roof venting w/small insulated soffits
Hi all,
I posted essentially this question a few weeks ago and didn’t really get much response so I’ll try again.
I’ve got a colonial with very small soffits – 3 – 3.5″ wide. The 2nd floor ceiling line is flat over the main portion and sloped to follow the roofline at the front and back (I believe this is called at tray ceiling? but I’ve seen capes like this). There is a variety of loose insulation in the attic and and the rafter bays (which are 3′ deep due to the sloping roof at the edges) are full of insulation.
We are reroofing and I’m trying to figure out how to ventilate. I read Gene Leger’s “Complete building construction” chapter on this but I’m afraid it didn’t shed much light because the soffits are much narrower than his examples and sounds like they could allow snow/rain penetration.
My questions:
1. How should this be ventilated?
2. If getting the insulation out of the rafter bays is crucial – how do I do that? I can’t reach the inside of the soffits from the attic – they are too far. There’s some fiberglass up there -is a shopvac a bad idea?
Thanks for any help.
-Bob
Replies
Well, one question I would ask myself is: Given my climate, the intended use of that space, etc., what is the best insulation strategy, how thick should that insulation be, and do I need a ventilated roof? There was a long debate here on the pluses and minuses of insulated, non-vented roofs vs. the current gold standard, i.e. uninsulated roofs with vents along the ridges and soffits.
For some good information to chew on, have a gander over at buildingscience.com and look into their roof section. Insulating the roof and closing the soffit vents makes a lot of sense, for example, if your home has AC, ducts, or heating equipment up there. The new roofing materials would have to withstand the slightly higher temperatures that a sealed roof entails, but the difference in Las Vegas was 17°F, IIRC.
Your local code and inspector might also weigh in on this, so I'd take a closer look at those factors also.
For what it's worth, we Corbonded the roof/rafter bays and couldn't be happier. Our AC air handler up there is sitting in conditioned space, duct losses don't cool the roof as much, and the sprinkler system is safe from freezing conditions as well. On the outside, we applied DuraSlate, a recycled PVC slate material which has also held up great, so far.
RayMoore has had great success in TX with externally applied insulation, i.e. the PERSIST system, so I'd look into that also. It doesn't really get hotter or more humid than down there...
If providing a spec-grade ventilated roof is necessary, I would put screens over the soffit entrances and lead U-shaped vent pipes down to them. Attach as necessary to clear the insulation between the conditioned space and the roof. More importantly, seal all ceiling/floor penetrations to prevent air movement into the roof space. Fiberglass does not do that job, only foam and other non-porous materials will.
We live in Massachusetts and we don't plan on using the attic for living area - maybe just storage and hopefully the eventual ac ducts.I've read some of the arguements for superinsulation+no venting but I can't see how it would retrofit into this house - the rafter bays are 3' deep or so and the rafters themselves aren't wide enough to get a high R-value. Plus the money is an issue and I think venting and using the existing insulation system as is would be cheaper.Mainly I don't know how to get the blown-in insulation out of the rafter bays.
if the roof is being removed consider putting in spray foam before you put the new roof on. you may need to pull up the sheathing but getting a good insulation in place is worth the extra time to get to the areas you can't reach from the attic area.
Edited 8/3/2005 12:20 am ET by FoamMan