Encapsulated Crawlspaces/Attics
jdclassen
| Posted in General Questions on
Often I am asked if is ok to leave the existing insulation in a crawlspace ceiling, or attic floor when encapsulating with foam insulation on the foundation or roof-deck. My answer is always that the performance is actually better when the insulation is removed, but I struggle to understand and explain why other than that I read it somewhere (which I cannot seem to find anymore)
Does anyone have a link to an article or some research on this subject?
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Replies
Jeff,
I am not an expert. When this question has been posted before, it's my recollection that the existing insulation can stay in place when converting a ventilated space to a conditioned space. But the homeowner may want to remove the old insulation if it is problematic (asbestos) or in poor condition (rodent dropping, for example).
In my townhouse, I had the old blown in cellulose vacuumed out before foaming the roof sheathing. It was pretty nasty looking, and I was concerned about indoor air quality.
Hey Steve,
Did you inform your HOA and, if so, what was their reaction? I wish I could foam the underside of my roof sheathing. Ahh to dream. :)
Joe Lstiburek has a column on his website about how warm air holds more dust than cold air, so if you have spot where warm air cools you will get dust deposits. Most old insulation is leaky, so it's full of these spots, and typically old insulation is incredibly dusty, even batt insulation. It's just nasty stuff.
Hey Steve,
Did you inform your HOA and, if so, what was their reaction? I wish I could foam the underside of my roof sheathing. Ahh to dream. :)
Our HOA doesn't care about what you do inside your unit as long as the changes don't affect other units or the exterior. Of course, some folks blew a gasket when the HVAC designer accidentally spec'd a commercial Daikin outdoor unit (one digit difference between the commercial and residential units, same performance but the commercial unit is twice as tall and therefore harder to hide behind landscaping).
Nice!
>"My answer is always that the performance is actually better when the insulation is removed, but I struggle to understand and explain why other than that I read it somewhere (which I cannot seem to find anymore)"
The reason that you're struggling is that it isn't actually true. Removing the insulation at the attic floor does NOT improve the overall thermal performance of the building, and in many/most cases removing the floor insulation will reduce overall thermal performance.
But once the roof is insulated the floor-R no longer counts toward code compliance, only the roof-R does. This is because the attic space is a potentially large thermal bypass of the floor insulation layer.
The only time it would be necessary to remove is if the roof insulation is insufficient for dew point control in the attic space, creating a potential mold hazard in the attic. But in most cases to create a dew point control hazard the R-value at the roof would have to be well below IRC code minimums. With R30 at the roof deck and R30 at the attic floor it's only begins to be a potential mold hazard for the attic in IECC climate zone 7 or higher. But with a code minimum R49 at the roof deck and R30 at the attic floor it's not a problem even in zone 7.
In my experience old insulation is nearly always gross crap that you just want out of your home. Plus it's in the way when you want to do just about anything. I've never regretted ripping it out and throwing in the nearest dumpster.
When you leave crawlspace overhead insulation in place, the crawlspace will be cooler than without, which can cause condensation and mold risk. Be careful with semi-conditioned spaces.