooms, indoor hot tub or hanging wet cloths to dry in basement?
Either you keep the roof warm so not to reach the dew point - difficult in the winter or reduce the RH. (Ventilation)
There is really no reason why properly installed insulation can't touch sheathing provide one uses a whole house approch.

Even in a cold attic above a tightly sealed house, you can have condensation in colder climates. Think of a relatively warm, sunny day in winter heating up the attic air as it exchanges with outside air. Warmer air can hold more moisture. At night when the temperature drops a lot, the attic will cool down quickly, especially on a clear night (radiational cooling), with a good chance of hitting the dewpoint for that attic air.

This is true, it does happen, but it tends to not last very long as temperatures even out between inside and out, generally making it a non-event. 

The leaky houses are the problem (in winter, up north), where an endless stream of warm moist air can get up into the attic. 

Moisture comes from inside & outside (not including the famous bath fans & dryers that vent into the attic) - a ventilated roof is to help reduce minimal loads that do get in to these areas. http://thehtrc.com/2011/air-sealing-attic-baffles (piece on attic baffles & includes link to ice dams / attic condensation)

Piece on ways of controlling humidity - http://thehtrc.com/2015/faq-window-wall-condensation-iced 

properly installed insulation = read article above as it would have baffles

I would imagine that they did that so that they didn't have to worry about wasting insulation getting onto the soffits (assuming they were installing loosefill and not batts)

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