Insulate a finished attic, after the fact

[jpdx]

I have a century-old home that had it's attic finished decades ago with sheetrock. Unfortunately, there is no insulation anywhere. What is the best strategy to insulate a space after it has been finished?

I am familiar with the blown-in insulation, but would like to avoid this for a few reasons: ventilation, settling, and I really don't want to patch all those holes!

I do have crawlspace access to the knee walls and can easily add bats there. I suppose I could pre-cut some ridged foam insulation in short lengths and shimmy them into the rafter cavities via the crawlspace. This would allow a gap above the foam for ventilation.

Appreciate your help...

[Linea_Norway]

I have a century-old home that had it's attic finished decades ago with sheetrock. Unfortunately, there is no insulation anywhere. What is the best strategy to insulate a space after it has been finished?

I am familiar with the blown-in insulation, but would like to avoid this for a few reasons: ventilation, settling, and I really don't want to patch all those holes!

I do have crawlspace access to the knee walls and can easily add bats there. I suppose I could pre-cut some ridged foam insulation in short lengths and shimmy them into the rafter cavities via the crawlspace. This would allow a gap above the foam for ventilation.

Appreciate your help...

Here in Norway they recommand insulating cold attics with something like Glava mats. This is insulation mat on rols that you can easily cut in the right size. You could put several layers on top of each other to create 12 inches stick of insulation.
This is a very profitable way to insulate your house.

[paddedhat]

If you're planing on a long term commitment to the place, you have at least two options. First, and the cheapest from a DIY perspective, is to tear the ceiling rock out, properly install baffle vents, insulate and install new sheetrock. Second would be to get a pro involved, to dense pack the space with cellulose.  The "need" for ventilation is not absolute. There are successes in everything from dense packing with cellulose or chopped fiberglass, to spray foam applications that are done without venting. Finally. I assume you have a cape cod style space with sheetrocked kneewalls. If this is the case, insulating the slope and the kneewall is the worst possible option. Ignoring the kneewall, and treating it like any other interior wall is best. This would mean that you insulate the rafters from top to bottom.

[jpdx]

Thank you, Paddedhat. More info: there are already several vents at the peak of the roof.  I may get a pro involved, but I want to limit cutting into the sheetrock due to potential asbestos in the joint compound. Also in one room the sheetrock was installed over wood planks, so that would be a lot of work to demo. I see what you are saying about insulating the rafters top to bottom but why is it a bad option to insulate the slope and knee wall?

[paddedhat]

It a bad option for a few reasons. First it keeps the knee wall attic space outside of the "building envelope" which means that the knee wall and the floor/ceiling below are exterior surfaces, from an insulation standpoint. Now, you have rafters without insulation, what is the chance that the ceiling below is properly air sealed and insulated to high standards?  About none, eh?  In a typical older home you may be able to effectively insulate an existing rafter with r-21 to r-30 batts, but there is a 99% chance that that knee wall is framed with 2x4s for a max of r-15. Next you have to place a finished panel over the back of the knee wall, and if it's a code inspected situation, this covering has to be fire rated. The reason is that batt insulation is not effective if it can be exposed to "air washing" of the exterior surface. IOW, without sheathing on a wall, the performance of the insulation is greatly compromised. Next the access doors to the knee space are now technically exterior doors, since they lead to the exterior. Are your doors insulated, weatherstripped and  ready to perform as outside doors?  No? Probably not, and if they typical hollow core doors they can actually sweat and frost on the interior surface if it gets cold enough. 

As for asbestos exposure in sheetrock mud, I would really do a bit more research before I got too freaked by the potential hazards present. First, unless we are talking about popcorn or other heavy textured surfaces, it is entirely possible to drill strategic holes in your ceiling and never get near the joints, or compound. Second a LOT of the information online is garbage, since it is "sponsored" by law firms, and others, who are seeking litigants for mesothelioma cases. When it comes to US government info. on the hazards, they would literally be the last source I would trust, having studied all the bullshit they pulled to create and support a billion dollar a year Radon removal scam. Just my opinion, but when it comes to old, smooth finish, painted sheetrock, I'll stick with the Canadian Government standard of basic dust control protocol, with dust masks, vacuuming, etc.

[jpdx]

Thank you for explaining that so well. I learned a lot!

[MrSal]

Thank you for explaining that so well. I learned a lot!

Yes paddehat is great! He exposes everything so detailed all the time. So much value on his recs I have done a lot of things where he gave his input and I followed it! A person to listen to!

[robartsd]

Attic ventilation is needed if you live in an area where snow can accumulate on the roof. Without ventilation to keep the roof surface cold, the bottom layer of snow on the roof can melt then re-freeze on the eaves creating ice dams and leading to water damage at the wall. If snow accumulation is not a problem, then attic ventilation is less critical, but still can be useful for helping roofing material dry out completely. Paddedhat makes good points about insulating the space behind the knee wall.

In concept, I like the idea of putting in the precut insullation behind the sheetrock, but I think that would actually lead to a lot of frustration.

If you don't mind the added thickness of the surface, you could use rigid foam over the existing sheetrock and an new layer of sheetrock for the new finished surface (extra long screws would be needed).

[paddedhat]

Thank you for explaining that so well. I learned a lot!

Yes paddehat is great! He exposes everything so detailed all the time. So much value on his recs I have done a lot of things where he gave his input and I followed it! A person to listen to!

Thanks for the kind comment.

[paddedhat]

Attic ventilation is needed if you live in an area where snow can accumulate on the roof. Without ventilation to keep the roof surface cold, the bottom layer of snow on the roof can melt then re-freeze on the eaves creating ice dams and leading to water damage at the wall. If snow accumulation is not a problem, then attic ventilation is less critical, but still can be useful for helping roofing material dry out completely. Paddedhat makes good points about insulating the space behind the knee wall.

In concept, I like the idea of putting in the precut insullation behind the sheetrock, but I think that would actually lead to a lot of frustration.

If you don't mind the added thickness of the surface, you could use rigid foam over the existing sheetrock and an new layer of sheetrock for the new finished surface (extra long screws would be needed).

I've done this, and it doesn't actually suck too bad to do. I would install a 1-1/2 or 2" layer of foil faced poly-iso, spray foam any gaps, and tape all the seams with silver foil tape. I would them flat strap, perpendicular to the rafters, on 16" centers with 1x3 pine, screwed to the rafters, as a base for sheetrock. 

[dilinger]

I've done this, and it doesn't actually suck too bad to do. I would install a 1-1/2 or 2" layer of foil faced poly-iso, spray foam any gaps, and tape all the seams with silver foil tape. I would them flat strap, perpendicular to the rafters, on 16" centers with 1x3 pine, screwed to the rafters, as a base for sheetrock.

This is the way to go, though I'd use multiple sheets of 1" polyiso with offset seams for airsealing.  You can often get cheap used or manufacturer reject polyiso on craiglist for this sort of thing; the used stuff often has screw holes but is otherwise in great shape, and multiple layers blocks air entry through those holes.

[paddedhat]

I've done this, and it doesn't actually suck too bad to do. I would install a 1-1/2 or 2" layer of foil faced poly-iso, spray foam any gaps, and tape all the seams with silver foil tape. I would them flat strap, perpendicular to the rafters, on 16" centers with 1x3 pine, screwed to the rafters, as a base for sheetrock.

This is the way to go, though I'd use multiple sheets of 1" polyiso with offset seams for airsealing.  You can often get cheap used or manufacturer reject polyiso on craiglist for this sort of thing; the used stuff often has screw holes but is otherwise in great shape, and multiple layers blocks air entry through those holes.

Good info. The manufacturer defect option is interesting. I have a friend who insulated a pole barn for almost nothing, using factory reject garage door insulation panels. He paid a few bucks a truckload for 1" polyiso that the manufacturer would of ended up paying a waste hauler to take to a landfill. Probably not high enough quality, and too many seams, to use in a house, but it works great in a barn.