Ideal loft humidity for PIV system

[noddy997]

Looking to fit a PIV but my loft has 75% humidity

I got heating pipes and tanks up there which are lagged except the cold water header tank.

I've installed some soffit vents and that hasn't decreased the RH.

I do seem to have a lot of loft insulation, will removing some help?

Will the PIV just be blowing the humid air into the house?

Is there an ideal RH level for lofts particularly when installing a PIV?

 

[RandomGrinch]

You may be looking at this incorrectly.
Relative humidity is, well, relative!
It provides a measure of the saturation of the air at a given pressure and temperature.
As cold air can hold less moisture than warm air - a high RH in the cold loft air, would equate to a low RH, when that air mass is warmed up to a comfortable indoor temperature.
Ideally your loft should have an RH close to that of the outdoor air.
The loft should be considered as a completely separated space from the house; it should be sealed from any sources of water vapour coming from the house, I.e. downlights should have moisture seals and the loft hatch should also fit well; the ceiling should have as much insulation as is reasonably possible.
The loft itself should have adequate ventilation, preferably allowing for a cross flow of air within the loft.
Any water tanks should have lids and be lagged.

 

[foxhole]

Looking to fit a PIV but my loft has 75% humidity

I got heating pipes and tanks up there which are lagged except the cold water header tank.

I've installed some soffit vents and that hasn't decreased the RH.

I do seem to have a lot of loft insulation, will removing some help?

Will the PIV just be blowing the humid air into the house?

Is there an ideal RH level for lofts particularly when installing a PIV?

Loft humidity levels are irrelevant .

 

[noddy997]

Loft humidity levels are irrelevant .

But won't the PIV push the same humid air into the landing?

 

[noddy997]

You may be looking at this incorrectly.
Relative humidity is, well, relative!
It provides a measure of the saturation of the air at a given pressure and temperature.
As cold air can hold less moisture than warm air - a high RH in the cold loft air, would equate to a low RH, when that air mass is warmed up to a comfortable indoor temperature.
Ideally your loft should have an RH close to that of the outdoor air.
The loft should be considered as a completely separated space from the house; it should be sealed from any sources of water vapour coming from the house, I.e. downlights should have moisture seals and the loft hatch should also fit well; the ceiling should have as much insulation as is reasonably possible.
The loft itself should have adequate ventilation, preferably allowing for a cross flow of air within the loft.
Any water tanks should have lids and be lagged.

I seem to have lack of ventilation and excessive insulation. I have suffered from oxidised cables in the loft which have result in intermittent heating and hot water. I changed a fan in the loft yesterday and the unused earth cable wss
and found to be oxidised!!

 

[RandomGrinch]

I seem to have lack of ventilation and excessive insulation.

Excessive insulation isn't an issue.
As before, ideally your loft needs to be a separated space from your home.
If you have oxidation issues with cable in the loft, then that can be caused by condensation, or damp.
To help, you need decent ventilation; you need to ensure there are no roof leaks; and you need to ensure no humid air from downstairs moves through unsealed gaps into the loft - again, as earlier, cooler air can hold less water vapour - warm moist air from downstairs will pass the dew point, when cooled in the loft.

But won't the PIV push the same humid air into the landing?

If there are condensation issues in the loft, this should be sorted, but even with a humidity of 70%; pushing that air downstairs into the warm shouldn't dramatically raise the humidity.

For example, very simplistically, if your loft has an RH of 70% at 5C, that would.equate to a vapour concentration of 4.75g/m3

At 20C, that same vapour concentration would have an RH of only 27%

 

[noddy997]

Excessive insulation isn't an issue.
As before, ideally your loft needs to be a separated space from your home.
If you have oxidation issues with cable in the loft, then that can be caused by condensation, or damp.
To help, you need decent ventilation; you need to ensure there are no roof leaks; and you need to ensure no humid air from downstairs moves through unsealed gaps into the loft - again, as earlier, cooler air can hold less water vapour - warm moist air from downstairs will pass the dew point, when cooled in the loft.


If there are condensation issues in the loft, this should be sorted, but even with a humidity of 70%; pushing that air downstairs into the warm shouldn't dramatically raise the humidity.

For example, very simplistically, if your loft has an RH of 70% at 5C, that would.equate to a vapour concentration of 4.75g/m3

At 20C, that same vapour concentration would have an RH of only 27%

How are you doing those calculations? Is there a calculator?

 

[david3456]

How are you doing those calculations? Is there a calculator?

https://www.calculator.net/dew-point-calculator.html Enter loft temperature and humidity and it gives the dew point. Enter temperature and humidity from readings inside your house and you get the interior dewpoint. If the former is lower than the latter, the PIV should do something useful. If the other way round the PIV wil do nothing.

Take readings at different times of the day though. In mine, the loft dewpoint is high during the day but low at night. I don't understand the mechanism for this but it's possible that a warm attic encourages evaporation from the water tanks. There is attic smell when the dewpoint is high.I have a system running Home Assistant and turn the PIV on or off according to conditions (Vent Axia instructions say nothing about not fitting a switch). I also measure CO2, VOCs and particulates. Raised CO2 is from breathing and does not come in from outside; the PIV helps reduce build up slightly. Attic smell does not seem to be accompanied by raised VOCs or particulates.