Exterior Insulation the Best Approach in Retrofit

Originally published by: The Chronicle HeraldMarch 29, 2019

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Adding a layer of rigid foam on the outside of a building provides the single biggest boost in wall insulation for an existing home. Doing this before a residing job provides big benefits.

Q: What’s the best way to add insulation to the outside of my house? My friend is planning to apply vertical vinyl siding and wants to upgrade energy performance as part of the job. What’s the best plan of attack?

A: Assuming your friend is working from bare wall sheathing outwards, the best approach I know of is to apply extruded polystyrene foam to the outside walls, then the siding. This kind of foam is completely waterproof and retains insulation values quite well over time.

Normally, with wood siding, I recommend some kind of drainage space behind the siding, but that’s less important with vinyl than wood. Since the job involves vertical siding, some kind of horizontal strapping slightly thicker than the total thickness of foam will be needed to secure the siding. A one-and-a-half-inch thickness of foam would be a minimum. I’d probably go two inches.

The main challenge with this plan is the need to extend window and door sills and frames to accommodate the thickness of foam, waterproofing those extensions made to cover them. Custom-bent aluminum capping is the best approach I know of for this. Is there any chance of convincing your friend to use something other than vinyl? It’s economical, but weak. I often see broken vinyl siding.

Attic frost and condensation

Click to enlarge. The condensation on the side of this attic opening indicates leakage of air into the attic from inside the house. Leaky attic hatches are the #1 cause of energy loss and condensation in attics. - Steve Maxwell

Q: Is the condensation in my attic a problem? Earlier this winter I heard a faint drip-drip sound coming from the attic of my 17-year- old bungalow. When I opened the attic hatch, I found the edges of the hole wet. There’s also lots of frost on the plumbing vent pipe as it goes through the attic. My handyman tells me there’s no need for action. What do you say?

A: If this were my attic, I’d do something. The source of the moisture you see is indoor air that’s leaking into the unheated attic space. As this air cools, it loses some of its ability to hold water. This water comes out as liquid or frost, depending on how cold the surface involved is. One reason that unheated attics like yours are ventilated is so moisture from wintertime condensation can leave harmlessly as the weather warms up and the summer gets hot. The thing is, air leaks and the resulting condensation is never a good thing. A little leakage is inevitable, but the less the better. The fact that you could hear some water dripping tells me that too much condensation is building up.

Generally speaking, the number one source of leaked air in attics is the attic hatch. I can see from the photos you sent that it’s insulated on top (that’s good), but it’s also probably leaking air around the edges. The fit looks tight, but the fact that there’s condensation around the edges proves that air leaks are happening. Indoor air could be making its way into your attic through electrical boxes for light fixtures, but attic hatches are typically the big culprit.

One simple way to ensure a 100 per cent seal around the hatch is by applying peelable caulking to the seam when the hatch is closed. The best stuff is clear and seals well, though it can be peeled off whenever you want.

One other way of tackling this issue is to increase the amount of ventilation area in your roof. Next time you have the roof shingled, have a ridge vent added. Also, it’s not clear in the photos you sent, but your attic insulation might be blocking the free flow of air through the vents at the eaves of your roof. While limiting the amount of leaked air entering your attic is one part of the fix, boosting air circulation through the attic is another. Ask your handyman to check that the soffit vents are clear, then have the hatch sealed up.


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