Does an attic need to be vented? (slate roof)

We’re in the process of insulating and renovating an unfinished attic space in a 1915 home. Our assumption going into the project was that we would allow space for ventilation behind the insulation (under the roof) so that air could pass from the soffit vents to the ridge vent.

Well, it turns out that we actually don’t have soffit vents, and we don’t have a ridge vent, either. Looking up an down our street, it appears that all the homes on our street don’t have soffit vents.

So, the GC has suggested using closed cell or open cell foam to fully insulate the roof, and forget about ventilation altogether.

Because we have a slate roof, putting in a ridge vent would be very costly. I’m not really sure if it would even be useful, because it wouldn’t have anything to ventilate.

This is a pretty dramatic step, so I’m looking to collect as many opinions as possible before dramatically altering our roof in this way. We live in the NE, so avoiding ice dams is an issue.

How about opening the soffits and adding gable vents instead of ridge vents…just a thought.

What’s the slope of your roof?

Add a double set of gable end vents (low down near the bottom of the gable end and near the peak). Drill 2/3/4 inch holes on the soffits and add aluminum or plastic vent covers.
Partial ventilation is better than no ventilation.

Gable vents were my first idea, but I’ve heard pretty negative things about gable vents from quite a few professionals.

The slope of the roof is pretty steep, the roof is about 12 feet high and 25 feet wide.

I have seen many unvented 100+ year old attics ,they have survived this long so as the saying goes if it ain’t broke why fix it.
The slope sounds great to me to step to walk , I expect snow will not stay much.


Without knowing your location, site orientation and the specifics of your roof design, all bets are off as far as any of us really providing fact based info.

He’s some info from some of the most knowledgeable for general ideas on how to design your attic retrofit.

Click: BS

Appears some of my cohorts disagree with current technology…oh well

I agree with Roy. If it ain’t broke, don’t try to fix it.

I would seal any opening in the attic floor which would allow warm/moist air into the attic area. Slate is not air tight it breathes. I would not spray foam on the underside of the roof. Bad idea.

Very good site.

                 Roof ventilation is a big issue today in the roofing industry,                     and it should be. Asphalt roofing shingles nailed or                     stapled onto plywood decks are roofs systems that will                     not breathe – they suffocate the roof, and proper                     ventilation is imperative. Otherwise the plywood will                     delaminate and the roof will fall apart. Slate roofs                     attached to board decking, on the other hand, are breathable                     roof systems. They are not air-tight – they’re                     water tight. Ventilation may be necessary to prevent                     condensation occurring under the roof sheathing from                     warm inside air leaking into the roof space. This is                     easily achieved with gable vents or roof vents, air                     spaces between the insulation and the sheathing, and                     ventilated soffits. However, most older residential                     slate roofs had no particular ventilation systems associated                     with them. After a century, most are still in good                     working order. Such is the advantage of a roof that                     can breathe on its own. When these older slate roofs                     are retrofitted with roof insulation, care must be                     made to ensure that warm air will not come in contact                     with cold roof sheathing. Aluminum ridge vents should                     be avoided on slate roofs. Such vents are designed                     to be used with asphalt shingle roofs — as such,                     they’re cheaply made, they do not fit the ridge                     of a slate roof well, and they interfere with the normal                     maintenance of a slate roof.                   

strange how different areas say different things .

Added Roy
JLC has a great post on insulating roofs I will try and find it

Interesting information. I definitely feel convinced that an unvented attic would be the best for my situation (Massachusetts, front gabled roof with a 12/12 pitch). Now I just need to figure out the most cost effective way of getting there.

My thought was to do a couple inches of closed cell foam, and then fill the rest of the rafter bays with spray in place cellulose. But, after talking with one insulation guy, he told me that wouldn’t be much cheaper than just doing the whole thing in cc foam. I’ll wait to see the actual prices. I’m a little wary I’m going to have significant sticker shock.

have you looked into these products