Is roof condensastion REALLY an issue in a big old victorian?
Hey Gang,
Attic guy here again.
INTRO: I’m trying to justify NOT installing rafter (and soffit, and ridge) vents in my attic/roof. I think I want to go with with rockwool insulation (8″ R32) Here’s my justification, does it make sense?
So, in regard to insulating a vaulted attic, where the attic becomes part of the conditioned envelope. I understand the issue with condensation problems, and I’m pretty well versed on the two main options (vented vs un-vented). But these best practices, seem to exist in situations where you’ve got new construction and a substantially “tight” general envelope in the entire house. (i hope)
What about situation like mine. I’ve got this big old Victorian house with zero insulation anywhere. It’s basically swiss cheese. It doesn’t have any kind of membrane anywhere, it’s plaster lath on the inside and wood siding on the outside.
Warm moist air is created by living/breathing, taking showers and cooking food. But in a house like this, where you’ve got only two people living in it, and so much space, and such old-school construction, many many places for air to seep in and out.
The question is: Will simply placing insulation on the underside of my rafters create some new thermal situation where condensation could become a problem?
The only other possible factor I can think of besides the swiss cheese part is that precisely because these houses had NO insulation, they are cold as hell and by the time any hot steamy air reaches the underside of the roof deck its…cold dry air.. So it is possible that attic insulation could change this…The house becomes warmer and warmer air holds more water and then this could lead to issues…but I have no idea.
I’m about to say screw it, and just put the rockwool up there and be done with it. I don’t feel like enough hot air and moisture will be trapped by the time that air reaches the 3rd floor, to cause any problems. Do you.
Also this room will be air-conditioned….hmmm…..what about that…?
If you made it this far…thanks!
Matt
Replies
Where are you located? Makes a big difference
I'm in Pittsburgh, zone 5
I’m in chicago, with a bungalow. Pretty much the same situation. 1920s construction. Previous owners put fiberglass insulation right on the rafters, which I’m sure would throw some people here into a fit. Here’s the deal with mine, not sure if it will be the same with yours but the previous owners here for whatever reason did not insulate all the rafter bays. I’ve recently gone and pulled some of the insulation out, and there were no problems the decking looked the same in the rafter bays with or without insulation. I’m not saying that moisture might not be an issue in some situations, but it seems like we need a lot more info before we all run around saying if you insulate rafters that the sky will literally fall.
I appreciate you taking the time to answer. good info.
Tough question. I suspect you're thinking correctly about the large amount of natural ventilation. But frankly, air sealing (https://www.finehomebuilding.com/2013/10/23/video-series-air-seal-an-attic) is where you should start before insulating. That's where you'll get the most bang for your buck, even in a leaky old Vic. And as to the moisture, in my experience, wet attics are usually the result of wet foundations (https://www.finehomebuilding.com/1999/09/01/fixes-for-damp-moldy-houses). Water comes in, gets picked up by the air, which rises due to convection (https://www.finehomebuilding.com/2010/07/15/how-it-works-the-stack-effect), and when it hits the cool roof sheathing, the water condenses.
So, it's complicated. Dry the foundation. Fix the air leaks between there and the attic. Then think about insulating and roof venting.
Thanks for the reply. Good links here as well.