Free loft insulation- costing me more to keep house cool..

[Obelix]

The loft space will be significantly hotter than the rest of the house if the roof is experiencing direct sunlight (like the windows you described), in which case opening the hatch will let hot air into the house from that space.

 

Hot air rises not falls - I can understand where you are coming from but every house I'm been in opening the roof hatch in summer results in a strong upward flow of air. Remember that the roof space is vented so a hot attic is continually pulling air at the eaves and it goes out the vents higher up.

[Cyclone]

Chimney like effect, that does make some sense.

 

But the space is still hotter than the rest of the house, so having it uninsulated would allow conduction through to the ceilings of the upper rooms and heat them up. Insulating it will keep the house cooler in the sun.

[Obelix]

For sure it will yes. When we had our house cavity insulated when I was still living at home, the biggest effect we noticed was not a warmer house in winter (although I reckon I shovelled less coal from then on) but we certainly noticed a much cooler inside during the summer heat. Once it cooled down ouside in the evening, we used to open the roof hatch and the cellar door and drew cool air all the way up the house from outside.

[Fogey]

Okay, so the house has a cavity wall in some parts at least, suggesting anything from the 1930's onwards. Are there any significant areas of vertical tiling on the external walls? (like roof tiling but on the wall) or areas of just cladding? These areas might be timber framed and so are unlikely to have benefitted from any kind of pumped cavity fill hence they will let heat out in winter and let heat in during the summer. These sorts of walls will need a different type of insulation applying which might require either the inner or outer face of the wall be removed first.

 

Lofts (like all cavities) should really be ventilated. As warm moist air rises through the ceiling and through the insulation it comes into contact with the cooler air and things like the roof joists/roof tiles/sarking membrane and the moisture condenses. The ventilation should be there to remove the moist air to prevent condensation forming.

 

Cavity wall insulation is a pretty good insulation method generally, the inner masonry leaf acts as a thermal store which helps regulate the temperature across the day, the insulation in the cavity makes sure the heat released by the inner leaf when things cool down goes back into the house rather than to the outside world. It also reduces solar heat gain through the wall generally. However, if you're having cavities filled after construction then a surveyor should be employed to carry out an assessment to show the risk of moisture forming in the wall build up. If moist air is likely to condense in the insulation or inner leaf then it will cause some expensive problems. Even condensing in the outer leaf might be an issue, softer materials like sandstone or poor quality bricks might quickly deteriorate in the face of months of moisture forming inside them. Modern cavity walls generally include an air gap to reduce the risk of moisture build up.

 

Windows that receive sunlight will increase solar gain and single glazing will generally let in/generate more heat than double or triple glazing. Solar films or solar control glass can help although they might affect daylight levels and can tint the light coming through the glass (like your windows are made from the glass in sunglasses). Thick curtains drawn during the day will help reduce solar gain. External solar shading or some form of shutter will help even more but you might need planning permission and they could look awful.

 

Opening windows on opposite sides of the house (or in different walls) and wedging the doors open in-between them should help with cross ventilation which should be significantly stronger than any stack effect formed by heat rising, especially where an external breeze contributes (lecturers on my degree claimed 36x more air flow for an equivalent open area). If you wedge your internal doors open then think about what that means for you if a fire breaks out. At the very least you should make sure there is a protected route between your bedrooms and an external door. Your kitchen and possibly any space with a boiler should not have a door left open onto this route.

[TimmyR]

The loft space will be significantly hotter than the rest of the house if the roof is experiencing direct sunlight (like the windows you described), in which case opening the hatch will let hot air into the house from that space.

 

That only happens because there is no airflow. If the loft hatch is always open then it'll stay cool in the loft.

[spilldig]

Another thing to take into consideration is what happened with our neighbours. After never having a problem with their guttering,the first year after having loft insulation it snowed. When the snow slid of the roof it brought the guttering down. When they mentioned this to the builder who did the repair he asked if they had loft insulation. Apparently it doesn't happen without loft insulation because the snow thaws slowly from the heat in the loft.It was plastic guttering though. I suppose wooden guttering might have stood the weight.

[Cyclone]

That snow melting on the roof is you paying to heat up a whole lot of frozen water.

[spilldig]

That snow melting on the roof is you paying to heat up a whole lot of frozen water.

 

That should go without saying. That's why I didn't bother saying it.

[Cyclone]

Not everyone would necessarily twig that though.

[Bojolo]

Insulation slows down the flow of heat form one side to the other side, if the inside of the house is warmer than the outside then it helps to keeps the house warm, if the outside is warmer than inside then the inside will stay cooler.

Flasks help to keep hot liquids hot and cold liquid cold, don't use a flask and the liquid will quickly change temperature to match the temperature of its surroundings, the same applies to your house.

 

You are correct. However, its a poor model for a house. A house contains appliances that generate heat. Windows where sun shines through and heats up the inside. The liquid in your flask doesn't generate heat. Our house is very warm upstairs in summer because that's where all our PCs are and any heat generated in the rooms below (wide screen tv and oven etc) rises through the uninsulated ceiling but hangs around on the second floor because it cannot easily escape the insulated layer above the ceiling i.e. the loft. So it is quite possible that his house is warmer in summer.