Loading and Insulating Old 2×4 Rafters
Good Day,
I’m dealing with a very old, shabby home that was built with 2×4 rafters (~20 inch spacing) and 3/4 tongue & groove roof sheathing. Currently the attic is very poorly insulated and I was planning to re-insulate with 3.5 inches of closed cell foam between the rafters and 4 inches of polyiso board above the sheathing in a few years when I re-roof.
My concern here is loading. That’s a substantial amount of weight to add to the roof. Is this feasible with the current roof and, if not, is there anything I can do to make it workable. I’m thinking of maybe adding support posts between the rafters and the joists so that they only span ~7 ft, or even less if that’s viable.
I appreciate any insight here. I’m located in Zone 4/5 on the east coast, so can expected significant snow events (that usually melt quickly).
Dave
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Replies
Dave,
I think that in order for the exterior insulation to have the intended effect, you'd need to have the attic sealed fairly tightly. Shabby may have been an exaggeration, but if it's drafty up there, the hot air will still find a way in. Are you planning on turning this into livable space?
2x4 rafters don't give you a lot in terms of space or load capacity, I think you'll end up making a lot of sacrifices and spending a little more trying to make it work, in terms of the total assembly R value. As far as load/support goes, that can be solved with supports in the right places. Sistering new rafters to the current is one avenue of gaining strength, as is adding a knee wall to help support the load.
What's the roof pitch? 7 ft is a pretty short span.
Kyle,
Thanks for the feedback. The reason that I'm trying to bring the attic into the envelope is that I have no subfloor or mechanical room so my furnace, well tank and piping are all in the attic. The only reason this hasn't caused frozen pipes is that the furnace keeps the space above freezing all winter. But I'd really like to move to a heat-pump system (and stop the horrific heat loss from a furnace in an uninsulated environment) so I'm trying to figure how to make that possible.
The current span of the rafters is 15' at about 6/12 or 7/12 pitch. Yes, knee wall was what I was thinking, which would reduce the span to 7'.
I'm pretty game to give this a try. If you or anyone has other suggestions or ideas on what I could do I'd love to hear them. Another idea I had was closed cell foam between the rafter and then polyiso below that, screwed into the rafters.
Thanks!
Dave,
I'm going to 'think out loud" here as I type, so don't be alarmed if I go back and forth with myself a bit. I would think that cost wise, it would be more efficient to add a knee wall to shorten the span, than to sister a beefier rafter to every side.
If you first only consider the loads being handled, the max. deflection on most type of beams with uniformly distributed loads scales with the length of the beam raised to the 4th power (L^4). Meaning a span of half the length results in (0.5 * L)^4 , or 1/16 of the original deflection. Assuming they're still in good shape, and there's no water damage, rot, insects, and the original fasteners are intact, and the tongue and groove is still in good shape, a 2x4 knee wall can be a cost effective solution, in terms of load support. So far, so good, but what about insulation from that assembly?
Back of the envelope on the R value from this assembly - 4 inches of polyiso (derated due to cold temperatures in the winter) = 22, 3/4 inches of wood = 0.5R, 3 inches (realisticly, they wont get it flush all the way to the edge of the rafters in a nice neat plane) with 20" spacing is somewhere around 14. That gives you (22 + 0.5 + 14) ~ R36. This falls short of the 2018 IECC specification of R49 for climate zone 4, or 5.
2018 IECC (https://codes.iccsafe.org/content/iecc2018/chapter-4-re-residential-energy-efficiency?site_type=public)
You would need another two inches of polyiso to meet those requirements, if you used the full R value rating, and included the r value of the 3/4" tongue and groove itself. Putting it on top means you'd need at least 8" screws (the price of which would realistically double the total cost of the project), and good luck hitting a 2x4 dead center every time with 6" of anything in the way. Screwing it on from underneath would require a layer of drywall as a fire block, which adds weight and deflection back to what we were trying to solve in the first place.
This is where I think the 2x4's fall short. It's not necessarily the load, it's the problems with the insulation values. Not to mention an especially vulnerable area, where the wall meets the roof at the birds mouth - it's the thinnest there out of all places. Not even sistering rafters will help you here, it's a pinch point that constrains the whole project.
I hope some of those thoughts are enough to chew on. If I were being perfectly honest I'd say that you're in for a new roof, rafters, decking, membrane, and all, to bring it up to code. Right now that's an expensive proposition, but with the cost of all the polyiso eliminated, thicker rafters, and no knee wall, it might not be as bad as you think. The upside to that is that you would end up with a much tighter assembly than you would have ever gotten with the tongue and groove, even if you'd taped the polyiso.
It's a hard proposition, I'm glad to not be in your shoes!
Hi Kyle,
Thanks for your thoughtful response. Definitely a lot to chew on. Replace the roof!! Yikes! I hear you, but that's a lot of money. Maybe I could just replace the rafters and save the decking. But that's only a few thousand in savings.
I was thinking if knee-wall + furring out the rafters would give me the depth I need to insulate between the rafters. I think 7" of close cell foam would give me R42, then only 1" of polyiso would do it....
I hadn't thought about furring out the rafters. That + a knee wall may give you the depth you need, if it doesn't eat too far into your headspace.
How long are the spans of those 2x4 rafters? You need to make sure they can handle the snow load for your area, and that snow load varies even within the same climate zone -- you need to know the snow load for YOUR SPECIFIC AREA. Be VERY careful with this. Roofs sometimes collapse from excessive snow load. The SAFETY of the structure is more imporant than insulation. Work out the structural issues FIRST, then think about insulation.
The easiest way to beef up that roof would be to sister in some deeper rafters. How deep depends on snow load, so get your snow load numbers, then size things for that and the dead load of the extra insulation you're planning to add. If you're concerned about how the new, beefed-up rafters will transfer all those loads, get an engineer involved to help you put together a good plan. This is somethng that needs to be done right. Make sure if/when you sister new rafters up there, that you put a bead of sealant between the old and new rafters for air sealing.
There are other ways to reinforce a roof, methods that basically involve supporting the rafters in other places so that they have shorter effective spans, but you need to know a lot more about the rest of the structure to know if that's an option here.
BTW, it doesn't matter how fast the snow melts for roof load -- if it's up there, it's up there.
Bill
Is the area open enough to just build a big box around your equipment? Then you could just insulate the box, and the floor of the remaining attic to keep the pipes warm.
Tom,
This is so simple I feel kind of foolish for not thinking about it. My piping runs over the
Joists so I would need to lay batting or something over them to ensure they weren’t in the middle of the cellulose and properly heated from below. But by far the easiest and cheapest solution. Thanks!
The problem with ducts/air handler in the attic is not just heat loss through insulated ducting. The bigger issue, and the larger source of energy loss, is that there are large air leaks especially around any fittings registers or plenums. Putting insulation over ducting won't help with this.
You can encapsulate all your HVAC ducting in a layer of SPF which would address both problems but your would still have to build an air tight mechanical room over your air handler. Depending on the layout of your attic, this could be simper and need less SPF than trying to insulate and condition your whole attic.
There is a lot of information out there about this, google "encapsulated ducts":
https://basc.pnnl.gov/resource-guides/encapsulated-ducts#edit-group-scope
There are a number of older homes with similar rafter size and span around me. The ones without any knee walls or collar ties tend to have bent rafters. Collar ties alone seem to do a good enough job to reduce deflection but tend to be in the way if you want to use the attic as storage.
Since you have oddball rafter spacing, one option could be to cross strap the underside of the rafters with 2x4s or 2x6 on edge.
You can spray foam the existing rafter space and install batts into the cross strapping. The cross strapping does most of the job of eliminating thermal bridging without having to install rigid above the roof. If you install mineral wool batts, these can also be the thermal barrier for the foam saving you the cost of intumescent coating.
This would get you a similar R value assembly as the SPF+rigid without having to do any work above the roof and should be cheaper to install.
Having the cross strapping at 16" OC would also mean that it would be much easier to finish the space down the road.
Hey thanks I like this idea. It like furring out the rafter but also adding rigidity. One question is what would you secure the strapping to do that it actually adds support? I feel like the ends need to be supported. Or maybe I’m missing something?
You need to support the existing roof with either collar ties, knee walls or both. That gives you the additional structural support.
The cross strapping (that is 2x4/2x6 at 90degrees to the rafters) is there to provide extra insulation space. It does add a bit of structural bridging between adjacent rafters, which might let you reduce the number of collar ties, but it doesn't add support.
The cross strapping is either toe nailed on or attached with long wood screws to the rafters.
Ah, got it. Makes more sense. Thanks again for the insight. I can see how this instead of furring strips reduces thermal bridging. Not sure if I’ll ever finish the attic but having 16 OC would definitely make it easier if I ever get that far.
When you say the span is 15', is that overall?....eave to eave, or eave to the roof peak? technically, a rafter span is from the eave to the peak.
A 15 foot span with a 2x4, even an "honest" 2x4, is an awfully long span. You're going to get deflection with that, especially with a heavy snow load.
It's also possible to tie 2x4s onto the lower edge of the existing ones to make a sort of DIY I-joist. This is a lesser known way to stiffen beams compared with the more common way of sistering a second one along side the original.
Bill
Hi Bill,
Span is eave to peak. It may be a little less I’m going to check tmw. But def over 10’. Question: if the rafter is sagging a little do I need to jack it up before sistering ina joist or adding a knee wall?
Thanks