DIY Doctor UK DIY Forums

Hallo all,

I would be grateful for some advice on upgrading the insulation in my house. I will be doing it myself and as such won't need to involve BC. The goal is to make a significant reduction in my fuel bills/increase in comfort whilst not taking too much space out of the upstairs rooms. I need to avoid spending more than I have to, as well.

It's a chalet bungalow & for various reasons probably easier to insulate the roof rather than the ceilings. The rafters are 100 x 50mm and the underfelt is non-breathable, which means I can only put max. 50mm of PIR board between the rafters. I really don't want to lose more space than I have to from the underside of the sloping bit of the ceilings as the rooms are small enough as it is. I'm thinking to put another 50mm of PIR board under the rafters, which appears to give a U value of around 0.25. It looks like this would be roughly twice as good as what is currently there, 100mm of fibreglass blanket between the underside of the roofing felt and the top of the plasterboard below (I'm not sure how the lack of an air gap will have affected the levels of condensation but from what I can see it all looks ok).

It looks like an alternative to the PIR board might be to use a multifoil under the rafters (which looks like it would take up a similar amount of space to the PIR board for a similar U value) and leave the fibreglass in place. Looking around the internet there seem to be a variety of different opinions on multifoil ranging from snake oil to wonder material and I wonder which material I am better off using? I'm leaning towards PIR board, which seems to be more of a guaranteed result.

I will also need some guidance on the finer details of installing whatever I end up with (for example how to apply the insulation around the purlins, what tape to use, etc) and would be grateful for a pointer towards where I could find this info.

Thanks very much in advance,

Chris

Hi macavity,
It is difficult to envisage the construction of your roof where yo can access all the undersides of the rafters. Assuming you can do this and you insulate under the rafters you will still need to deal with damp air in the roof space. If your room in the roof is a box you would do better to upgrade its insulation.
Regards S

Stoney,

Thanks for your reply. I haven't worked with PIR board before, or done such an extensive insulation job so your advice is really useful.

Re the construction of the house, in order to access part of the rafters I will have to rip out the plasterboard from the upstairs rooms where the roof forms part of the wall/ceiling & redecorate after. No way around this if I want to improve the insulation, eh. Other than that the rafters are accessible from the roof spaces, most of which I can get in to.

Re moist air in the roof space I'm not sure if you mean the space left above the PIR board under the tiles/felt or the space in the loft that would be under the PIR board and above the ceilings.

If above the PIR board then I suppose I'm relying on the air flow in the 50mm air gap to control the moisture.

If under the PIR, is it likely to be a problem? I'm thinking the air in the loft space will have a similar moisture content to the rooms below and a similar dew point as presumably it will be at a similar temperature to the rest of the house.

I could indeed insulate across the ceilings rather than to the apex of the roof, which would be cheaper as I would have the room to use fibreglass blanket instead of PIR board, but there's things in the loft that would get in the way (2 x empty galvanised water tanks) which don't fit through the hatch so I can't get them out without major ceiling surgery, and also pipes which could freeze up if outside the insulated bit of the house.

If moist air is going to be an issue then maybe I can build something to sit the tanks on above the insulation & do a thorough job of lagging the pipes. Presumably the join between the fibreglass and PIR board insulation would be ok if I'm careful to overlap one with the other?

Thanks again for your advice and I'll wait to see what you say before I decide what to do with the loft.

All the best,

Chris