Attic insulation: slate roof/double brick walls/no soffit vents
Hello,
I have a 1939 brick home with a slate roof.
I have attached a rough sketch of how I believe the walls/roof are constructed. The ceiling and walls are all plaster on lath…walls are on furring strips.
All the second story rooms have about a 2′ tall section of sloped ceiling along the building perimeter — the ceiling is raised above the height of the brick walls.
At some point, someone had blown in cellulose into the attic and shot it, very spottily, into the 6″ tall void that is formed between the roof deck and plaster/lath above that sloped portion of the ceiling. This runs all the way out to the roof fascia.
My home does not have any soffit vents. There is a single exhaust fan in the attic, but no ventilation otherwise (save for gaps/cracks in finish).
My question relates to that void space — what is my best option to insulate the attic while retaining proper ventilation and preventing things like ice dams (which we’ve never had, in my experience) from occurring.
I’m inclined to err on the side of minimal change — my home has stood for 80 years without many major issues. I would like to increase the energy efficiency, however, and know that making changes to do so could require some modifications beyond just stuffing more insulation in the attic.
Thanks, in advance, for any advice.
Best,
Chris
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Replies
Chris, I would suggest vacuuming out the insulation in those rafter bays, sliding Accuvent or another semi-rigid insulation baffle down the slope, and dense-packing with cellulose insulation. Preferably with 1 1/2" thick baffles.
Elsewhere in the attic provide venting equivalent to at least 1/150 of the floor area. It can be a combination of ridge and gable vents. If your house is 24' x 36', you should have at least 5.76 ft² in vent area.
While you're up there, air-seal any penetrations between the living space and the attic.
4.5" of insulation will give you about R-16, or R-12 when framing is factored in. Pretty low, but much more than you have now. This will increase the risk of ice dams forming, though--depending on your location, there may be enough heat loss to melt snow high on the roof but not enough to fully melt it when it reaches the eaves. Higher R-values mean less heat loss and lower risk of ice dams. Can you add rigid insulation to the interior of the ceiling? It could be foam, or something more interesting (with lower embodied carbon and health risks) like cork or wood fiber insulation.
Michael,
Thanks for the thoughtful and thorough reply!
I had typed up a lengthy response last night, but I wasn't logged in while doing so and the website glitched when I eventually did log in...looks like the response never went through..
Basically, I had already started on some of this work before seeing your response -- looks like we were on the same page.
I removed all the old, compressed cellulose from my attic (it was packed under fiberglass batting, also compressed...). I have picked up baffles and will insulate the areas in question after installing baffles.
I am in the process of gutting my bathroom...I'll probably insulate with batting now that the ceiling is open. I may just go ahead and remove that portion of the ceiling in the three remaining rooms -- one of the three had water damage from a long term roof leak caused by a face nailed slate. When I replaced the slate and the decking after purchasing the home, the lath in that portion of the ceiling was completely rotted away.