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I'd like to do a bad attic insulation job - will this work?

I'm only sort of kidding. House is very large, 100 year old brick center hall colonial in historic district in central Virginia. No exterior modifications will be allowed. Venting is by gable vents, around 4 sqft at west end, 8 sqft at east end. No soffit or other vents are present or possible due to historic district regulations (not to mention practicality). Large 3rd floor is built into the slate roof with dormers. Ceiling of 3rd floor is not even close to sealed. Extensive hvac in attic is not sealed or insulated even vaguely correctly.

Currently there is some mineral wool between the rafters forming the 3rd floor wall/ceiling. I propose to fill all rafters with wet blown cellulose, seal the gable vents, and insulate exposed brick gable on interior with flash and rigid foam. I will then add both a return and a feed from the hvac to the attic, making it conditioned space, and dealing with moisture.

I am not willing to have high density foam sprayed, as the failure risk is way too high, hence the need for mechanical moisture control - there's no practical way to spray reasonable lifts.. I think that's effectively what's happening now, due to the woefully insufficient venting and poorly sealed hvac and 3rd floor ceiling. In fact, I'd bet the interesting Venturi design of the gable venting (previously fan assisted) effectively depressurizes the attic nicely.

Obviously this is not ideal. But I'm not going to be able to make this perfect, or even close, considering my constraints, which are ironclad. Moisture permeable assembly and mechanical ventilation/ humidity control seem to be the only solution - no million dollar renovations removing 24 squares of 12/12 slate are practicable...

My thought is that interior moisture in the winter will be dealt with by the heat pump/low ambient humidity, and the high exterior humidity in the summer will be somewhat mitigated by the high heat load of the insulated slate roof. I of course expect no slate failure from roof overheating... Any moisture making its way into the attic through the insulation will be handled by the hvac, which is sized for dehumidification

Asked by Charles F
Posted May 28, 2012 11:31 AM ET
Edited May 28, 2012 11:41 AM ET


8 Answers

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I'm a little worried about one aspect of your plan -- "I propose to fill all rafters with wet blown cellulose." Do you mean the sloping ceiling above occupied space, or the rafters in your attic? In either case, you shouldn't use cellulose unless you have a vent channel between your insulation and your roof sheathing. For more information, see How to Build an Insulated Cathedral Ceiling.

You seem to be quite worried about moisture in your attic; why? Most attics are dry, especially attics with slate roofing. Are there signs of moisture problems? If so, you need to address the moisture issues. In older homes, wet attics are usually a sign of basement moisture issues.

Answered by Martin Holladay
Posted May 29, 2012 5:02 AM ET


Hi Martin, thanks for the reply.

I don't have a moisture problem currently in the attic, but expect I will after I fill the rafters, which partially cover the occupied space, and partially cover the attic. Hence the need for mechanical dehumidification.

I fear I wasn't clear above - I know how to do a cathedral ceiling, but the standard approach is not possible in this case. As far as I can tell I can either A) put no insulation, and vent most of my hvac to the outdoors, or B) put insulation without venting, and provide mechanical means of handling any resulting moisture.

Vapor impermeable Spray foam is not possible, for obvious technical reasons, nor is venting, so I'm pretty much left with providing vapor permeability and mechanical dehumidification, as I see it. Obviously this is "bad" but is also real world - waste some energy, or a lot of energy?

Answered by Charles F
Posted May 29, 2012 11:14 AM ET


If you want to create an unvented conditioned attic, and you can't include ventilation channels, then you need to use spray polyurethane foam. I'm not sure why you don't want to use close-cell spray foam. However, if you prefer open-cell spray foam, you can use open-cell.

If you install spray foam on the underside of a roof with skip sheathing and slate roofing, you need to first install a physical barrier to prevent the spray foam from gumming up the back side of the slates. You could use cardboard, plywood, or rigid foam pieces to be sure the spray foam doesn't contact the slates.

Answered by Martin Holladay
Posted May 29, 2012 11:26 AM ET


Though I like closed cell foam in its place, I'm extremely hesitant to install closed cell foam in very large passes, which would be the only practical way to do it in this case. Given the issues closed cell has had with large lifts - fire (minor) and odor (major) I'm unwilling to take the risk.

I have no similar problem with open cell, but I can't put the barrier in place without destroying truly huge amounts of lath and plaster, which is not practical nor very green. And though open cell is vapor permeable, what I'm worried about is moisture - the bane of unvented attic systems. I think that cellulose is more moisture permeable than open cell., giving me more opportunity to dry an assembly I'm about to wet.

I'm not sure that putting insulation without a vapor barrier in this case is going to be disastrous, even done passively, as I think solar heat loading and the gaps in the roofing will do something to mitigate vapor drive from the exterior in the summer, and ambient humidity in the house in the winter isn't that bad (house is incredibly leaky, as you might imagine, which is it's own project... but it does dehumify nicely... and plaster and brick are great moisture buffers)

That said, I think its conceivable that by insulating I will create a moisture problem, which needs to be handled. Given the HVAC is already in the attic, it would be very easy to make official what I expect is already happening by making it conditioned space.

I think my proposed solution is already in effect in a de facto manner. Though the attic is vented, its around 12 sqft of gable venting, no soffit for a 6000 sqft house - I can't imagine that's much more than nominal, just enough to depressurize the attic and draw up even more conditioned air than is already leaking out of the unsealed hvac. The space between the rafters over the 3rd floor rooms is already filled with mineral wool. I just plan on making it "official" by covering the gables with closed cell foam (full access available)

As I said, this is a "bad" insulation job. I'm just trying to avoid No insulation, which is even worse, and is the current situation.

Answered by Charles F
Posted May 29, 2012 12:17 PM ET


I agree with you that skip sheathing and slate roofing are vapor-permeable; that means that you don't have to worry about the accumulation of moisture in your roof sheathing.

I don't know whether it's advisable to install cellulose insulation under slate roofing, however. I can easily imagine some of the cellulose finding its way through cracks between the slates and showing up on your roof.

Answered by Martin Holladay
Posted May 29, 2012 12:22 PM ET


Now theres something that hadnt occurred to me. Wouldnt rain/wind take care of that issue though?

Per my readings of lstiburek, I'm less concerned with drying outward through the slate, which would appear to be a winter issue (low exterior humidity) and more concerned with moisture vapor drive into the house in a wet virginia summer - high external humidity, low internal humidity equals a strong chance of inward vapor drive, no?

Given how long humid summers last here, thats what i see as the main issue. I dont want to create a wet attic situation where i used to have none. But neither do i want no insulation (the standard local solution).

Answered by Charles F
Posted May 29, 2012 1:39 PM ET


There's a good reason that slate roofing is usually installed over skip sheathing: the air gaps behind the slates help them dry after they get wet. If you succeed in installing cellulose under the slate roofing, the back side of the slates will make your cellulose wet every time it rains.

You really want to maintain an air gap between the back of the slates and the insulation layer.

Answered by Martin Holladay
Posted May 29, 2012 1:43 PM ET


I agree. But practically speaking that's impossible, unless I don't insulate at all. Which I think is the worst solution. Hence the mechanical dehumification through hvac. It will still result in some energy savings over no insulation at all.

Answered by Charles F
Posted May 31, 2012 11:51 AM ET

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