How best to heat and ventilate this small building?

[Tony K]

Evening all. 

As part of my SB I have built a (very) small garden office, around 2.5m x 2m. As per the attached detail, it is single skin blockwork with 100mm PIR insulation inside. Though it is omitted from the detail, the outside will be wrapped in breathable membrane and then clad in timber, and I will use batons between the PIR and the plasterboard internally to create a service void. 

 

The building has power but no plumbing. It has a glazed door but no other openings. There are no trickle vents in the door and no space to add them. 

 

I need both to heat this room and ventilate it. I am thinking of a small electric heater for the former (I know they are not cheap to run, but I don't think I will need to use it much given the size of the space), but I am a bit stumped when it comes to ventilating it. 

 

I have considered a simple air-brick or open vent in the wall (maybe one at each end to get a through-draft), but will that not cost me a lot of heat?

 

I am not sure if there is any logic to an electric extractor fan. Would that not also require an open vent to draw fresh air in?

 

Any suggestions gratefully received!

 

Thanks.

 

 

Garden Office Details (1).pdf

[Mr Punter]

A 4" wall vent would do it.  Have it adjustable.  Without heat recovery there is always a balance between heating cost and ventilation.

[Tony K]

11 minutes ago, Mr Punter said:

A 4" wall vent would do it.  Have it adjustable.  Without heat recovery there is always a balance between heating cost and ventilation.

 

Thanks. Are you thinking of this kind of thing?

 

Timloc AeroCore Through-Wall Vent Set with Baffle Terracotta - 127 x 350 (dia x length)

 

Also, is there any benefit to fitting one at each end of the room to get a through-pull?

[JohnMo]

Ventilation get a Greenwood CV2 GIP from eBay etc, about £50. Set to lowest speed will not heat it running, will just pull air though the small gaps in the building fabric.

 

You need a 100mm hole through the wall.

 

Heating electric or join it to house heating or a through wall heat pump heater.

[SteamyTea]

If you change the air every half hour, that is about 30 kg/hour.

A kg of air takes 1 kJ/kg.K to heat.

So taking a really cold day, with a 30K temp difference, that would be 1MJ of energy.

0.3 kWh, or 10p.

There will be losses through the fabric, but I cannot work them out at the moment as too tired. Maybe when I get home.

[Tony K]

1 minute ago, JohnMo said:

Ventilation get a Greenwood CV2 GIP from eBay etc, about £50. Set to lowest speed will not heat it running, will just pull air though the small gaps in the building fabric.

 

You need a 100mm hole through the wall.

 

Heating electric or join it to house heating or a through wall heat pump heater.

 

Do they make a noise at all?

[JohnMo]

On minimum flow rate I had to take the cover off to check it was running just couldn't hear it. Not all fans are the same. Had a different fan and was definitely noisy although advertised as silent.

 

Just needs a live and neutral and it does smart humidity sensing.

[Russell griffiths]

My wife has a similar sized office in the garden, it has no ventilation apart from the door leading in, I have never noticed any form of musty smell or dampness in it anywhere. 
it’s highly insulated in the wall floor and roof, the air gets changed ever time she comes inside to make a cuppa. 
heating is via a small oil filled rad on a timer that comes on an hour before she starts work, she’s been in it since covid and loves it. 

[Tony K]

12 minutes ago, Russell griffiths said:

My wife has a similar sized office in the garden, it has no ventilation apart from the door leading in, I have never noticed any form of musty smell or dampness in it anywhere. 
it’s highly insulated in the wall floor and roof, the air gets changed ever time she comes inside to make a cuppa. 
heating is via a small oil filled rad on a timer that comes on an hour before she starts work, she’s been in it since covid and loves it. 

 

I think perhaps my main concern is that I plan to have bookshelves in there and actually put books on them. Knowing me, that will be where most of them stay for years at a time, and I don't want them to get damaged while I'm ignoring them!

[JohnMo]

19 minutes ago, Russell griffiths said:

she’s been in it since covid and loves it.

You should really let her out, that 5 years now.

[SteamyTea]

Just knocked up a quick spreadsheet.  I assumed the insulation has a k-Value of 0.03 W.m^-1.K^-1, is 0.1m thick, all surfaces covered, ground temperature is 8°C and airf temperature is 7°C, then the total losses, if heated for 24 hours, will be 3 kWh/day. Air changes an hour are 2.

 

If you have a time of usage electricity tariff i.e. E7, pop a second hand storage heater in, a 10 kWh one (1.5 kW input, 0.6 kW output).

[SteamyTea]

33 minutes ago, JohnMo said:

You should really let her out, that 5 years now.

Interesting series about it this week on R4.