Difficult insulation retrofit
I have been reading through many of the very informative blog posts about insulation retrofitting. We have kind of a difficult mess to work with. Our house is one and a half stories and we have previously insulated the sloped part of the roof, both sides of the knee-wall and then over the top of the flat part – above the ceiling. We have a conglomeration of materials because we tried to have foam sprayed between the rafters, both where they are exposed and also up in the cavities where they are not exposed. This was many years ago – at least 15 and I don’t know whether it was closed or open cell foam – all I know is that it was polyurethane foam. It was a dismal failure trying to spray foam up in the cavities and so we aborted it half way through. We ended up opening up the cavities from inside the rooms and scraping out all of the foam that was put into them. What I now see that we should have done THEN was have foam sprayed in between the now open cavities but we had had it with this guy because we’d tried to get some money back from him before we decided to proceed by doing a tear-out from inside. So, what we ended up doing was putting EPS in the open cavities (foaming them in for a tight fit). We have 2X6 rafters, so about 5 1/2 – 6″ thick EPS, plus 1 inch of Thermax on the interior then drywall over that. However, on one side of the house, the foam that had been sprayed in between the rafters was okay so we just put Thermax and drywall over it and didn’t vent that side of the roof (the south side). On the other half of the house we put the EPS plus Thermax but we used thinner billets of EPS such that we left around a half inch airspace behind/above them to ventilate at least half of our roof. We have soffit vents (unboxed soffits – the vents are perforated strips cut into the blockers between the rafters (I don’t know what you call those) and then we have a ridge vent. Above the ceiling we have a conglomeration of cellulose plus some fiberglass bats laid over top that had previously been in between the rafters.
So, now we need a new roof and realize we have another chance to address our insulation situation from above. We do also have some air leakage as it was difficult to do a good job of sealing around the partitions when we did the tear-out and retrofit from inside. I am hoping maybe by tearing off the roof sheathing in the problem areas we can address it from above but we are hoping to avoid doing a complete tear-off. What would you suggest? We are in Zone 6.
Diane Schneider
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Replies
Diane,
Q. "What would you suggest?"
A. It's hard to say, because I don't know whether your existing insulation is well installed or not, or how big your budget is.
If you have 5.5 inches of EPS, that's about R-20 -- not much. Improving your roof insulation by adding more rigid foam insulation on top of your existing sheathing is always possible. But I don't know how much of your sloped roof is insulated, and how much covers a vented attic.
It's about 2/3 sloped roof and 1/3 covering a vented attic. As far as budget goes - I'd like to know what the ideal solution would be and cost that out and then scale back from there if we can't afford it. We are also debating the merits of metal roofing verses asphalt shingles if that makes any difference or if you have advice about that aspect.
Thanks!
Diane
Diane,
In Zone 6, the minimum code R-value for ceilings is R-49, so you need another R-29 just to barely meet code. If you want to hit that number, you need to install at least 4.5 inches of rigid polyisocyanurate foam on your roof. Let's install 6 inches -- after all, you asked about the "ideal" roof.
Metal roofing is a better choice than asphalt shingles.
If we were to install that much insulation would you think it better to use 1 or 2 inch thick sheets, staggering the seams, or use something like Nailbase (assuming we could afford it)? And what ABOUT an airspace? Do we need one? And what about the airspace UNDER the sheathing we have on half our roof? Can we just close that off and still insulate above it over the sheathing?
Why is metal better than asphalt shingles and what about an airspace directly under that to prevent "sweating"? Is that necessary?
Oh, and one more question: A builder neighbor of ours is trying to sell us on the idea of P2000 insulation saying that 1 inch of it is equivalent to R25 or something. Any thoughts on that stuff?
Diane
Diane,
It's hard to answer all your questions, many of which are wide-ranging and basic.
1. If you install 6 inches of polyiso on top of your roof sheathing, you can safely seal the tops and bottoms of your ventilation chutes.
2. You can use 2-in.-thick layers of polyiso with staggered seams, or nailbase, or SIPs; your choice. You can price out these different options locally.
3. The distributors of P2000 insulation have often been cited for exaggerations and scams; avoid them like the plague. Here's more information: Beware of R-Value Crooks.
In your recommendation that we need at least 4.5 inches of polyiso were you taking into account the 1 inch of polyiso we already have on the inside of the rafters or doesn't that count for some reason?
I am also wondering, if you insulate up the entire outside of the roof over the flat/ceiling part that is insulated, what about the gable-end walls that are not insulated? Are you getting the "good" out of the insulation up over the peak when there would be heat loss out the walls/ends?
Diane,
Sorry, I forgot that you had 1 inch of polyiso. (Your original question was rather long.) So you can subtract 1 inch from the roof recommendation -- you're down to 3.5 inches.
And you guessed correctly about your gable walls. If you insulate your sloped roof, you will also need to insulate the gable walls of your attic.
Martin,
A few months ago, you wrote about the Chainsaw Retrofit. In one of the links, there were pictures of a secondary roof assembly built on top of the original roof. The cavities were filed with batt insulation. Would this be a practical and economical alternative for Diane? - Jim
Jim,
You are referring to a retrofit by Harold Orr.
I'm not a fan of fiberglass batts, but Harold Orr has used them for exterior retrofits. When used this way, they still have all of the disadvantages of fiberglass batts.
If they are being used in an insulated sloped roof assembly, fiberglass batts need two tight air barriers -- one below the batts and one above -- as well as a ventilation channel above the top air barrier.
Martin,
Would Roxul or cellulose work better? - Jim
Jim,
Perhaps. But the quality of the air barrier details will matter more than the choice between fiberglass and mineral wool.