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Insulation, AC unit questions

Hi I'm looking for some help regarding insulation and ac units in old homes.
Our house was built in 1891.

Before we moved in, the upstairs was cooled/heated by window units. We installed a new ac unit in the attic sometime after we moved in. 1st problem, they installed a 1.5 ton unit instead of a 2 ton unit which is what we apparently need. We've had issues with this unit starting a couple of years after it was installed. I've tried to convince my mum to have the unit taken out and replaced with the correct one but to no avail. Currently, it has been "fixed" but it stays blistering hot upstairs during the day. Long term - shouldn't we change to the 2 ton unit? Or will fixing other issues (insulation, windows) make this unit work better for us?

Also, when they opened up the attic to create space for the unit we also reinsulated up there. That was about 8 years ago. Since then, the insulation that was up there has disintegrated. I've read a few articles about spray insulation vs the regular pink insulation and have seen mixed opinions. We aren't looking to insulate anywhere else but the attic. Is it okay for a house this old, to use spray foam insulation? I want a solution that will last but I also don't want to affect the integrity of the house since apparently spray insulation can't be removed from the wood.

Also, our windows are very leaky so we will be adding storm windows to every window in the house to help with heat/cool air loss.

If anyone has any knowledge, articles, or links to other threads that would be helpful I would really appreciate it!

Re: Insulation, AC unit questions

A larger unit may not solve your issues. Attics get very hot during the day so the first step is to make sure that it is well ventilated. Without seeing your attic first hand, I'd guess that yours isn't, but that is based on most attics not being vented very well. A fan in the gable may help your AC to work better than anything else.

The next thing to look at is the duct work. A long, leaky, uninsulated duct in a hot attic will make the AC almost useless. The ductwork needs to have all the seams sealed with a mastic, then at least 2 inches (R-6) of fiberglass insulation wrapped around them. The insulation must have no gaps in it.

Then concentrate on the heat barrier between the attic and the rooms below. The pink stuff is just fine, and considering how hot an attic gets, I would be concerned that any foam insulation would break down over time much quicker in a hot attic than it would in a wall.

Fiberglass is really stable, if your insulation is breaking down like you say, then I'd suspect that it is cellulose and not fiberglass. But fiberglass will settle over time and appear to be thinner. You need to have the fiberglass deep enough to cover over the tops of your joists. If you have a floor in your attic, then filling the cavity more than 2/3 rds the way up does not add anything. If you don't need an attic floor for ventilation, then remove it and insulate at least 2 inches above the joists.

With a properly vented and insulated attic with sealed and insulated ducts, your AC should work adequately. You want the smallest AC that will work so that it runs longer. By running longer, it keeps the humidity level inside the house lower helping you to feel even cooler than the temperature may indicate. This will also help with your energy bill.

Re: Insulation, AC unit questions

Something else to remember is that giving it less work to do will increase it's ability to maintain comfort. Caulking and better windows along with the rest already mentioned might just be enough to get it back into it's performance limits. I'd start by having an Insulator assess the situation visually, then bring in a HVAC contractor to do the same for that part. Both should be free or very cheap and give you some idea of what to do here.

Before sealing any deals, get at least three quotes to ensure that you're getting good advice and a good deal.


Re: Insulation, AC unit questions

I am not a big fan of powered roof ventilators where A/C is concerned. They can actually create a negative pressure in the attic and suck the cool air right out of the house through all the areas prone to leaks. The average house leaks like a sieve. Older homes are even worse. A house of this age is probably balloon framed and has hugh amounts of air moving through the walls to the attic. Every ceiling fixture, outlet, attic hatch door or attic door is a potential air leak where cool air can exit to the lower air pressure in the attic.

Ideally, you want perimeter soffit vents and exit vents as high up on the peak as possible. This creates a natural convection current of air without creating a negative pressure. The attic temperaturee, in the best case scenario would be about the same as the ambient outside temp. Your insuilation protects you from the heat or cold in the attic.

Re: Insulation, AC unit questions
keith3267 wrote:

By running longer, it keeps the humidity level inside the house lower helping you to feel even cooler than the temperature may indicate. This will also help with your energy bill.

By running longer do you mean keep it constantly on 24/7? How does that help with the energy bill? If you wouldn't mind explaining this to me I would really appreciate it!

Re: Insulation, AC unit questions

Thanks guys! Great suggestions! I'm going to get on this as soon as I can. I'll try and take a picture of the space this weekend maybe that will help better clarify what I'm working with.

Re: Insulation, AC unit questions


A too big unit will too rapidly cool the house down, leaving it cool and clammy. The unit that has to run a little longer is taking the humidity out too.

The more premium units will have two stage cooling that uses the one uinit to keep the house comfortable, but with a back up unit that kicks in when it is really hot.

Re: Insulation, AC unit questions

The 24/7 would be for the hottest week of the year. The rest of the time it would cycle on and off, but the on cycles would be longer than they would be for a larger unit.

Every time a unit cycles off, then back on, there is something called cycling costs. When the unit comes on, it has to first cool itself down, then cool down the ducts and then it can start cooling the room. These are the cycling costs, the time the unit runs before the cool air is felt in the rooms. The fewer time a unit cycles, the less it costs to operate.

The two and three speed and variable speed units extend the number of hot weeks that the unit can run 24/7, and then run at the lowest possible speed, which costs less than the higher speeds, as long as possible.

Re: Insulation, AC unit questions

I installed a Lowe's $90 power attic ventilator in a two-storey wood frame 1920s house with success. Of course you have to have fresh air supplies by installing roof vents down low on the roof including the front of the house if a hip roof. After that, you can set the thermostat/humidistat to kick on when you want. My house was literally 20 degrees cooler after installing the fan. AS for sucking air from the foundation or through electrical outlets downstairs via chimney effect, I found the fan took the air from the path of least resistance - the roof vents. Easy to install and worth trying. Saves huge money.

Re: Insulation, AC unit questions
cocteau3 wrote:

I installed a Lowe's $90 power attic ventilator in a two-storey wood frame 1920s house with success. Of course you have to have fresh air supplies by installing roof vents down low on the roof including the front of the house if a hip roof.

Was the fan installed at one of the gable ends or pierced through the roof ?

My "cape cod" attic (2nd floor is shared finished space & attic) is much warmer than outside air -- especially during the summer. Overall, the 2nd floor is 6-7 degrees warmer (summer) than the 1st floor despite my attempt to direct more A/C air flow to the 2nd floor (it's the opposite during the winter due to only R13 3" insulation all around (walls & ceiling joists). My intuition suggests that a fan would be useful in exchanging the air, to equalize the hotter attic air with outside air.

But my gables ends are not accessible (I have 7/8" plasterboard ceiling/wall so breaking through would be messy), and not sure if a roof mounted fan is a good idea.

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